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 bytecode 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 bytecode. The violations pointed out by the verifier pass indicate bugs in transformation passes or input to the parser.
LLVM uses three different forms of identifiers, for different purposes:
LLVM requires that values start with a '%' sign 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 '%' character.
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:
...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 *)* ; Global variable / Function body section separator implementation ; 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.
Due to a limitation in the current LLVM assembly parser (it is limited by one-token lookahead), modules are split into two pieces by the "implementation" keyword. Global variable prototypes and definitions must occur before the keyword, and function definitions must occur after it. Function prototypes may occur either before or after it. In the future, the implementation keyword may become a noop, if the parser gets smarter.
All Global Variables and Functions have one of the following types of linkage:
The next two types of linkage are targeted for Microsoft Windows platform only. They are designed to support importing (exporting) symbols from (to) DLLs.
_imp__ and the function or variable name.
_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.
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:
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:
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 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.
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 with an initializer, section, and alignment:
%G = constant float 1.0, 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 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, and an optional alignment.
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 program 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 functions are identified by their name and type signature. Hence, two functions with the same name but different parameter lists or return values are considered different functions, and LLVM will resolve references to each appropriately.
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.
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 type so two functions types that differ only by the parameter attributes are different function types.
Parameter attributes are simple keywords that follow the type specified. If multiple parameter attributes are needed, they are space separated. For example:
%someFunc = i16 (i8 sext %someParam) zext
%someFunc = i16 (i8 zext %someParam) zext
Note that the two function types above are unique because the parameter has a different attribute (sext in the first one, zext in the second). Also note that the attribute for the function result (zext) comes immediately after the argument list.
Currently, only the following parameter attributes are defined:
The current motivation for parameter attributes is to enable the sign and zero extend information necessary for the C calling convention to be passed from the front end to LLVM. The zext and sext attributes are used by the code generator to perform the required extension. However, parameter attributes are an orthogonal feature to calling conventions and may be used for other purposes in the future.
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.
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 primitive types are the fundamental building blocks of the LLVM system. The current set of primitive types is as follows:
|
|
These different primitive types fall into a few useful classifications:
| Classification | Types |
|---|---|
| integer | i1, i8, i16, i32, i64 |
| floating point | float, double |
| first class | i1, i8, i16, i32, i64, float, double, pointer,packed |
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 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.
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.
[<# elements> x <elementtype>]
The number of elements is a constant integer value; elementtype may be any type with a size.
|
[40 x i32 ] [41 x i32 ] [40 x i8] |
Array of 40 32-bit integer values. Array of 41 32-bit integer values. Array of 40 8-bit integer values. |
Here are some examples of multidimensional arrays:
|
[3 x [4 x i32]] [12 x [10 x float]] [2 x [3 x [4 x i16]]] |
3x4 array of 32-bit integer values. 12x10 array of single precision floating point values. 2x3x4 array of 16-bit integer values. |
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.
The function type can be thought of as a function signature. It consists of a return type and a list of formal parameter types. Function types are usually used to build virtual function tables (which are structures of pointers to functions), for indirect function calls, and when defining a function.
The return type of a function type cannot be an aggregate type.
<returntype> (<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.
| i32 (i32) | function taking an i32, returning an i32 |
| float (i16 sext, i32 *) * | Pointer to a function that takes an i16 that should be sign extended and a pointer to i32, returning float. |
| i32 (i8*, ...) | A vararg function that takes at least one pointer to i8 (char in C), which returns an integer. This is the signature for printf in LLVM. |
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.
{ <type list> }
|
{ i32, i32, i32 } { float, i32 (i32) * } |
a triple of three i32 values A pair, where the first element is a float and the second element is a pointer to a function that takes an i32, returning an i32. |
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.
< { <type list> } >
|
< { i32, i32, i32 } > < { float, i32 (i32) * } > |
a triple of three i32 values A pair, where the first element is a float and the second element is a pointer to a function that takes an i32, returning an i32. |
As in many languages, the pointer type represents a pointer or reference to another object, which must live in memory.
<type> *
|
[4x i32]* i32 (i32 *) * |
A pointer to array of
four i32 values A pointer to a function that takes an i32*, returning an i32. |
A packed type is a simple derived type that represents a vector of elements. Packed types are used when multiple primitive data are operated in parallel using a single instruction (SIMD). A packed 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 ...). Packed types are considered first class.
< <# elements> x <elementtype> >
The number of elements is a constant integer value; elementtype may be any integer or floating point type.
|
<4 x i32> <8 x float> <2 x i64> |
Packed vector of 4 32-bit integer values. Packed vector of 8 floating-point values. Packed vector of 2 64-bit integer values. |
Opaque types are used to represent unknown types in the system. This corresponds (for example) to the C notion of a foward declared structure type. In LLVM, opaque types can eventually be resolved to any type (not just a structure type).
opaque
| opaque |
An opaque type. |
LLVM has several different basic types of constants. This section describes them all and their syntax.
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.
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:
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.
ret <type> <value> ; Return a value from a non-void function ret void ; Return from void function
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 a value and then causes control flow, and one that just causes control flow to occur.
The 'ret' instruction may return any 'first class' type. Notice that a function is not well formed if there exists a 'ret' instruction inside of the function that returns a value that does not match the return type of the function.
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.
ret i32 5 ; Return an integer value of 5 ret void ; Return from a void function
br i1 <cond>, label <iftrue>, label <iffalse>
br label <dest> ; Unconditional branch
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.
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.
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.
Test:
%cond = icmp eq, i32 %a, %b
br i1 %cond, label %IfEqual, label %IfUnequal
IfEqual:
ret i32 1
IfUnequal:
ret i32 0
switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]
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.
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.
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.
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.
