This document is a reference manual for the LLVM assembly language. LLVM is an SSA based representation that provides type safety, low-level operations, flexibility, and the capability of representing 'all' high-level languages cleanly. It is the common code representation used throughout all phases of the LLVM compilation strategy.

The LLVM code representation is designed to be used in three different forms: as an in-memory compiler IR, as an on-disk bitcode representation (suitable for fast loading by a Just-In-Time compiler), and as a human readable assembly language representation. This allows LLVM to provide a powerful intermediate representation for efficient compiler transformations and analysis, while providing a natural means to debug and visualize the transformations. The three different forms of LLVM are all equivalent. This document describes the human readable representation and notation.

The LLVM representation aims to be light-weight and low-level while being expressive, typed, and extensible at the same time. It aims to be a "universal IR" of sorts, by being at a low enough level that high-level ideas may be cleanly mapped to it (similar to how microprocessors are "universal IR's", allowing many source languages to be mapped to them). By providing type information, LLVM can be used as the target of optimizations: for example, through pointer analysis, it can be proven that a C automatic variable is never accessed outside of the current function... allowing it to be promoted to a simple SSA value instead of a memory location.

It is important to note that this document describes 'well formed' LLVM assembly language. There is a difference between what the parser accepts and what is considered 'well formed'. For example, the following instruction is syntactically okay, but not well formed:

%x = add i32 1, %x

...because the definition of %x does not dominate all of its uses. The LLVM infrastructure provides a verification pass that may be used to verify that an LLVM module is well formed. This pass is automatically run by the parser after parsing input assembly and by the optimizer before it outputs bitcode. The violations pointed out by the verifier pass indicate bugs in transformation passes or input to the parser.

LLVM identifiers come in two basic types: global and local. Global identifiers (functions, global variables) begin with the @ character. Local identifiers (register names, types) begin with the % character. Additionally, there are three different formats for identifiers, for different purposes:

1. Named values are represented as a string of characters with their prefix. For example, %foo, @DivisionByZero, %a.really.long.identifier. The actual regular expression used is '[%@][a-zA-Z$._][a-zA-Z$._0-9]*'. Identifiers which require other characters in their names can be surrounded with quotes. In this way, anything except a " character can be used in a named value.
2. Unnamed values are represented as an unsigned numeric value with their prefix. For example, %12, @2, %44.
3. Constants, which are described in a section about constants, below.

LLVM requires that values start with a prefix for two reasons: Compilers don't need to worry about name clashes with reserved words, and the set of reserved words may be expanded in the future without penalty. Additionally, unnamed identifiers allow a compiler to quickly come up with a temporary variable without having to avoid symbol table conflicts.

Reserved words in LLVM are very similar to reserved words in other languages. There are keywords for different opcodes ('add', 'bitcast', 'ret', etc...), for primitive type names ('void', 'i32', etc...), and others. These reserved words cannot conflict with variable names, because none of them start with a prefix character ('%' or '@').

Here is an example of LLVM code to multiply the integer variable '%X' by 8:

The easy way:

%result = mul i32 %X, 8

After strength reduction:

%result = shl i32 %X, i8 3

And the hard way:

add i32 %X, %X           ; yields {i32}:%0
add i32 %0, %0           ; yields {i32}:%1
%result = add i32 %1, %1

This last way of multiplying %X by 8 illustrates several important lexical features of LLVM:

1. Comments are delimited with a ';' and go until the end of line.
2. Unnamed temporaries are created when the result of a computation is not assigned to a named value.
3. Unnamed temporaries are numbered sequentially

...and it also shows a convention that we follow in this document. When demonstrating instructions, we will follow an instruction with a comment that defines the type and name of value produced. Comments are shown in italic text.

LLVM programs are composed of "Module"s, each of which is a translation unit of the input programs. Each module consists of functions, global variables, and symbol table entries. Modules may be combined together with the LLVM linker, which merges function (and global variable) definitions, resolves forward declarations, and merges symbol table entries. Here is an example of the "hello world" module:

; Declare the string constant as a global constant...
@.LC0 = internal constant [13 x i8] c"hello world\0A\00"          ; [13 x i8]*

; External declaration of the puts function
declare i32 @puts(i8 *)                                            ; i32(i8 *)* 

; Definition of main function
define i32 @main() {                                                 ; i32()* 
        ; Convert [13x i8 ]* to i8  *...
        %cast210 = getelementptr [13 x i8 ]* @.LC0, i64 0, i64 0 ; i8 *

        ; Call puts function to write out the string to stdout...
        call i32 @puts(i8 * %cast210)                              ; i32
        ret i32 0
}

This example is made up of a global variable named ".LC0", an external declaration of the "puts" function, and a function definition for "main".

In general, a module is made up of a list of global values, where both functions and global variables are global values. Global values are represented by a pointer to a memory location (in this case, a pointer to an array of char, and a pointer to a function), and have one of the following linkage types.

All Global Variables and Functions have one of the following types of linkage:

internal

Global values with internal linkage are only directly accessible by objects in the current module. In particular, linking code into a module with an internal global value may cause the internal to be renamed as necessary to avoid collisions. Because the symbol is internal to the module, all references can be updated. This corresponds to the notion of the 'static' keyword in C.

linkonce:

Globals with "linkonce" linkage are merged with other globals of the same name when linkage occurs. This is typically used to implement inline functions, templates, or other code which must be generated in each translation unit that uses it. Unreferenced linkonce globals are allowed to be discarded.

weak:

"weak" linkage is exactly the same as linkonce linkage, except that unreferenced weak globals may not be discarded. This is used for globals that may be emitted in multiple translation units, but that are not guaranteed to be emitted into every translation unit that uses them. One example of this are common globals in C, such as "int X;" at global scope.

appending:

"appending" linkage may only be applied to global variables of pointer to array type. When two global variables with appending linkage are linked together, the two global arrays are appended together. This is the LLVM, typesafe, equivalent of having the system linker append together "sections" with identical names when .o files are linked.

extern_weak:

The semantics of this linkage follow the ELF model: the symbol is weak until linked, if not linked, the symbol becomes null instead of being an undefined reference.

externally visible:

If none of the above identifiers are used, the global is externally visible, meaning that it participates in linkage and can be used to resolve external symbol references.

The next two types of linkage are targeted for Microsoft Windows platform only. They are designed to support importing (exporting) symbols from (to) DLLs.

dllimport:

"dllimport" linkage causes the compiler to reference a function or variable via a global pointer to a pointer that is set up by the DLL exporting the symbol. On Microsoft Windows targets, the pointer name is formed by combining _imp__ and the function or variable name.

dllexport:

"dllexport" linkage causes the compiler to provide a global pointer to a pointer in a DLL, so that it can be referenced with the dllimport attribute. On Microsoft Windows targets, the pointer name is formed by combining _imp__ and the function or variable name.

For example, since the ".LC0" variable is defined to be internal, if another module defined a ".LC0" variable and was linked with this one, one of the two would be renamed, preventing a collision. Since "main" and "puts" are external (i.e., lacking any linkage declarations), they are accessible outside of the current module.

It is illegal for a function declaration to have any linkage type other than "externally visible", dllimport, or extern_weak.

Aliases can have only external, internal and weak linkages.

LLVM functions, calls and invokes can all have an optional calling convention specified for the call. The calling convention of any pair of dynamic caller/callee must match, or the behavior of the program is undefined. The following calling conventions are supported by LLVM, and more may be added in the future:

"ccc" - The C calling convention:

This calling convention (the default if no other calling convention is specified) matches the target C calling conventions. This calling convention supports varargs function calls and tolerates some mismatch in the declared prototype and implemented declaration of the function (as does normal C).

"fastcc" - The fast calling convention:

This calling convention attempts to make calls as fast as possible (e.g. by passing things in registers). This calling convention allows the target to use whatever tricks it wants to produce fast code for the target, without having to conform to an externally specified ABI. Implementations of this convention should allow arbitrary tail call optimization to be supported. This calling convention does not support varargs and requires the prototype of all callees to exactly match the prototype of the function definition.

"coldcc" - The cold calling convention:

This calling convention attempts to make code in the caller as efficient as possible under the assumption that the call is not commonly executed. As such, these calls often preserve all registers so that the call does not break any live ranges in the caller side. This calling convention does not support varargs and requires the prototype of all callees to exactly match the prototype of the function definition.

"cc <n>" - Numbered convention:

Any calling convention may be specified by number, allowing target-specific calling conventions to be used. Target specific calling conventions start at 64.

More calling conventions can be added/defined on an as-needed basis, to support pascal conventions or any other well-known target-independent convention.

All Global Variables and Functions have one of the following visibility styles:

"default" - Default style:

On ELF, default visibility means that the declaration is visible to other modules and, in shared libraries, means that the declared entity may be overridden. On Darwin, default visibility means that the declaration is visible to other modules. Default visibility corresponds to "external linkage" in the language.

"hidden" - Hidden style:

Two declarations of an object with hidden visibility refer to the same object if they are in the same shared object. Usually, hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so no other module (executable or shared library) can reference it directly.

"protected" - Protected style:

On ELF, protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind to the local symbol. That is, the symbol cannot be overridden by another module.

Global variables define regions of memory allocated at compilation time instead of run-time. Global variables may optionally be initialized, may have an explicit section to be placed in, and may have an optional explicit alignment specified. A variable may be defined as "thread_local", which means that it will not be shared by threads (each thread will have a separated copy of the variable). A variable may be defined as a global "constant," which indicates that the contents of the variable will never be modified (enabling better optimization, allowing the global data to be placed in the read-only section of an executable, etc). Note that variables that need runtime initialization cannot be marked "constant" as there is a store to the variable.

LLVM explicitly allows declarations of global variables to be marked constant, even if the final definition of the global is not. This capability can be used to enable slightly better optimization of the program, but requires the language definition to guarantee that optimizations based on the 'constantness' are valid for the translation units that do not include the definition.

As SSA values, global variables define pointer values that are in scope (i.e. they dominate) all basic blocks in the program. Global variables always define a pointer to their "content" type because they describe a region of memory, and all memory objects in LLVM are accessed through pointers.

A global variable may be declared to reside in a target-specifc numbered address space. For targets that support them, address spaces may affect how optimizations are performed and/or what target instructions are used to access the variable. The default address space is zero. The address space qualifier must precede any other attributes.

LLVM allows an explicit section to be specified for globals. If the target supports it, it will emit globals to the section specified.

An explicit alignment may be specified for a global. If not present, or if the alignment is set to zero, the alignment of the global is set by the target to whatever it feels convenient. If an explicit alignment is specified, the global is forced to have at least that much alignment. All alignments must be a power of 2.

For example, the following defines a global in a numbered address space with an initializer, section, and alignment:

@G = constant float 1.0 addrspace(5), section "foo", align 4

LLVM function definitions consist of the "define" keyord, an optional linkage type, an optional visibility style, an optional calling convention, a return type, an optional parameter attribute for the return type, a function name, a (possibly empty) argument list (each with optional parameter attributes), an optional section, an optional alignment, an optional garbage collector name, an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM function declarations consist of the "declare" keyword, an optional linkage type, an optional visibility style, an optional calling convention, a return type, an optional parameter attribute for the return type, a function name, a possibly empty list of arguments, an optional alignment, and an optional garbage collector name.

A function definition contains a list of basic blocks, forming the CFG for the function. Each basic block may optionally start with a label (giving the basic block a symbol table entry), contains a list of instructions, and ends with a terminator instruction (such as a branch or function return).

The first basic block in a function is special in two ways: it is immediately executed on entrance to the function, and it is not allowed to have predecessor basic blocks (i.e. there can not be any branches to the entry block of a function). Because the block can have no predecessors, it also cannot have any PHI nodes.

LLVM allows an explicit section to be specified for functions. If the target supports it, it will emit functions to the section specified.

An explicit alignment may be specified for a function. If not present, or if the alignment is set to zero, the alignment of the function is set by the target to whatever it feels convenient. If an explicit alignment is specified, the function is forced to have at least that much alignment. All alignments must be a power of 2.

Aliases act as "second name" for the aliasee value (which can be either function, global variable, another alias or bitcast of global value). Aliases may have an optional linkage type, and an optional visibility style.

Syntax:

@<Name> = [Linkage] [Visibility] alias <AliaseeTy> @<Aliasee>

The return type and each parameter of a function type may have a set of parameter attributes associated with them. Parameter attributes are used to communicate additional information about the result or parameters of a function. Parameter attributes are considered to be part of the function, not of the function type, so functions with different parameter attributes can have the same function type.

Parameter attributes are simple keywords that follow the type specified. If multiple parameter attributes are needed, they are space separated. For example:

declare i32 @printf(i8* noalias , ...) nounwind
declare i32 @atoi(i8*) nounwind readonly

Note that any attributes for the function result (nounwind, readonly) come immediately after the argument list.

Currently, only the following parameter attributes are defined:

zeroext

This indicates that the parameter should be zero extended just before a call to this function.

signext

This indicates that the parameter should be sign extended just before a call to this function.

inreg

This indicates that the parameter should be placed in register (if possible) during assembling function call. Support for this attribute is target-specific

byval

This indicates that the pointer parameter should really be passed by value to the function. The attribute implies that a hidden copy of the pointee is made between the caller and the callee, so the callee is unable to modify the value in the callee. This attribute is only valid on llvm pointer arguments. It is generally used to pass structs and arrays by value, but is also valid on scalars (even though this is silly).

sret

This indicates that the pointer parameter specifies the address of a structure that is the return value of the function in the source program. Loads and stores to the structure are assumed not to trap. May only be applied to the first parameter.

noalias

This indicates that the parameter does not alias any global or any other parameter. The caller is responsible for ensuring that this is the case, usually by placing the value in a stack allocation.

noreturn

This function attribute indicates that the function never returns. This indicates to LLVM that every call to this function should be treated as if an unreachable instruction immediately followed the call.

nounwind

This function attribute indicates that no exceptions unwind out of the function. Usually this is because the function makes no use of exceptions, but it may also be that the function catches any exceptions thrown when executing it.

nest

This indicates that the parameter can be excised using the trampoline intrinsics.

readonly

This function attribute indicates that the function has no side-effects except for producing a return value or throwing an exception. The value returned must only depend on the function arguments and/or global variables. It may use values obtained by dereferencing pointers.

readnone

A readnone function has the same restrictions as a readonly function, but in addition it is not allowed to dereference any pointer arguments or global variables.

Each function may specify a garbage collector name, which is simply a string.

define void @f() gc "name" { ...

The compiler declares the supported values of name. Specifying a collector which will cause the compiler to alter its output in order to support the named garbage collection algorithm.

Modules may contain "module-level inline asm" blocks, which corresponds to the GCC "file scope inline asm" blocks. These blocks are internally concatenated by LLVM and treated as a single unit, but may be separated in the .ll file if desired. The syntax is very simple:

module asm "inline asm code goes here"
module asm "more can go here"

The strings can contain any character by escaping non-printable characters. The escape sequence used is simply "\xx" where "xx" is the two digit hex code for the number.