; 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 ]
<result> = invoke [cconv] <ptr to function ty> %<function ptr val>(<function args>) to label <normal label> unwind label <exception label>
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.
This instruction requires several arguments:
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.
%retval = invoke i32 %Test(i32 15) to label %Continue
unwind label %TestCleanup ; {i32}:retval set
%retval = invoke coldcc i32 %Test(i32 15) to label %Continue
unwind label %TestCleanup ; {i32}:retval set
unwind
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.
The 'unwind' intrinsic 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.
unreachable
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.
The 'unreachable' instruction has no defined semantics.
Binary operators are used to do most of the computation in a program. They require two operands, execute an operation on them, and produce a single value. The operands might represent multiple data, as is the case with the packed data type. The result value of a binary operator is not necessarily the same type as its operands.
There are several different binary operators:
<result> = add <ty> <var1>, <var2> ; yields {ty}:result
The 'add' instruction returns the sum of its two operands.
The two arguments to the 'add' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.
The value produced is the integer or floating point sum of the two operands.
<result> = add i32 4, %var ; yields {i32}:result = 4 + %var
<result> = sub <ty> <var1>, <var2> ; yields {ty}:result
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.
The two arguments to the 'sub' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.
The value produced is the integer or floating point difference of the two operands.
<result> = sub i32 4, %var ; yields {i32}:result = 4 - %var
<result> = sub i32 0, %val ; yields {i32}:result = -%var
<result> = mul <ty> <var1>, <var2> ; yields {ty}:result
The 'mul' instruction returns the product of its two operands.
The two arguments to the 'mul' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.
The value produced is the integer or floating point product of the two operands.
Because the operands are the same width, the result of an integer multiplication is the same whether the operands should be deemed unsigned or signed.
<result> = mul i32 4, %var ; yields {i32}:result = 4 * %var
<result> = udiv <ty> <var1>, <var2> ; yields {ty}:result
The 'udiv' instruction returns the quotient of its two operands.
The two arguments to the 'udiv' instruction must be integer values. Both arguments must have identical types. This instruction can also take packed versions of the values in which case the elements must be integers.
The value produced is the unsigned integer quotient of the two operands. This instruction always performs an unsigned division operation, regardless of whether the arguments are unsigned or not.
<result> = udiv i32 4, %var ; yields {i32}:result = 4 / %var
<result> = sdiv <ty> <var1>, <var2> ; yields {ty}:result
The 'sdiv' instruction returns the quotient of its two operands.
The two arguments to the 'sdiv' instruction must be integer values. Both arguments must have identical types. This instruction can also take packed versions of the values in which case the elements must be integers.
The value produced is the signed integer quotient of the two operands. This instruction always performs a signed division operation, regardless of whether the arguments are signed or not.
<result> = sdiv i32 4, %var ; yields {i32}:result = 4 / %var
<result> = fdiv <ty> <var1>, <var2> ; yields {ty}:result
The 'fdiv' instruction returns the quotient of its two operands.
The two arguments to the 'div' instruction must be floating point values. Both arguments must have identical types. This instruction can also take packed versions of the values in which case the elements must be floating point.
The value produced is the floating point quotient of the two operands.
<result> = fdiv float 4.0, %var ; yields {float}:result = 4.0 / %var
<result> = urem <ty> <var1>, <var2> ; yields {ty}:result
The 'urem' instruction returns the remainder from the unsigned division of its two arguments.
The two arguments to the 'urem' instruction must be integer values. Both arguments must have identical types.
This instruction returns the unsigned integer remainder of a division. This instruction always performs an unsigned division to get the remainder, regardless of whether the arguments are unsigned or not.
<result> = urem i32 4, %var ; yields {i32}:result = 4 % %var
<result> = srem <ty> <var1>, <var2> ; yields {ty}:result
The 'srem' instruction returns the remainder from the signed division of its two operands.
The two arguments to the 'srem' instruction must be integer values. Both arguments must have identical types.
This instruction returns the remainder of a division (where the result has the same sign as the divisor), not the modulus (where the result has the same sign as the dividend) of a value. For more information about the difference, see The Math Forum.
<result> = srem i32 4, %var ; yields {i32}:result = 4 % %var
<result> = frem <ty> <var1>, <var2> ; yields {ty}:result
The 'frem' instruction returns the remainder from the division of its two operands.
The two arguments to the 'frem' instruction must be floating point values. Both arguments must have identical types.
This instruction returns the remainder of a division.
<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, execute an operation on them, and produce a single value. The resulting value of the bitwise binary operators is always the same type as its first operand.
<result> = and <ty> <var1>, <var2> ; yields {ty}:result
The 'and' instruction returns the bitwise logical and of its two operands.
The two arguments to the 'and' instruction must be integer values. Both arguments must have identical types.
The truth table used for the 'and' instruction is:
| In0 | In1 | Out |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
<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
<result> = or <ty> <var1>, <var2> ; yields {ty}:result
The 'or' instruction returns the bitwise logical inclusive or of its two operands.
The two arguments to the 'or' instruction must be integer values. Both arguments must have identical types.
The truth table used for the 'or' instruction is:
| In0 | In1 | Out |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
<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
<result> = xor <ty> <var1>, <var2> ; yields {ty}:result
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.
The two arguments to the 'xor' instruction must be integer values. Both arguments must have identical types.
The truth table used for the 'xor' instruction is:
| In0 | In1 | Out |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
<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
<result> = shl <ty> <var1>, i8 <var2> ; yields {ty}:result
The 'shl' instruction returns the first operand shifted to the left a specified number of bits.
The first argument to the 'shl' instruction must be an integer type. The second argument must be an 'i8' type.
The value produced is var1 * 2var2.