The inline asm code is simply printed to the machine code .s file when assembly code is generated.

A module may specify a target specific data layout string that specifies how data is to be laid out in memory. The syntax for the data layout is simply:

    target datalayout = "layout specification"

The layout specification consists of a list of specifications separated by the minus sign character ('-'). Each specification starts with a letter and may include other information after the letter to define some aspect of the data layout. The specifications accepted are as follows:

E

Specifies that the target lays out data in big-endian form. That is, the bits with the most significance have the lowest address location.

e

Specifies that hte target lays out data in little-endian form. That is, the bits with the least significance have the lowest address location.

p:size:abi:pref

This specifies the size of a pointer and its abi and preferred alignments. All sizes are in bits. Specifying the pref alignment is optional. If omitted, the preceding : should be omitted too.

isize:abi:pref

This specifies the alignment for an integer type of a given bit size. The value of size must be in the range [1,2^23).

vsize:abi:pref

This specifies the alignment for a vector type of a given bit size.

fsize:abi:pref

This specifies the alignment for a floating point type of a given bit size. The value of size must be either 32 (float) or 64 (double).

asize:abi:pref

This specifies the alignment for an aggregate type of a given bit size.

When constructing the data layout for a given target, LLVM starts with a default set of specifications which are then (possibly) overriden by the specifications in the datalayout keyword. The default specifications are given in this list:

• E - big endian
• p:32:64:64 - 32-bit pointers with 64-bit alignment
• i1:8:8 - i1 is 8-bit (byte) aligned
• i8:8:8 - i8 is 8-bit (byte) aligned
• i16:16:16 - i16 is 16-bit aligned
• i32:32:32 - i32 is 32-bit aligned
• i64:32:64 - i64 has abi alignment of 32-bits but preferred alignment of 64-bits
• f32:32:32 - float is 32-bit aligned
• f64:64:64 - double is 64-bit aligned
• v64:64:64 - 64-bit vector is 64-bit aligned
• v128:128:128 - 128-bit vector is 128-bit aligned
• a0:0:1 - aggregates are 8-bit aligned

When llvm is determining the alignment for a given type, it uses the following rules:

1. If the type sought is an exact match for one of the specifications, that specification is used.
2. If no match is found, and the type sought is an integer type, then the smallest integer type that is larger than the bitwidth of the sought type is used. If none of the specifications are larger than the bitwidth then the the largest integer type is used. For example, given the default specifications above, the i7 type will use the alignment of i8 (next largest) while both i65 and i256 will use the alignment of i64 (largest specified).
3. If no match is found, and the type sought is a vector type, then the largest vector type that is smaller than the sought vector type will be used as a fall back. This happens because <128 x double> can be implemented in terms of 64 <2 x double>, for example.

The LLVM type system is one of the most important features of the intermediate representation. Being typed enables a number of optimizations to be performed on the IR directly, without having to do extra analyses on the side before the transformation. A strong type system makes it easier to read the generated code and enables novel analyses and transformations that are not feasible to perform on normal three address code representations.

The types fall into a few useful classifications:

The first class types are perhaps the most important. Values of these types are the only ones which can be produced by instructions, passed as arguments, or used as operands to instructions. This means that all structures and arrays must be manipulated either by pointer or by component.

The primitive types are the fundamental building blocks of the LLVM system.

Overview:

The void type does not represent any value and has no size.

Syntax:

  void

Overview:

The label type represents code labels.

Syntax:

  label

The real power in LLVM comes from the derived types in the system. This is what allows a programmer to represent arrays, functions, pointers, and other useful types. Note that these derived types may be recursive: For example, it is possible to have a two dimensional array.

Overview:

The integer type is a very simple derived type that simply specifies an arbitrary bit width for the integer type desired. Any bit width from 1 bit to 2^23-1 (about 8 million) can be specified.

Syntax:

  iN

The number of bits the integer will occupy is specified by the N value.

Examples:

Overview:

The array type is a very simple derived type that arranges elements sequentially in memory. The array type requires a size (number of elements) and an underlying data type.

Syntax:

  [<# elements> x <elementtype>]

The number of elements is a constant integer value; elementtype may be any type with a size.

Examples:

Here are some examples of multidimensional arrays:

Note that 'variable sized arrays' can be implemented in LLVM with a zero length array. Normally, accesses past the end of an array are undefined in LLVM (e.g. it is illegal to access the 5th element of a 3 element array). As a special case, however, zero length arrays are recognized to be variable length. This allows implementation of 'pascal style arrays' with the LLVM type "{ i32, [0 x float]}", for example.

Overview:

The function type can be thought of as a function signature. It consists of a return type and a list of formal parameter types. The return type of a function type is a scalar type, a void type, or a struct type. If the return type is a struct type then all struct elements must be of first class types, and the struct must have at least one element.

Syntax:

  <returntype list> (<parameter list>)

...where '<parameter list>' is a comma-separated list of type specifiers. Optionally, the parameter list may include a type ..., which indicates that the function takes a variable number of arguments. Variable argument functions can access their arguments with the variable argument handling intrinsic functions. '<returntype list>' is a comma-separated list of first class type specifiers.

Examples:

Overview:

The structure type is used to represent a collection of data members together in memory. The packing of the field types is defined to match the ABI of the underlying processor. The elements of a structure may be any type that has a size.

Structures are accessed using 'load and 'store' by getting a pointer to a field with the 'getelementptr' instruction.

Syntax:

  { <type list> }

Examples:

Overview:

The packed structure type is used to represent a collection of data members together in memory. There is no padding between fields. Further, the alignment of a packed structure is 1 byte. The elements of a packed structure may be any type that has a size.

Structures are accessed using 'load and 'store' by getting a pointer to a field with the 'getelementptr' instruction.

Syntax:

  < { <type list> } > 

Examples:

Overview:

As in many languages, the pointer type represents a pointer or reference to another object, which must live in memory. Pointer types may have an optional address space attribute defining the target-specific numbered address space where the pointed-to object resides. The default address space is zero.

Syntax:

  <type> *

Examples:

Overview:

A vector type is a simple derived type that represents a vector of elements. Vector types are used when multiple primitive data are operated in parallel using a single instruction (SIMD). A vector type requires a size (number of elements) and an underlying primitive data type. Vectors must have a power of two length (1, 2, 4, 8, 16 ...). Vector types are considered first class.

Syntax:

  < <# elements> x <elementtype> >

The number of elements is a constant integer value; elementtype may be any integer or floating point type.

Examples:

Overview:

Opaque types are used to represent unknown types in the system. This corresponds (for example) to the C notion of a forward declared structure type. In LLVM, opaque types can eventually be resolved to any type (not just a structure type).

Syntax:

  opaque

Examples:

LLVM has several different basic types of constants. This section describes them all and their syntax.

Boolean constants

The two strings 'true' and 'false' are both valid constants of the i1 type.

Integer constants

Standard integers (such as '4') are constants of the integer type. Negative numbers may be used with integer types.

Floating point constants

Floating point constants use standard decimal notation (e.g. 123.421), exponential notation (e.g. 1.23421e+2), or a more precise hexadecimal notation (see below). The assembler requires the exact decimal value of a floating-point constant. For example, the assembler accepts 1.25 but rejects 1.3 because 1.3 is a repeating decimal in binary. Floating point constants must have a floating point type.

Null pointer constants

The identifier 'null' is recognized as a null pointer constant and must be of pointer type.

The one non-intuitive notation for constants is the optional hexadecimal form of floating point constants. For example, the form 'double 0x432ff973cafa8000' is equivalent to (but harder to read than) 'double 4.5e+15'. The only time hexadecimal floating point constants are required (and the only time that they are generated by the disassembler) is when a floating point constant must be emitted but it cannot be represented as a decimal floating point number. For example, NaN's, infinities, and other special values are represented in their IEEE hexadecimal format so that assembly and disassembly do not cause any bits to change in the constants.

Aggregate constants arise from aggregation of simple constants and smaller aggregate constants.

Structure constants

Structure constants are represented with notation similar to structure type definitions (a comma separated list of elements, surrounded by braces ({})). For example: "{ i32 4, float 17.0, i32* @G }", where "@G" is declared as "@G = external global i32". Structure constants must have structure type, and the number and types of elements must match those specified by the type.

Array constants

Array constants are represented with notation similar to array type definitions (a comma separated list of elements, surrounded by square brackets ([])). For example: "[ i32 42, i32 11, i32 74 ]". Array constants must have array type, and the number and types of elements must match those specified by the type.

Vector constants

Vector constants are represented with notation similar to vector type definitions (a comma separated list of elements, surrounded by less-than/greater-than's (<>)). For example: "< i32 42, i32 11, i32 74, i32 100 >". Vector constants must have vector type, and the number and types of elements must match those specified by the type.

Zero initialization

The string 'zeroinitializer' can be used to zero initialize a value to zero of any type, including scalar and aggregate types. This is often used to avoid having to print large zero initializers (e.g. for large arrays) and is always exactly equivalent to using explicit zero initializers.

The addresses of global variables and functions are always implicitly valid (link-time) constants. These constants are explicitly referenced when the identifier for the global is used and always have pointer type. For example, the following is a legal LLVM file:

@X = global i32 17
@Y = global i32 42
@Z = global [2 x i32*] [ i32* @X, i32* @Y ]

The string 'undef' is recognized as a type-less constant that has no specific value. Undefined values may be of any type and be used anywhere a constant is permitted.

Undefined values indicate to the compiler that the program is well defined no matter what value is used, giving the compiler more freedom to optimize.

Constant expressions are used to allow expressions involving other constants to be used as constants. Constant expressions may be of any first class type and may involve any LLVM operation that does not have side effects (e.g. load and call are not supported). The following is the syntax for constant expressions:

trunc ( CST to TYPE )

Truncate a constant to another type. The bit size of CST must be larger than the bit size of TYPE. Both types must be integers.

zext ( CST to TYPE )

Zero extend a constant to another type. The bit size of CST must be smaller or equal to the bit size of TYPE. Both types must be integers.

sext ( CST to TYPE )

Sign extend a constant to another type. The bit size of CST must be smaller or equal to the bit size of TYPE. Both types must be integers.

fptrunc ( CST to TYPE )

Truncate a floating point constant to another floating point type. The size of CST must be larger than the size of TYPE. Both types must be floating point.

fpext ( CST to TYPE )

Floating point extend a constant to another type. The size of CST must be smaller or equal to the size of TYPE. Both types must be floating point.

fptoui ( CST to TYPE )

Convert a floating point constant to the corresponding unsigned integer constant. TYPE must be a scalar or vector integer type. CST must be of scalar or vector floating point type. Both CST and TYPE must be scalars, or vectors of the same number of elements. If the value won't fit in the integer type, the results are undefined.

fptosi ( CST to TYPE )

Convert a floating point constant to the corresponding signed integer constant. TYPE must be a scalar or vector integer type. CST must be of scalar or vector floating point type. Both CST and TYPE must be scalars, or vectors of the same number of elements. If the value won't fit in the integer type, the results are undefined.

uitofp ( CST to TYPE )

Convert an unsigned integer constant to the corresponding floating point constant. TYPE must be a scalar or vector floating point type. CST must be of scalar or vector integer type. Both CST and TYPE must be scalars, or vectors of the same number of elements. If the value won't fit in the floating point type, the results are undefined.

sitofp ( CST to TYPE )

Convert a signed integer constant to the corresponding floating point constant. TYPE must be a scalar or vector floating point type. CST must be of scalar or vector integer type. Both CST and TYPE must be scalars, or vectors of the same number of elements. If the value won't fit in the floating point type, the results are undefined.

ptrtoint ( CST to TYPE )

Convert a pointer typed constant to the corresponding integer constant TYPE must be an integer type. CST must be of pointer type. The CST value is zero extended, truncated, or unchanged to make it fit in TYPE.

inttoptr ( CST to TYPE )

Convert a integer constant to a pointer constant. TYPE must be a pointer type. CST must be of integer type. The CST value is zero extended, truncated, or unchanged to make it fit in a pointer size. This one is really dangerous!

bitcast ( CST to TYPE )

Convert a constant, CST, to another TYPE. The size of CST and TYPE must be identical (same number of bits). The conversion is done as if the CST value was stored to memory and read back as TYPE. In other words, no bits change with this operator, just the type. This can be used for conversion of vector types to any other type, as long as they have the same bit width. For pointers it is only valid to cast to another pointer type.

getelementptr ( CSTPTR, IDX0, IDX1, ... )

Perform the getelementptr operation on constants. As with the getelementptr instruction, the index list may have zero or more indexes, which are required to make sense for the type of "CSTPTR".

select ( COND, VAL1, VAL2 )

Perform the select operation on constants.

icmp COND ( VAL1, VAL2 )

Performs the icmp operation on constants.

fcmp COND ( VAL1, VAL2 )

Performs the fcmp operation on constants.

vicmp COND ( VAL1, VAL2 )

Performs the vicmp operation on constants.

vfcmp COND ( VAL1, VAL2 )

Performs the vfcmp operation on constants.

extractelement ( VAL, IDX )

Perform the extractelement operation on constants.

insertelement ( VAL, ELT, IDX )

Perform the insertelement operation on constants.

shufflevector ( VEC1, VEC2, IDXMASK )

Perform the shufflevector operation on constants.

OPCODE ( LHS, RHS )

Perform the specified operation of the LHS and RHS constants. OPCODE may be any of the binary or bitwise binary operations. The constraints on operands are the same as those for the corresponding instruction (e.g. no bitwise operations on floating point values are allowed).

LLVM supports inline assembler expressions (as opposed to Module-Level Inline Assembly) through the use of a special value. This value represents the inline assembler as a string (containing the instructions to emit), a list of operand constraints (stored as a string), and a flag that indicates whether or not the inline asm expression has side effects. An example inline assembler expression is:

i32 (i32) asm "bswap $0", "=r,r"

Inline assembler expressions may only be used as the callee operand of a call instruction. Thus, typically we have:

%X = call i32 asm "bswap $0", "=r,r"(i32 %Y)

Inline asms with side effects not visible in the constraint list must be marked as having side effects. This is done through the use of the 'sideeffect' keyword, like so:

call void asm sideeffect "eieio", ""()

TODO: The format of the asm and constraints string still need to be documented here. Constraints on what can be done (e.g. duplication, moving, etc need to be documented).

The LLVM instruction set consists of several different classifications of instructions: terminator instructions, binary instructions, bitwise binary instructions, memory instructions, and other instructions.

As mentioned previously, every basic block in a program ends with a "Terminator" instruction, which indicates which block should be executed after the current block is finished. These terminator instructions typically yield a 'void' value: they produce control flow, not values (the one exception being the 'invoke' instruction).

There are six different terminator instructions: the 'ret' instruction, the 'br' instruction, the 'switch' instruction, the 'invoke' instruction, the 'unwind' instruction, and the 'unreachable' instruction.