<result> = shl i32 4, i8 %var ; yields {i32}:result = 4 << %var
<result> = shl i32 4, i8 2 ; yields {i32}:result = 16
<result> = shl i32 1, i8 10 ; yields {i32}:result = 1024
<result> = lshr <ty> <var1>, i8 <var2> ; yields {ty}:result
The 'lshr' instruction (logical shift right) returns the first operand shifted to the right a specified number of bits.
The first argument to the 'lshr' instruction must be an integer type. The second argument must be an 'i8' type.
This instruction always performs a logical shift right operation. The var2 most significant bits will be filled with zero bits after the shift.
<result> = lshr i32 4, i8 1 ; yields {i32}:result = 2
<result> = lshr i32 4, i8 2 ; yields {i32}:result = 1
<result> = lshr i8 4, i8 3 ; yields {i8 }:result = 0
<result> = lshr i8 -2, i8 1 ; yields {i8 }:result = 0x7FFFFFFF
<result> = ashr <ty> <var1>, i8 <var2> ; yields {ty}:result
The 'ashr' instruction (arithmetic shift right) returns the first operand shifted to the right a specified number of bits.
The first argument to the 'ashr' instruction must be an integer type. The second argument must be an 'i8' type.
This instruction always performs an arithmetic shift right operation, regardless of whether the arguments are signed or not. The var2 most significant bits will be filled with the sign bit of var1.
<result> = ashr i32 4, i8 1 ; yields {i32}:result = 2
<result> = ashr i32 4, i8 2 ; yields {i32}:result = 1
<result> = ashr i8 4, i8 3 ; yields {i8}:result = 0
<result> = ashr i8 -2, i8 1 ; yields {i8 }:result = -1
LLVM supports several instructions to represent vector operations in a target-independent manner. This 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.
<result> = extractelement <n x <ty>> <val>, i32 <idx> ; yields <ty>
The 'extractelement' instruction extracts a single scalar element from a packed vector at a specified index.
The first operand of an 'extractelement' instruction is a value of packed type. The second operand is an index indicating the position from which to extract the element. The index may be a variable.
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.
%result = extractelement <4 x i32> %vec, i32 0 ; yields i32
<result> = insertelement <n x <ty>> <val>, <ty> <elt>, i32 <idx> ; yields <n x <ty>>
The 'insertelement' instruction inserts a scalar element into a packed vector at a specified index.
The first operand of an 'insertelement' instruction is a value of packed 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.
The result is a packed 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.
%result = insertelement <4 x i32> %vec, i32 1, i32 0 ; yields <4 x i32>
<result> = shufflevector <n x <ty>> <v1>, <n x <ty>> <v2>, <n x i32> <mask> ; yields <n x <ty>>
The 'shufflevector' instruction constructs a permutation of elements from two input vectors, returning a vector of the same type.
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.
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.
%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.
<result> = malloc <type>[, i32 <NumElements>][, align <alignment>] ; yields {type*}:result
The 'malloc' instruction allocates memory from the system heap and returns a pointer to it.
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. If an 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.
Memory is allocated using the system "malloc" function, and a pointer is returned.
%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
free <type> <value> ; yields {void}
The 'free' instruction returns memory back to the unused memory heap to be reallocated in the future.
'value' shall be a pointer value that points to a value that was allocated with the 'malloc' instruction.
Access to the memory pointed to by the pointer is no longer defined after this instruction executes.
%array = malloc [4 x i8] ; yields {[4 x i8]*}:array free [4 x i8]* %array
<result> = alloca <type>[, i32 <NumElements>][, align <alignment>] ; yields {type*}:result
The 'alloca' instruction allocates memory on the current stack frame of the procedure that is live until the current function returns to its caller.
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. If an 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.
Memory is allocated; a pointer is returned. '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.
%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
<result> = load <ty>* <pointer>
<result> = volatile load <ty>* <pointer>
The 'load' instruction is used to read from memory.
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 location of memory pointed to is loaded.
%ptr = alloca i32 ; yields {i32*}:ptr store i32 3, i32* %ptr ; yields {void} %val = load i32* %ptr ; yields {i32}:val = i32 3
store <ty> <value>, <ty>* <pointer> ; yields {void}
volatile store <ty> <value>, <ty>* <pointer> ; yields {void}
The 'store' instruction is used to write to memory.
There are two arguments to the 'store' instruction: a value to store and an address in which to store it. The type of the '<pointer>' operand must be a pointer to the 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 contents of memory are updated to contain '<value>' at the location specified by the '<pointer>' operand.
%ptr = alloca i32 ; yields {i32*}:ptr store i32 3, i32* %ptr ; yields {void} %val = load i32* %ptr ; yields {i32}:val = i32 3
<result> = getelementptr <ty>* <ptrval>{, <ty> <idx>}*
The 'getelementptr' instruction is used to get the address of a subelement of an aggregate data structure.
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, and will be sign extended to 64-bit values.
For example, let's consider a C code fragment and how it gets compiled to LLVM:
struct RT {
char A;
i32 B[10][20];
char C;
};
struct ST {
i32 X;
double Y;
struct RT Z;
};
define i32 *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 }
implementation
define i32* %foo(%ST* %s) {
entry:
%reg = getelementptr %ST* %s, i32 1, i32 2, i32 1, i32 5, i32 13
ret i32* %reg
}
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 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 rules 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.
; 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.
<result> = trunc <ty> <value> to <ty2> ; yields ty2
The 'trunc' instruction truncates its operand to the type ty2.
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.
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.
%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
<result> = zext <ty> <value> to <ty2> ; yields ty2
The 'zext' instruction zero extends its operand to type ty2.
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.