Syntax:

  ret <type> <value>       ; Return a value from a non-void function
  ret void                 ; Return from void function
  ret <type> <value>, <type> <value>  ; Return two values from a non-void function 

Overview:

The 'ret' instruction is used to return control flow (and a value) from a function back to the caller.

There are two forms of the 'ret' instruction: one that returns value(s) and then causes control flow, and one that just causes control flow to occur.

Arguments:

The 'ret' instruction may return zero, one or multiple values. The type of each return value must be a 'first class' type. Note that a function is not well formed if there exists a 'ret' instruction inside of the function that returns values that do not match the return type of the function.

Semantics:

When the 'ret' instruction is executed, control flow returns back to the calling function's context. If the caller is a "call" instruction, execution continues at the instruction after the call. If the caller was an "invoke" instruction, execution continues at the beginning of the "normal" destination block. If the instruction returns a value, that value shall set the call or invoke instruction's return value. If the instruction returns multiple values then these values can only be accessed through a 'getresult ' instruction.

Example:

  ret i32 5                       ; Return an integer value of 5
  ret void                        ; Return from a void function
  ret i32 4, i8 2                 ; Return two values 4 and 2  

Syntax:

  br i1 <cond>, label <iftrue>, label <iffalse>
  br label <dest>          ; Unconditional branch

Overview:

The 'br' instruction is used to cause control flow to transfer to a different basic block in the current function. There are two forms of this instruction, corresponding to a conditional branch and an unconditional branch.

Arguments:

The conditional branch form of the 'br' instruction takes a single 'i1' value and two 'label' values. The unconditional form of the 'br' instruction takes a single 'label' value as a target.

Semantics:

Upon execution of a conditional 'br' instruction, the 'i1' argument is evaluated. If the value is true, control flows to the 'iftrue' label argument. If "cond" is false, control flows to the 'iffalse' label argument.

Example:

Test:
  %cond = icmp eq, i32 %a, %b
  br i1 %cond, label %IfEqual, label %IfUnequal
IfEqual:
  ret i32 1
IfUnequal:
  ret i32 0

Syntax:

  switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]

Overview:

The 'switch' instruction is used to transfer control flow to one of several different places. It is a generalization of the 'br' instruction, allowing a branch to occur to one of many possible destinations.

Arguments:

The 'switch' instruction uses three parameters: an integer comparison value 'value', a default 'label' destination, and an array of pairs of comparison value constants and 'label's. The table is not allowed to contain duplicate constant entries.

Semantics:

The switch instruction specifies a table of values and destinations. When the 'switch' instruction is executed, this table is searched for the given value. If the value is found, control flow is transfered to the corresponding destination; otherwise, control flow is transfered to the default destination.

Implementation:

Depending on properties of the target machine and the particular switch instruction, this instruction may be code generated in different ways. For example, it could be generated as a series of chained conditional branches or with a lookup table.

Example:

 ; Emulate a conditional br instruction
 %Val = zext i1 %value to i32
 switch i32 %Val, label %truedest [i32 0, label %falsedest ]

 ; Emulate an unconditional br instruction
 switch i32 0, label %dest [ ]

 ; Implement a jump table:
 switch i32 %val, label %otherwise [ i32 0, label %onzero 
                                      i32 1, label %onone 
                                      i32 2, label %ontwo ]

Syntax:

  <result> = invoke [cconv] <ptr to function ty> <function ptr val>(<function args>) 
                to label <normal label> unwind label <exception label>

Overview:

The 'invoke' instruction causes control to transfer to a specified function, with the possibility of control flow transfer to either the 'normal' label or the 'exception' label. If the callee function returns with the "ret" instruction, control flow will return to the "normal" label. If the callee (or any indirect callees) returns with the "unwind" instruction, control is interrupted and continued at the dynamically nearest "exception" label. If the callee function returns multiple values then individual return values are only accessible through a 'getresult' instruction.

Arguments:

This instruction requires several arguments:

1. The optional "cconv" marker indicates which calling convention the call should use. If none is specified, the call defaults to using C calling conventions.
2. 'ptr to function ty': shall be the signature of the pointer to function value being invoked. In most cases, this is a direct function invocation, but indirect invokes are just as possible, branching off an arbitrary pointer to function value.
3. 'function ptr val': An LLVM value containing a pointer to a function to be invoked.
4. 'function args': argument list whose types match the function signature argument types. If the function signature indicates the function accepts a variable number of arguments, the extra arguments can be specified.
5. 'normal label': the label reached when the called function executes a 'ret' instruction.
6. 'exception label': the label reached when a callee returns with the unwind instruction.

Semantics:

This instruction is designed to operate as a standard 'call' instruction in most regards. The primary difference is that it establishes an association with a label, which is used by the runtime library to unwind the stack.

This instruction is used in languages with destructors to ensure that proper cleanup is performed in the case of either a longjmp or a thrown exception. Additionally, this is important for implementation of 'catch' clauses in high-level languages that support them.

Example:

  %retval = invoke i32 @Test(i32 15) to label %Continue
              unwind label %TestCleanup              ; {i32}:retval set
  %retval = invoke coldcc i32 %Testfnptr(i32 15) to label %Continue
              unwind label %TestCleanup              ; {i32}:retval set

Syntax:

  unwind

Overview:

The 'unwind' instruction unwinds the stack, continuing control flow at the first callee in the dynamic call stack which used an invoke instruction to perform the call. This is primarily used to implement exception handling.

Semantics:

The 'unwind' instruction causes execution of the current function to immediately halt. The dynamic call stack is then searched for the first invoke instruction on the call stack. Once found, execution continues at the "exceptional" destination block specified by the invoke instruction. If there is no invoke instruction in the dynamic call chain, undefined behavior results.

Syntax:

  unreachable

Overview:

The 'unreachable' instruction has no defined semantics. This instruction is used to inform the optimizer that a particular portion of the code is not reachable. This can be used to indicate that the code after a no-return function cannot be reached, and other facts.

Semantics:

The 'unreachable' instruction has no defined semantics.

Binary operators are used to do most of the computation in a program. They require two operands of the same type, execute an operation on them, and produce a single value. The operands might represent multiple data, as is the case with the vector data type. The result value has the same type as its operands.

There are several different binary operators:

Syntax:

  <result> = add <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'add' instruction returns the sum of its two operands.

Arguments:

The two arguments to the 'add' instruction must be either integer or floating point values. This instruction can also take vector versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point sum of the two operands.

If an integer sum has unsigned overflow, the result returned is the mathematical result modulo 2ⁿ, where n is the bit width of the result.

Because LLVM integers use a two's complement representation, this instruction is appropriate for both signed and unsigned integers.

Example:

  <result> = add i32 4, %var          ; yields {i32}:result = 4 + %var

Syntax:

  <result> = sub <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'sub' instruction returns the difference of its two operands.

Note that the 'sub' instruction is used to represent the 'neg' instruction present in most other intermediate representations.

Arguments:

The two arguments to the 'sub' instruction must be either integer or floating point values. This instruction can also take vector versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point difference of the two operands.

If an integer difference has unsigned overflow, the result returned is the mathematical result modulo 2ⁿ, where n is the bit width of the result.

Because LLVM integers use a two's complement representation, this instruction is appropriate for both signed and unsigned integers.

Example:

  <result> = sub i32 4, %var          ; yields {i32}:result = 4 - %var
  <result> = sub i32 0, %val          ; yields {i32}:result = -%var

Syntax:

  <result> = mul <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'mul' instruction returns the product of its two operands.

Arguments:

The two arguments to the 'mul' instruction must be either integer or floating point values. This instruction can also take vector versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point product of the two operands.

If the result of an integer multiplication has unsigned overflow, the result returned is the mathematical result modulo 2ⁿ, where n is the bit width of the result.

Because LLVM integers use a two's complement representation, and the result is the same width as the operands, this instruction returns the correct result for both signed and unsigned integers. If a full product (e.g. i32xi32->i64) is needed, the operands should be sign-extended or zero-extended as appropriate to the width of the full product.

Example:

  <result> = mul i32 4, %var          ; yields {i32}:result = 4 * %var

Syntax:

  <result> = udiv <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'udiv' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'udiv' instruction must be integer values. Both arguments must have identical types. This instruction can also take vector versions of the values in which case the elements must be integers.

Semantics:

The value produced is the unsigned integer quotient of the two operands.

Note that unsigned integer division and signed integer division are distinct operations; for signed integer division, use 'sdiv'.

Division by zero leads to undefined behavior.

Example:

  <result> = udiv i32 4, %var          ; yields {i32}:result = 4 / %var

Syntax:

  <result> = sdiv <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'sdiv' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'sdiv' instruction must be integer values. Both arguments must have identical types. This instruction can also take vector versions of the values in which case the elements must be integers.

Semantics:

The value produced is the signed integer quotient of the two operands rounded towards zero.

Note that signed integer division and unsigned integer division are distinct operations; for unsigned integer division, use 'udiv'.

Division by zero leads to undefined behavior. Overflow also leads to undefined behavior; this is a rare case, but can occur, for example, by doing a 32-bit division of -2147483648 by -1.

Example:

  <result> = sdiv i32 4, %var          ; yields {i32}:result = 4 / %var

Syntax:

  <result> = fdiv <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'fdiv' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'fdiv' instruction must be floating point values. Both arguments must have identical types. This instruction can also take vector versions of floating point values.

Semantics:

The value produced is the floating point quotient of the two operands.

Example:

  <result> = fdiv float 4.0, %var          ; yields {float}:result = 4.0 / %var

Syntax:

  <result> = urem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'urem' instruction returns the remainder from the unsigned division of its two arguments.

Arguments:

The two arguments to the 'urem' instruction must be integer values. Both arguments must have identical types. This instruction can also take vector versions of the values in which case the elements must be integers.

Semantics:

This instruction returns the unsigned integer remainder of a division. This instruction always performs an unsigned division to get the remainder.

Note that unsigned integer remainder and signed integer remainder are distinct operations; for signed integer remainder, use 'srem'.

Taking the remainder of a division by zero leads to undefined behavior.

Example:

  <result> = urem i32 4, %var          ; yields {i32}:result = 4 % %var

Syntax:

  <result> = srem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'srem' instruction returns the remainder from the signed division of its two operands. This instruction can also take vector versions of the values in which case the elements must be integers.

Arguments:

The two arguments to the 'srem' instruction must be integer values. Both arguments must have identical types.

Semantics:

This instruction returns the remainder of a division (where the result has the same sign as the dividend, var1), not the modulo operator (where the result has the same sign as the divisor, var2) of a value. For more information about the difference, see The Math Forum. For a table of how this is implemented in various languages, please see Wikipedia: modulo operation.

Note that signed integer remainder and unsigned integer remainder are distinct operations; for unsigned integer remainder, use 'urem'.

Taking the remainder of a division by zero leads to undefined behavior. Overflow also leads to undefined behavior; this is a rare case, but can occur, for example, by taking the remainder of a 32-bit division of -2147483648 by -1. (The remainder doesn't actually overflow, but this rule lets srem be implemented using instructions that return both the result of the division and the remainder.)

Example:

  <result> = srem i32 4, %var          ; yields {i32}:result = 4 % %var

Syntax:

  <result> = frem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'frem' instruction returns the remainder from the division of its two operands.

Arguments:

The two arguments to the 'frem' instruction must be floating point values. Both arguments must have identical types. This instruction can also take vector versions of floating point values.

Semantics:

This instruction returns the remainder of a division. The remainder has the same sign as the dividend.

Example:

  <result> = frem float 4.0, %var          ; yields {float}:result = 4.0 % %var

Bitwise binary operators are used to do various forms of bit-twiddling in a program. They are generally very efficient instructions and can commonly be strength reduced from other instructions. They require two operands of the same type, execute an operation on them, and produce a single value. The resulting value is the same type as its operands.

Syntax:

  <result> = shl <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'shl' instruction returns the first operand shifted to the left a specified number of bits.

Arguments:

Both arguments to the 'shl' instruction must be the same integer type. 'var2' is treated as an unsigned value.

Semantics:

The value produced is var1 * 2^var2 mod 2ⁿ, where n is the width of the result. If var2 is (statically or dynamically) negative or equal to or larger than the number of bits in var1, the result is undefined.

Example:

  <result> = shl i32 4, %var   ; yields {i32}: 4 << %var
  <result> = shl i32 4, 2      ; yields {i32}: 16
  <result> = shl i32 1, 10     ; yields {i32}: 1024
  <result> = shl i32 1, 32     ; undefined

Syntax:

  <result> = lshr <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'lshr' instruction (logical shift right) returns the first operand shifted to the right a specified number of bits with zero fill.

Arguments:

Both arguments to the 'lshr' instruction must be the same integer type. 'var2' is treated as an unsigned value.

Semantics:

This instruction always performs a logical shift right operation. The most significant bits of the result will be filled with zero bits after the shift. If var2 is (statically or dynamically) equal to or larger than the number of bits in var1, the result is undefined.

Example:

  <result> = lshr i32 4, 1   ; yields {i32}:result = 2
  <result> = lshr i32 4, 2   ; yields {i32}:result = 1
  <result> = lshr i8  4, 3   ; yields {i8}:result = 0
  <result> = lshr i8 -2, 1   ; yields {i8}:result = 0x7FFFFFFF 
  <result> = lshr i32 1, 32  ; undefined

Syntax:

  <result> = ashr <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'ashr' instruction (arithmetic shift right) returns the first operand shifted to the right a specified number of bits with sign extension.

Arguments:

Both arguments to the 'ashr' instruction must be the same integer type. 'var2' is treated as an unsigned value.

Semantics:

This instruction always performs an arithmetic shift right operation, The most significant bits of the result will be filled with the sign bit of var1. If var2 is (statically or dynamically) equal to or larger than the number of bits in var1, the result is undefined.

Example:

  <result> = ashr i32 4, 1   ; yields {i32}:result = 2
  <result> = ashr i32 4, 2   ; yields {i32}:result = 1
  <result> = ashr i8  4, 3   ; yields {i8}:result = 0
  <result> = ashr i8 -2, 1   ; yields {i8}:result = -1
  <result> = ashr i32 1, 32  ; undefined

Syntax:

  <result> = and <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'and' instruction returns the bitwise logical and of its two operands.

Arguments:

The two arguments to the 'and' instruction must be integer values. Both arguments must have identical types.

Semantics:

The truth table used for the 'and' instruction is:

Example:

  <result> = and i32 4, %var         ; yields {i32}:result = 4 & %var
  <result> = and i32 15, 40          ; yields {i32}:result = 8
  <result> = and i32 4, 8            ; yields {i32}:result = 0

Syntax:

  <result> = or <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'or' instruction returns the bitwise logical inclusive or of its two operands.

Arguments:

The two arguments to the 'or' instruction must be integer values. Both arguments must have identical types.

Semantics:

The truth table used for the 'or' instruction is:

Example:

  <result> = or i32 4, %var         ; yields {i32}:result = 4 | %var
  <result> = or i32 15, 40          ; yields {i32}:result = 47
  <result> = or i32 4, 8            ; yields {i32}:result = 12

Syntax:

  <result> = xor <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'xor' instruction returns the bitwise logical exclusive or of its two operands. The xor is used to implement the "one's complement" operation, which is the "~" operator in C.