The zext fills the high order bits of the value with zero bits until it reaches the size of the destination type, ty2. When the the operand and the type are the same size, no bit filling is done and the cast is considered a no-op cast because no bits change (only the type changes).
When zero extending from i1, the result will always be either 0 or 1.
%X = zext i32 257 to i64 ; yields i64:257 %Y = zext i1 true to i32 ; yields i32:1
<result> = sext <ty> <value> to <ty2> ; yields ty2
The 'sext' sign extends value to the type ty2.
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.
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 the the operand and the type are the same size, no bit filling is done and the cast is considered a no-op cast because no bits change (only the type changes).
When sign extending from i1, the extension always results in -1 or 0.
%X = sext i8 -1 to i16 ; yields i16 :65535 %Y = sext i1 true to i32 ; yields i32:-1
<result> = fptrunc <ty> <value> to <ty2> ; yields ty2
The 'fptrunc' instruction truncates value to type ty2.
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.
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.
%X = fptrunc double 123.0 to float ; yields float:123.0 %Y = fptrunc double 1.0E+300 to float ; yields undefined
<result> = fpext <ty> <value> to <ty2> ; yields ty2
The 'fpext' extends a floating point value to a larger floating point value.
The 'fpext' instruction takes a floating point value to cast, and a floating point type to cast it to. The source type must be smaller than the destination type.
The 'fpext' instruction extends the value from a smaller floating point type to a larger floating point type. The fpext cannot be used to make a no-op cast because it always changes bits. Use bitcast to make a no-op cast for a floating point cast.
%X = fpext float 3.1415 to double ; yields double:3.1415 %Y = fpext float 1.0 to float ; yields float:1.0 (no-op)
<result> = fp2uint <ty> <value> to <ty2> ; yields ty2
The 'fp2uint' converts a floating point value to its unsigned integer equivalent of type ty2.
The 'fp2uint' instruction takes a value to cast, which must be a floating point value, and a type to cast it to, which must be an integer type.
The 'fp2uint' instruction converts its floating point operand into the nearest (rounding towards zero) unsigned integer value. If the value cannot fit in ty2, the results are undefined.
When converting to i1, the conversion is done as a comparison against zero. If the value was zero, the i1 result will be false. If the value was non-zero, the i1 result will be true.
%X = fp2uint double 123.0 to i32 ; yields i32:123 %Y = fp2uint float 1.0E+300 to i1 ; yields i1:true %X = fp2uint float 1.04E+17 to i8 ; yields undefined:1
<result> = fptosi <ty> <value> to <ty2> ; yields ty2
The 'fptosi' instruction converts floating point value to type ty2.
The 'fptosi' instruction takes a value to cast, which must be a floating point value, and a type to cast it to, which must also be an integer type.
The 'fptosi' instruction converts its floating point operand into the nearest (rounding towards zero) signed integer value. If the value cannot fit in ty2, the results are undefined.
When converting to i1, the conversion is done as a comparison against zero. If the value was zero, the i1 result will be false. If the value was non-zero, the i1 result will be true.
%X = fptosi double -123.0 to i32 ; yields i32:-123 %Y = fptosi float 1.0E-247 to i1 ; yields i1:true %X = fptosi float 1.04E+17 to i8 ; yields undefined:1
<result> = uitofp <ty> <value> to <ty2> ; yields ty2
The 'uitofp' instruction regards value as an unsigned integer and converts that value to the ty2 type.
The 'uitofp' instruction takes a value to cast, which must be an integer value, and a type to cast it to, which must be a floating point type.
The 'uitofp' instruction interprets its operand as an unsigned integer quantity and converts it to the corresponding floating point value. If the value cannot fit in the floating point value, the results are undefined.
%X = uitofp i32 257 to float ; yields float:257.0 %Y = uitofp i8 -1 to double ; yields double:255.0
<result> = sitofp <ty> <value> to <ty2> ; yields ty2
The 'sitofp' instruction regards value as a signed integer and converts that value to the ty2 type.
The 'sitofp' instruction takes a value to cast, which must be an integer value, and a type to cast it to, which must be a floating point type.
The 'sitofp' instruction interprets its operand as a signed integer quantity and converts it to the corresponding floating point value. If the value cannot fit in the floating point value, the results are undefined.
%X = sitofp i32 257 to float ; yields float:257.0 %Y = sitofp i8 -1 to double ; yields double:-1.0
<result> = ptrtoint <ty> <value> to <ty2> ; yields ty2
The 'ptrtoint' instruction converts the pointer value to the integer type ty2.
The 'ptrtoint' instruction takes a value to cast, which must be a pointer value, and a type to cast it to ty2, which must be an integer type.
The 'ptrtoint' instruction converts value to integer type ty2 by interpreting the pointer value as an integer and either truncating or zero extending that value to the size of the integer type. If value is smaller than ty2 then a zero extension is done. If value is larger than ty2 then a truncation is done. If they are the same size, then nothing is done (no-op cast).
%X = ptrtoint i32* %X to i8 ; yields truncation on 32-bit %Y = ptrtoint i32* %x to i64 ; yields zero extend on 32-bit
<result> = inttoptr <ty> <value> to <ty2> ; yields ty2
The 'inttoptr' instruction converts an integer value to a pointer type, ty2.
The 'inttoptr' instruction takes an integer value to cast, and a type to cast it to, which must be a pointer type.
The 'inttoptr' instruction converts value to type ty2 by applying either a zero extension or a truncation depending on the size of the integer value. If value is larger than the size of a pointer then a truncation is done. If value is smaller than the size of a pointer then a zero extension is done. If they are the same size, nothing is done (no-op cast).