Arguments:

The two arguments to the 'xor' instruction must be integer values. Both arguments must have identical types.

Semantics:

The truth table used for the 'xor' instruction is:

Example:

  <result> = xor i32 4, %var         ; yields {i32}:result = 4 ^ %var
  <result> = xor i32 15, 40          ; yields {i32}:result = 39
  <result> = xor i32 4, 8            ; yields {i32}:result = 12
  <result> = xor i32 %V, -1          ; yields {i32}:result = ~%V

LLVM supports several instructions to represent vector operations in a target-independent manner. These instructions cover the element-access and vector-specific operations needed to process vectors effectively. While LLVM does directly support these vector operations, many sophisticated algorithms will want to use target-specific intrinsics to take full advantage of a specific target.

Syntax:

  <result> = extractelement <n x <ty>> <val>, i32 <idx>    ; yields <ty>

Overview:

The 'extractelement' instruction extracts a single scalar element from a vector at a specified index.

Arguments:

The first operand of an 'extractelement' instruction is a value of vector type. The second operand is an index indicating the position from which to extract the element. The index may be a variable.

Semantics:

The result is a scalar of the same type as the element type of val. Its value is the value at position idx of val. If idx exceeds the length of val, the results are undefined.

Example:

  %result = extractelement <4 x i32> %vec, i32 0    ; yields i32

Syntax:

  <result> = insertelement <n x <ty>> <val>, <ty> <elt>, i32 <idx>    ; yields <n x <ty>>

Overview:

The 'insertelement' instruction inserts a scalar element into a vector at a specified index.

Arguments:

The first operand of an 'insertelement' instruction is a value of vector type. The second operand is a scalar value whose type must equal the element type of the first operand. The third operand is an index indicating the position at which to insert the value. The index may be a variable.

Semantics:

The result is a vector of the same type as val. Its element values are those of val except at position idx, where it gets the value elt. If idx exceeds the length of val, the results are undefined.

Example:

  %result = insertelement <4 x i32> %vec, i32 1, i32 0    ; yields <4 x i32>

Syntax:

  <result> = shufflevector <n x <ty>> <v1>, <n x <ty>> <v2>, <n x i32> <mask>    ; yields <n x <ty>>

Overview:

The 'shufflevector' instruction constructs a permutation of elements from two input vectors, returning a vector of the same type.

Arguments:

The first two operands of a 'shufflevector' instruction are vectors with types that match each other and types that match the result of the instruction. The third argument is a shuffle mask, which has the same number of elements as the other vector type, but whose element type is always 'i32'.

The shuffle mask operand is required to be a constant vector with either constant integer or undef values.

Semantics:

The elements of the two input vectors are numbered from left to right across both of the vectors. The shuffle mask operand specifies, for each element of the result vector, which element of the two input registers the result element gets. The element selector may be undef (meaning "don't care") and the second operand may be undef if performing a shuffle from only one vector.

Example:

  %result = shufflevector <4 x i32> %v1, <4 x i32> %v2, 
                          <4 x i32> <i32 0, i32 4, i32 1, i32 5>  ; yields <4 x i32>
  %result = shufflevector <4 x i32> %v1, <4 x i32> undef, 
                          <4 x i32> <i32 0, i32 1, i32 2, i32 3>  ; yields <4 x i32> - Identity shuffle.

A key design point of an SSA-based representation is how it represents memory. In LLVM, no memory locations are in SSA form, which makes things very simple. This section describes how to read, write, allocate, and free memory in LLVM.

Syntax:

  <result> = malloc <type>[, i32 <NumElements>][, align <alignment>]     ; yields {type*}:result

Overview:

The 'malloc' instruction allocates memory from the system heap and returns a pointer to it. The object is always allocated in the generic address space (address space zero).

Arguments:

The 'malloc' instruction allocates sizeof(<type>)*NumElements bytes of memory from the operating system and returns a pointer of the appropriate type to the program. If "NumElements" is specified, it is the number of elements allocated, otherwise "NumElements" is defaulted to be one. If a constant alignment is specified, the value result of the allocation is guaranteed to be aligned to at least that boundary. If not specified, or if zero, the target can choose to align the allocation on any convenient boundary.

'type' must be a sized type.

Semantics:

Memory is allocated using the system "malloc" function, and a pointer is returned. The result of a zero byte allocattion is undefined. The result is null if there is insufficient memory available.

Example:

  %array  = malloc [4 x i8 ]                    ; yields {[%4 x i8]*}:array

  %size   = add i32 2, 2                        ; yields {i32}:size = i32 4
  %array1 = malloc i8, i32 4                    ; yields {i8*}:array1
  %array2 = malloc [12 x i8], i32 %size         ; yields {[12 x i8]*}:array2
  %array3 = malloc i32, i32 4, align 1024       ; yields {i32*}:array3
  %array4 = malloc i32, align 1024              ; yields {i32*}:array4

Syntax:

  free <type> <value>                              ; yields {void}

Overview:

The 'free' instruction returns memory back to the unused memory heap to be reallocated in the future.

Arguments:

'value' shall be a pointer value that points to a value that was allocated with the 'malloc' instruction.

Semantics:

Access to the memory pointed to by the pointer is no longer defined after this instruction executes. If the pointer is null, the operation is a noop.

Example:

  %array  = malloc [4 x i8]                    ; yields {[4 x i8]*}:array
            free   [4 x i8]* %array

Syntax:

  <result> = alloca <type>[, i32 <NumElements>][, align <alignment>]     ; yields {type*}:result

Overview:

The 'alloca' instruction allocates memory on the stack frame of the currently executing function, to be automatically released when this function returns to its caller. The object is always allocated in the generic address space (address space zero).

Arguments:

The 'alloca' instruction allocates sizeof(<type>)*NumElements bytes of memory on the runtime stack, returning a pointer of the appropriate type to the program. If "NumElements" is specified, it is the number of elements allocated, otherwise "NumElements" is defaulted to be one. If a constant alignment is specified, the value result of the allocation is guaranteed to be aligned to at least that boundary. If not specified, or if zero, the target can choose to align the allocation on any convenient boundary.

'type' may be any sized type.

Semantics:

Memory is allocated; a pointer is returned. The operation is undefiend if there is insufficient stack space for the allocation. 'alloca'd memory is automatically released when the function returns. The 'alloca' instruction is commonly used to represent automatic variables that must have an address available. When the function returns (either with the ret or unwind instructions), the memory is reclaimed. Allocating zero bytes is legal, but the result is undefined.

Example:

  %ptr = alloca i32                              ; yields {i32*}:ptr
  %ptr = alloca i32, i32 4                       ; yields {i32*}:ptr
  %ptr = alloca i32, i32 4, align 1024           ; yields {i32*}:ptr
  %ptr = alloca i32, align 1024                  ; yields {i32*}:ptr

Syntax:

  <result> = load <ty>* <pointer>[, align <alignment>]
  <result> = volatile load <ty>* <pointer>[, align <alignment>]

Overview:

The 'load' instruction is used to read from memory.

Arguments:

The argument to the 'load' instruction specifies the memory address from which to load. The pointer must point to a first class type. If the load is marked as volatile, then the optimizer is not allowed to modify the number or order of execution of this load with other volatile load and store instructions.