%X = inttoptr i32 255 to i32* ; yields zero extend on 64-bit %X = inttoptr i32 255 to i32* ; yields no-op on 32-bit %Y = inttoptr i16 0 to i32* ; yields zero extend on 32-bit
<result> = bitcast <ty> <value> to <ty2> ; yields ty2
The 'bitcast' instruction converts value to type ty2 without changing any bits.
The 'bitcast' instruction takes a value to cast, which must be a first class value, and a type to cast it to, which must also be a first class type. The bit sizes of value and the destination type, ty2, must be identical. If the source type is a pointer, the destination type must also be a pointer.
The 'bitcast' instruction converts value to type ty2. It is always a no-op cast because no bits change with this conversion. The conversion is done as if the value had been stored to memory and read back as type ty2. Pointer types may only be converted to other pointer types with this instruction. To convert pointers to other types, use the inttoptr or ptrtoint instructions first.
%X = bitcast i8 255 to i8 ; yields i8 :-1 %Y = bitcast i32* %x to sint* ; yields sint*:%x %Z = bitcast <2xint> %V to i64; ; yields i64: %V
The instructions in this category are the "miscellaneous" instructions, which defy better classification.
<result> = icmp <cond> <ty> <var1>, <var2>
; yields {i1}:result
The 'icmp' instruction returns a boolean value based on comparison of its two integer operands.
The 'icmp' instruction takes three operands. The first operand is the condition code which indicates the kind of comparison to perform. It is not a value, just a keyword. The possibilities for the condition code are:
The remaining two arguments must be integer or pointer typed. They must also be identical types.
The 'icmp' compares var1 and var2 according to the condition code given as cond. The comparison performed always yields a i1 result, as follows:
If the operands are pointer typed, the pointer values are treated as integers and then compared.
<result> = icmp eq i32 4, 5 ; yields: result=false <result> = icmp ne float* %X, %X ; yields: result=false <result> = icmp ult i16 4, 5 ; yields: result=true <result> = icmp sgt i16 4, 5 ; yields: result=false <result> = icmp ule i16 -4, 5 ; yields: result=false <result> = icmp sge i16 4, 5 ; yields: result=false
<result> = fcmp <cond> <ty> <var1>, <var2>
; yields {i1}:result
The 'fcmp' instruction returns a boolean value based on comparison of its floating point operands.
The 'fcmp' instruction takes three operands. The first operand is the condition code which indicates the kind of comparison to perform. It is not a value, just a keyword. The possibilities for the condition code are:
In the preceding, ordered means that neither operand is a QNAN while unordered means that either operand may be a QNAN.
The val1 and val2 arguments must be floating point typed. They must have identical types.
In the foregoing, ordered means that neither operand is a QNAN and unordered means that either operand is a QNAN.
The 'fcmp' compares var1 and var2 according to the condition code given as cond. The comparison performed always yields a i1 result, as follows:
<result> = fcmp oeq float 4.0, 5.0 ; yields: result=false <result> = icmp one float 4.0, 5.0 ; yields: result=true <result> = icmp olt float 4.0, 5.0 ; yields: result=true <result> = icmp ueq double 1.0, 2.0 ; yields: result=false
<result> = phi <ty> [ <val0>, <label0>], ...
The 'phi' instruction is used to implement the φ node in the SSA graph representing the function.
The type of the incoming values are specified with the first type field. After this, the 'phi' instruction takes a list of pairs as arguments, with one pair for each predecessor basic block of the current block. Only values of first class type may be used as the value arguments to the PHI node. Only labels may be used as the label arguments.
There must be no non-phi instructions between the start of a basic block and the PHI instructions: i.e. PHI instructions must be first in a basic block.
At runtime, the 'phi' instruction logically takes on the value specified by the parameter, depending on which basic block we came from in the last terminator instruction.
Loop: ; Infinite loop that counts from 0 on up...
%indvar = phi i32 [ 0, %LoopHeader ], [ %nextindvar, %Loop ]
%nextindvar = add i32 %indvar, 1
br label %Loop
<result> = select i1 <cond>, <ty> <val1>, <ty> <val2> ; yields ty
The 'select' instruction is used to choose one value based on a condition, without branching.
The 'select' instruction requires a boolean value indicating the condition, and two values of the same first class type.
If the boolean condition evaluates to true, the instruction returns the first value argument; otherwise, it returns the second value argument.
%X = select i1 true, i8 17, i8 42 ; yields i8:17
<result> = [tail] call [cconv] <ty>* <fnptrval>(<param list>)
The 'call' instruction represents a simple function call.
This instruction requires several arguments:
The optional "tail" marker indicates whether the callee function accesses any allocas or varargs in the caller. If the "tail" marker is present, the function call is eligible for tail call optimization. Note that calls may be marked "tail" even if they do not occur before a ret instruction.
The optional "cconv" marker indicates which calling convention the call should use. If none is specified, the call defaults to using C calling conventions.
'ty': shall be the signature of the pointer to function value being invoked. The argument types must match the types implied by this signature. This type can be omitted if the function is not varargs and if the function type does not return a pointer to a function.
'fnptrval': An LLVM value containing a pointer to a function to be invoked. In most cases, this is a direct function invocation, but indirect calls are just as possible, calling an arbitrary pointer to function value.
'function args': argument list whose types match the function signature argument types. All arguments must be of first class type. If the function signature indicates the function accepts a variable number of arguments, the extra arguments can be specified.
The 'call' instruction is used to cause control flow to transfer to a specified function, with its incoming arguments bound to the specified values. Upon a 'ret' instruction in the called function, control flow continues with the instruction after the function call, and the return value of the function is bound to the result argument. This is a simpler case of the invoke instruction.