The optional constant "align" argument specifies the alignment of the operation (that is, the alignment of the memory address). A value of 0 or an omitted "align" argument means that the operation has the preferential alignment for the target. It is the responsibility of the code emitter to ensure that the alignment information is correct. Overestimating the alignment results in an undefined behavior. Underestimating the alignment may produce less efficient code. An alignment of 1 is always safe.

Semantics:

The location of memory pointed to is loaded.

Examples:

  %ptr = alloca i32                               ; yields {i32*}:ptr
  store i32 3, i32* %ptr                          ; yields {void}
  %val = load i32* %ptr                           ; yields {i32}:val = i32 3

Syntax:

  store <ty> <value>, <ty>* <pointer>[, align <alignment>]                   ; yields {void}
  volatile store <ty> <value>, <ty>* <pointer>[, align <alignment>]          ; yields {void}

Overview:

The 'store' instruction is used to write to memory.

Arguments:

There are two arguments to the 'store' instruction: a value to store and an address at which to store it. The type of the '<pointer>' operand must be a pointer to the first class type of the '<value>' operand. If the store is marked as volatile, then the optimizer is not allowed to modify the number or order of execution of this store with other volatile load and store instructions.

The optional constant "align" argument specifies the alignment of the operation (that is, the alignment of the memory address). A value of 0 or an omitted "align" argument means that the operation has the preferential alignment for the target. It is the responsibility of the code emitter to ensure that the alignment information is correct. Overestimating the alignment results in an undefined behavior. Underestimating the alignment may produce less efficient code. An alignment of 1 is always safe.

Semantics:

The contents of memory are updated to contain '<value>' at the location specified by the '<pointer>' operand.

Example:

  %ptr = alloca i32                               ; yields {i32*}:ptr
  store i32 3, i32* %ptr                          ; yields {void}
  %val = load i32* %ptr                           ; yields {i32}:val = i32 3

Syntax:

  <result> = getelementptr <ty>* <ptrval>{, <ty> <idx>}*

Overview:

The 'getelementptr' instruction is used to get the address of a subelement of an aggregate data structure.

Arguments:

This instruction takes a list of integer operands that indicate what elements of the aggregate object to index to. The actual types of the arguments provided depend on the type of the first pointer argument. The 'getelementptr' instruction is used to index down through the type levels of a structure or to a specific index in an array. When indexing into a structure, only i32 integer constants are allowed. When indexing into an array or pointer, only integers of 32 or 64 bits are allowed; 32-bit values will be sign extended to 64-bits if required.

For example, let's consider a C code fragment and how it gets compiled to LLVM:

struct RT {
  char A;
  int B[10][20];
  char C;
};
struct ST {
  int X;
  double Y;
  struct RT Z;
};

int *foo(struct ST *s) {
  return &s[1].Z.B[5][13];
}

The LLVM code generated by the GCC frontend is:

%RT = type { i8 , [10 x [20 x i32]], i8  }
%ST = type { i32, double, %RT }

define i32* %foo(%ST* %s) {
entry:
  %reg = getelementptr %ST* %s, i32 1, i32 2, i32 1, i32 5, i32 13
  ret i32* %reg
}

Semantics:

The index types specified for the 'getelementptr' instruction depend on the pointer type that is being indexed into. Pointer and array types can use a 32-bit or 64-bit integer type but the value will always be sign extended to 64-bits. Structure and packed structure types require i32 constants.

In the example above, the first index is indexing into the '%ST*' type, which is a pointer, yielding a '%ST' = '{ i32, double, %RT }' type, a structure. The second index indexes into the third element of the structure, yielding a '%RT' = '{ i8 , [10 x [20 x i32]], i8 }' type, another structure. The third index indexes into the second element of the structure, yielding a '[10 x [20 x i32]]' type, an array. The two dimensions of the array are subscripted into, yielding an 'i32' type. The 'getelementptr' instruction returns a pointer to this element, thus computing a value of 'i32*' type.

Note that it is perfectly legal to index partially through a structure, returning a pointer to an inner element. Because of this, the LLVM code for the given testcase is equivalent to:

  define i32* %foo(%ST* %s) {
    %t1 = getelementptr %ST* %s, i32 1                        ; yields %ST*:%t1
    %t2 = getelementptr %ST* %t1, i32 0, i32 2                ; yields %RT*:%t2
    %t3 = getelementptr %RT* %t2, i32 0, i32 1                ; yields [10 x [20 x i32]]*:%t3
    %t4 = getelementptr [10 x [20 x i32]]* %t3, i32 0, i32 5  ; yields [20 x i32]*:%t4
    %t5 = getelementptr [20 x i32]* %t4, i32 0, i32 13        ; yields i32*:%t5
    ret i32* %t5
  }

Note that it is undefined to access an array out of bounds: array and pointer indexes must always be within the defined bounds of the array type. The one exception for this rule is zero length arrays. These arrays are defined to be accessible as variable length arrays, which requires access beyond the zero'th element.

The getelementptr instruction is often confusing. For some more insight into how it works, see the getelementptr FAQ.

Example:

    ; yields [12 x i8]*:aptr
    %aptr = getelementptr {i32, [12 x i8]}* %sptr, i64 0, i32 1

The instructions in this category are the conversion instructions (casting) which all take a single operand and a type. They perform various bit conversions on the operand.

Syntax:

  <result> = trunc <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'trunc' instruction truncates its operand to the type ty2.

Arguments:

The 'trunc' instruction takes a value to trunc, which must be an integer type, and a type that specifies the size and type of the result, which must be an integer type. The bit size of value must be larger than the bit size of ty2. Equal sized types are not allowed.

Semantics:

The 'trunc' instruction truncates the high order bits in value and converts the remaining bits to ty2. Since the source size must be larger than the destination size, trunc cannot be a no-op cast. It will always truncate bits.

Example:

  %X = trunc i32 257 to i8              ; yields i8:1
  %Y = trunc i32 123 to i1              ; yields i1:true
  %Y = trunc i32 122 to i1              ; yields i1:false

Syntax:

  <result> = zext <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'zext' instruction zero extends its operand to type ty2.

Arguments:

The 'zext' instruction takes a value to cast, which must be of integer type, and a type to cast it to, which must also be of integer type. The bit size of the value must be smaller than the bit size of the destination type, ty2.

Semantics:

The zext fills the high order bits of the value with zero bits until it reaches the size of the destination type, ty2.

When zero extending from i1, the result will always be either 0 or 1.

Example:

  %X = zext i32 257 to i64              ; yields i64:257
  %Y = zext i1 true to i32              ; yields i32:1

Syntax:

  <result> = sext <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'sext' sign extends value to the type ty2.

Arguments:

The 'sext' instruction takes a value to cast, which must be of integer type, and a type to cast it to, which must also be of integer type. The bit size of the value must be smaller than the bit size of the destination type, ty2.

Semantics:

The 'sext' instruction performs a sign extension by copying the sign bit (highest order bit) of the value until it reaches the bit size of the type ty2.

When sign extending from i1, the extension always results in -1 or 0.

Example:

  %X = sext i8  -1 to i16              ; yields i16   :65535
  %Y = sext i1 true to i32             ; yields i32:-1

Syntax:

  <result> = fptrunc <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'fptrunc' instruction truncates value to type ty2.

Arguments:

The 'fptrunc' instruction takes a floating point value to cast and a floating point type to cast it to. The size of value must be larger than the size of ty2. This implies that fptrunc cannot be used to make a no-op cast.

Semantics:

The 'fptrunc' instruction truncates a value from a larger floating point type to a smaller floating point type. If the value cannot fit within the destination type, ty2, then the results are undefined.

Example:

  %X = fptrunc double 123.0 to float         ; yields float:123.0
  %Y = fptrunc double 1.0E+300 to float