%retval = call i32 %test(i32 %argc) call i32(i8 *, ...) *%printf(i8 * %msg, i32 12, i8 42); %X = tail call i32 %foo() %Y = tail call fastcc i32 %foo()
<resultval> = va_arg <va_list*> <arglist>, <argty>
The 'va_arg' instruction is used to access arguments passed through the "variable argument" area of a function call. It is used to implement the va_arg macro in C.
This instruction takes a va_list* value and the type of the argument. It returns a value of the specified argument type and increments the va_list to point to the next argument. Again, the actual type of va_list is target specific.
The 'va_arg' instruction loads an argument of the specified type from the specified va_list and causes the va_list to point to the next argument. For more information, see the variable argument handling Intrinsic Functions.
It is legal for this instruction to be called in a function which does not take a variable number of arguments, for example, the vfprintf function.
va_arg is an LLVM instruction instead of an intrinsic function because it takes a type as an argument.
See the variable argument processing section.
LLVM supports the notion of an "intrinsic function". These functions have well known names and semantics and are required to follow certain restrictions. Overall, these instructions represent an extension mechanism for the LLVM language that does not require changing all of the transformations in LLVM to add to the language (or the bytecode reader/writer, the parser, etc...).
Intrinsic function names must all start with an "llvm." prefix. This prefix is reserved in LLVM for intrinsic names; thus, functions may not be named this. Intrinsic functions must always be external functions: you cannot define the body of intrinsic functions. Intrinsic functions may only be used in call or invoke instructions: it is illegal to take the address of an intrinsic function. Additionally, because intrinsic functions are part of the LLVM language, it is required that they all be documented here if any are added.
To learn how to add an intrinsic function, please see the Extending LLVM Guide.
Variable argument support is defined in LLVM with the va_arg instruction and these three intrinsic functions. These functions are related to the similarly named macros defined in the <stdarg.h> header file.
All of these functions operate on arguments that use a target-specific value type "va_list". The LLVM assembly language reference manual does not define what this type is, so all transformations should be prepared to handle intrinsics with any type used.
This example shows how the va_arg instruction and the variable argument handling intrinsic functions are used.
define i32 %test(i32 %X, ...) {
; Initialize variable argument processing
%ap = alloca i8 *
%ap2 = bitcast i8** %ap to i8*
call void %llvm.va_start(i8* %ap2)
; Read a single integer argument
%tmp = va_arg i8 ** %ap, i32
; Demonstrate usage of llvm.va_copy and llvm.va_end
%aq = alloca i8 *
%aq2 = bitcast i8** %aq to i8*
call void %llvm.va_copy(i8 *%aq2, i8* %ap2)
call void %llvm.va_end(i8* %aq2)
; Stop processing of arguments.
call void %llvm.va_end(i8* %ap2)
ret i32 %tmp
}
declare void %llvm.va_start(i8* <arglist>)
The 'llvm.va_start' intrinsic initializes *<arglist> for subsequent use by va_arg.
The argument is a pointer to a va_list element to initialize.
The 'llvm.va_start' intrinsic works just like the va_start macro available in C. In a target-dependent way, it initializes the va_list element the argument points to, so that the next call to va_arg will produce the first variable argument passed to the function. Unlike the C va_start macro, this intrinsic does not need to know the last argument of the function, the compiler can figure that out.
declare void %llvm.va_end(i8* <arglist>)
The 'llvm.va_end' intrinsic destroys <arglist> which has been initialized previously with llvm.va_start or llvm.va_copy.
The argument is a va_list to destroy.
The 'llvm.va_end' intrinsic works just like the va_end macro available in C. In a target-dependent way, it destroys the va_list. Calls to llvm.va_start and llvm.va_copy must be matched exactly with calls to llvm.va_end.
declare void %llvm.va_copy(i8* <destarglist>, i8* <srcarglist>)
The 'llvm.va_copy' intrinsic copies the current argument position from the source argument list to the destination argument list.
The first argument is a pointer to a va_list element to initialize. The second argument is a pointer to a va_list element to copy from.
The 'llvm.va_copy' intrinsic works just like the va_copy macro available in C. In a target-dependent way, it copies the source va_list element into the destination list. This intrinsic is necessary because the llvm.va_begin intrinsic may be arbitrarily complex and require memory allocation, for example.
LLVM support for Accurate Garbage Collection requires the implementation and generation of these intrinsics. These intrinsics allow identification of GC roots on the stack, as well as garbage collector implementations that require read and write barriers. Front-ends for type-safe garbage collected languages should generate these intrinsics to make use of the LLVM garbage collectors. For more details, see Accurate Garbage Collection with LLVM.
declare void %llvm.gcroot(<ty>** %ptrloc, <ty2>* %metadata)
The 'llvm.gcroot' intrinsic declares the existence of a GC root to the code generator, and allows some metadata to be associated with it.
The first argument specifies the address of a stack object that contains the root pointer. The second pointer (which must be either a constant or a global value address) contains the meta-data to be associated with the root.
At runtime, a call to this intrinsics stores a null pointer into the "ptrloc" location. At compile-time, the code generator generates information to allow the runtime to find the pointer at GC safe points.
declare i8 * %llvm.gcread(i8 * %ObjPtr, i8 ** %Ptr)
The 'llvm.gcread' intrinsic identifies reads of references from heap locations, allowing garbage collector implementations that require read barriers.
The second argument is the address to read from, which should be an address allocated from the garbage collector. The first object is a pointer to the start of the referenced object, if needed by the language runtime (otherwise null).
The 'llvm.gcread' intrinsic has the same semantics as a load instruction, but may be replaced with substantially more complex code by the garbage collector runtime, as needed.
declare void %llvm.gcwrite(i8 * %P1, i8 * %Obj, i8 ** %P2)
The 'llvm.gcwrite' intrinsic identifies writes of references to heap locations, allowing garbage collector implementations that require write barriers (such as generational or reference counting collectors).
The first argument is the reference to store, the second is the start of the object to store it to, and the third is the address of the field of Obj to store to. If the runtime does not require a pointer to the object, Obj may be null.
The 'llvm.gcwrite' intrinsic has the same semantics as a store instruction, but may be replaced with substantially more complex code by the garbage collector runtime, as needed.
These intrinsics are provided by LLVM to expose special features that may only be implemented with code generator support.
declare i8 *%llvm.returnaddress(i32 <level>)
The 'llvm.returnaddress' intrinsic attempts to compute a target-specific value indicating the return address of the current function or one of its callers.
The argument to this intrinsic indicates which function to return the address for. Zero indicates the calling function, one indicates its caller, etc. The argument is required to be a constant integer value.
The 'llvm.returnaddress' intrinsic either returns a pointer indicating the return address of the specified call frame, or zero if it cannot be identified. The value returned by this intrinsic is likely to be incorrect or 0 for arguments other than zero, so it should only be used for debugging purposes.
Note that calling this intrinsic does not prevent function inlining or other aggressive transformations, so the value returned may not be that of the obvious source-language caller.
declare i8 *%llvm.frameaddress(i32 <level>)
The 'llvm.frameaddress' intrinsic attempts to return the target-specific frame pointer value for the specified stack frame.
The argument to this intrinsic indicates which function to return the frame pointer for. Zero indicates the calling function, one indicates its caller, etc. The argument is required to be a constant integer value.
The 'llvm.frameaddress' intrinsic either returns a pointer indicating the frame address of the specified call frame, or zero if it cannot be identified. The value returned by this intrinsic is likely to be incorrect or 0 for arguments other than zero, so it should only be used for debugging purposes.
Note that calling this intrinsic does not prevent function inlining or other aggressive transformations, so the value returned may not be that of the obvious source-language caller.
declare i8 *%llvm.stacksave()
The 'llvm.stacksave' intrinsic is used to remember the current state of the function stack, for use with llvm.stackrestore. This is useful for implementing language features like scoped automatic variable sized arrays in C99.
This intrinsic returns a opaque pointer value that can be passed to llvm.stackrestore. When an llvm.stackrestore intrinsic is executed with a value saved from llvm.stacksave, it effectively restores the state of the stack to the state it was in when the llvm.stacksave intrinsic executed. In practice, this pops any alloca blocks from the stack that were allocated after the llvm.stacksave was executed.
declare void %llvm.stackrestore(i8 * %ptr)
The 'llvm.stackrestore' intrinsic is used to restore the state of the function stack to the state it was in when the corresponding llvm.stacksave intrinsic executed. This is useful for implementing language features like scoped automatic variable sized arrays in C99.
See the description for llvm.stacksave.
declare void %llvm.prefetch(i8 * <address>,
i32 <rw>, i32 <locality>)
The 'llvm.prefetch' intrinsic is a hint to the code generator to insert a prefetch instruction if supported; otherwise, it is a noop. Prefetches have no effect on the behavior of the program but can change its performance characteristics.
address is the address to be prefetched, rw is the specifier determining if the fetch should be for a read (0) or write (1), and locality is a temporal locality specifier ranging from (0) - no locality, to (3) - extremely local keep in cache. The rw and locality arguments must be constant integers.
This intrinsic does not modify the behavior of the program. In particular, prefetches cannot trap and do not produce a value. On targets that support this intrinsic, the prefetch can provide hints to the processor cache for better performance.
declare void %llvm.pcmarker( i32 <id> )
The 'llvm.pcmarker' intrinsic is a method to export a Program Counter (PC) in a region of code to simulators and other tools. The method is target specific, but it is expected that the marker will use exported symbols to transmit the PC of the marker. The marker makes no guarantees that it will remain with any specific instruction after optimizations. It is possible that the presence of a marker will inhibit optimizations. The intended use is to be inserted after optimizations to allow correlations of simulation runs.
id is a numerical id identifying the marker.
This intrinsic does not modify the behavior of the program. Backends that do not support this intrinisic may ignore it.
declare i64 %llvm.readcyclecounter( )
The 'llvm.readcyclecounter' intrinsic provides access to the cycle counter register (or similar low latency, high accuracy clocks) on those targets that support it. On X86, it should map to RDTSC. On Alpha, it should map to RPCC. As the backing counters overflow quickly (on the order of 9 seconds on alpha), this should only be used for small timings.
When directly supported, reading the cycle counter should not modify any memory. Implementations are allowed to either return a application specific value or a system wide value. On backends without support, this is lowered to a constant 0.
LLVM provides intrinsics for a few important standard C library functions. These intrinsics allow source-language front-ends to pass information about the alignment of the pointer arguments to the code generator, providing opportunity for more efficient code generation.
declare void %llvm.memcpy.i32(i8 * <dest>, i8 * <src>,
i32 <len>, i32 <align>)
declare void %llvm.memcpy.i64(i8 * <dest>, i8 * <src>,
i64 <len>, i32 <align>)
The 'llvm.memcpy.*' intrinsics copy a block of memory from the source location to the destination location.
Note that, unlike the standard libc function, the llvm.memcpy.* intrinsics do not return a value, and takes an extra alignment argument.
The first argument is a pointer to the destination, the second is a pointer to the source. The third argument is an integer argument specifying the number of bytes to copy, and the fourth argument is the alignment of the source and destination locations.
If the call to this intrinisic has an alignment value that is not 0 or 1, then the caller guarantees that both the source and destination pointers are aligned to that boundary.
The 'llvm.memcpy.*' intrinsics copy a block of memory from the source location to the destination location, which are not allowed to overlap. It copies "len" bytes of memory over. If the argument is known to be aligned to some boundary, this can be specified as the fourth argument, otherwise it should be set to 0 or 1.
declare void %llvm.memmove.i32(i8 * <dest>, i8 * <src>,
i32 <len>, i32 <align>)
declare void %llvm.memmove.i64(i8 * <dest>, i8 * <src>,
i64 <len>, i32 <align>)
The 'llvm.memmove.*' intrinsics move a block of memory from the source location to the destination location. It is similar to the 'llvm.memcmp' intrinsic but allows the two memory locations to overlap.
Note that, unlike the standard libc function, the llvm.memmove.* intrinsics do not return a value, and takes an extra alignment argument.
The first argument is a pointer to the destination, the second is a pointer to the source. The third argument is an integer argument specifying the number of bytes to copy, and the fourth argument is the alignment of the source and destination locations.
If the call to this intrinisic has an alignment value that is not 0 or 1, then the caller guarantees that the source and destination pointers are aligned to that boundary.
The 'llvm.memmove.*' intrinsics copy a block of memory from the source location to the destination location, which may overlap. It copies "len" bytes of memory over. If the argument is known to be aligned to some boundary, this can be specified as the fourth argument, otherwise it should be set to 0 or 1.
declare void %llvm.memset.i32(i8 * <dest>, i8 <val>,
i32 <len>, i32 <align>)
declare void %llvm.memset.i64(i8 * <dest>, i8 <val>,
i64 <len>, i32 <align>)
The 'llvm.memset.*' intrinsics fill a block of memory with a particular byte value.
Note that, unlike the standard libc function, the llvm.memset intrinsic does not return a value, and takes an extra alignment argument.
The first argument is a pointer to the destination to fill, the second is the byte value to fill it with, the third argument is an integer argument specifying the number of bytes to fill, and the fourth argument is the known alignment of destination location.
If the call to this intrinisic has an alignment value that is not 0 or 1, then the caller guarantees that the destination pointer is aligned to that boundary.
The 'llvm.memset.*' intrinsics fill "len" bytes of memory starting at the destination location. If the argument is known to be aligned to some boundary, this can be specified as the fourth argument, otherwise it should be set to 0 or 1.
declare float %llvm.sqrt.f32(float %Val) declare double %llvm.sqrt.f64(double %Val)
The 'llvm.sqrt' intrinsics return the sqrt of the specified operand, returning the same value as the libm 'sqrt' function would. Unlike sqrt in libm, however, llvm.sqrt has undefined behavior for negative numbers (which allows for better optimization).
The argument and return value are floating point numbers of the same type.
This function returns the sqrt of the specified operand if it is a positive floating point number.
declare float %llvm.powi.f32(float %Val, i32 %power) declare double %llvm.powi.f64(double %Val, i32 %power)
The 'llvm.powi.*' intrinsics return the first operand raised to the specified (positive or negative) power. The order of evaluation of multiplications is not defined.
The second argument is an integer power, and the first is a value to raise to that power.
This function returns the first value raised to the second power with an unspecified sequence of rounding operations.
LLVM provides intrinsics for a few important bit manipulation operations. These allow efficient code generation for some algorithms.
declare i16 %llvm.bswap.i16(i16 <id>) declare i32 %llvm.bswap.i32(i32 <id>) declare i64 %llvm.bswap.i64(i64 <id>)
The 'llvm.bwsap' family of intrinsics is used to byteswap a 16, 32 or 64 bit quantity. These are useful for performing operations on data that is not in the target's native byte order.
The llvm.bswap.16 intrinsic returns an i16 value that has the high and low byte of the input i16 swapped. Similarly, the llvm.bswap.i32 intrinsic returns an i32 value that has the four bytes of the input i32 swapped, so that if the input bytes are numbered 0, 1, 2, 3 then the returned i32 will have its bytes in 3, 2, 1, 0 order. The llvm.bswap.i64 intrinsic extends this concept to 64 bits.
declare i8 %llvm.ctpop.i8 (i8 <src>) declare i16 %llvm.ctpop.i16(i16 <src>) declare i32 %llvm.ctpop.i32(i32 <src>) declare i64 %llvm.ctpop.i64(i64 <src>)
The 'llvm.ctpop' family of intrinsics counts the number of bits set in a value.
The only argument is the value to be counted. The argument may be of any integer type. The return type must match the argument type.
The 'llvm.ctpop' intrinsic counts the 1's in a variable.
declare i8 %llvm.ctlz.i8 (i8 <src>) declare i16 %llvm.ctlz.i16(i16 <src>) declare i32 %llvm.ctlz.i32(i32 <src>) declare i64 %llvm.ctlz.i64(i64 <src>)
The 'llvm.ctlz' family of intrinsic functions counts the number of leading zeros in a variable.
The only argument is the value to be counted. The argument may be of any integer type. The return type must match the argument type.
The 'llvm.ctlz' intrinsic counts the leading (most significant) zeros in a variable. If the src == 0 then the result is the size in bits of the type of src. For example, llvm.ctlz(i32 2) = 30.
declare i8 %llvm.cttz.i8 (i8 <src>) declare i16 %llvm.cttz.i16(i16 <src>) declare i32 %llvm.cttz.i32(i32 <src>) declare i64 %llvm.cttz.i64(i64 <src>)
The 'llvm.cttz' family of intrinsic functions counts the number of trailing zeros.
The only argument is the value to be counted. The argument may be of any integer type. The return type must match the argument type.
The 'llvm.cttz' intrinsic counts the trailing (least significant) zeros in a variable. If the src == 0 then the result is the size in bits of the type of src. For example, llvm.cttz(2) = 1.
The LLVM debugger intrinsics (which all start with llvm.dbg. prefix), are described in the LLVM Source Level Debugging document.