026_llvm.md


LLVM

LLVM is a statically typed intermediate representation and an associated toolchain for manipulating, optimizing and converting this intermediate form into native code.

So for example consider a simple function which takes two arguments, adds them, and xors the result. Writing in IR it would be formed as such:

Running this through the LLVM toolchain we can target our high level IR into multiple different assembly codes mapping onto various architectures and CPUs all from the same platform agnostic intermediate representation.

x86-64

test1:
        .cfi_startproc
        andl    %edx, %esi
        andl    %edx, %edi
        xorl    %esi, %edi
        movl    %edi, %eax
        ret

ARM

test1:
        and     r1, r2, r1
        and     r0, r2, r0
        eor     r0, r0, r1
        mov     pc, lr

PowerPC

.L.test1:
        .cfi_startproc
        and 4, 5, 4
        and 3, 5, 3
        xor 3, 3, 4
        blr 
        .long   0
        .quad   0

A uncommonly large amount of hardware manufacturers and software vendors (Adobe, AMD, Apple, ARM, Google, IBM, Intel, Mozilla, Qualcomm, Samsung, Xilinx) have come have converged on the LLVM toolchain as service agnostic way to talk about generating machine code.

What's even more impressive is that many of the advances in compiler optimizations and static analysis have been mechanized in the form of optimization passes so that all compilers written on top of the LLVM platform can take advantage of the same advanced optimizers that would often previously have to be developed independently.

Types

Primitive

i1     ; Boolean type
i8     ; char
i32    ; 32 bit integer
i64    ; 64 bit integer
float  ; 32 bit
double ; 64 bit

Arrays

[10 x float]      ; Array of 10 floats
[10 x [20 x i32]] ; Array of 10 arrays of 20 integers.

Structs

{float, i64}           ; structure
{float, {double, i3}}  ; nested structure
<{float, [2 x i3]}>    ; packed structure

Vectors

<4 x double>
<8 x float>

Pointers

float*        ; Pointer to a float
[25 x float]* ; Pointer to an array

The traditional void* pointer in C is a i8* pointer in LLVM with the appropriate casts.

Constants

[i1 true, i1 false]    ; constant bool array
<i32 42, i32 10>       ; constant vector
float 1.23421e+2       ; floating point constant
null                   ; null pointer constant

The zeroinitializer can be used to instantiate any type to the appropriate zero of any type.

<8 x float> zeroinitializer ; Zero vector

Named Types

%vec4 = type <4 x i32>
%pair = type { i32, i32 }

Recursive types declarations are supported.

%f = type { %f*, i32 }

Platform Information

target datalayout = "
   e-
   p      : 64  : 64   : 64-
   i1     : 8   : 8-
   i8     : 8   : 8-
   i16    : 16  : 16-
   i32    : 32  : 32-
   i64    : 64  : 64-
   f32    : 32  : 32-
   f64    : 64  : 64-
   v64    : 64  : 64-
   v128   : 128 : 128-
   a0     : 0   : 64-
   s0     : 64  : 64-
   f80    : 128 : 128-
   n8     : 16  : 32   : 64-
   S128
   "
target triple = "x86_64-unknown-linux-gnu"

Specifications are delimited by the minus sign -.

• The e indicates the platform is little-endian.
• The i<n> indicate the bitsize and alignment of the integer type.
• The f<n> indicate the bitsize and alignment of the floating point type.
• The p<n> indicate the bitsize and alignment of the pointer type.
• The v<n> indicate the bitsize and alignment of the vector type.
• The a<n> indicate the bitsize and alignment of the aggregate type.
• The n<n> indicate the widths of the CPU registers.
• The S<n> indicate the alignment of the stack.

Variables

Symbols used in an LLVM module are either global or local. Global symbols begin with @ and local symbols begin with %. All symbols must be defined or forward declared.

Instructions in LLVM are either numbered sequentially (%0, %1, ...) or given explicit variable names (%a, %foo, ..). For example the arguments to the following function are named values, while the result of the add instructions unnamed.

define i32 @add(i32 %a, i32 %b) {
  %1 = add i32 %a, %b
  ret i32 %1
}

Instructions

%result = add i32 10, 20

Logical

• shl
• lshr
• ashr
• and
• or
• xor

Binary Operators

• add
• fadd
• sub
• fsub
• mul
• fmul
• udiv
• sdiv
• fdiv
• urem
• srem
• frem

Comparison

op unsigned signed floating

---

lt ULT SLT OLT gt UGT SGT OGT le ULE SLE OLE ge UGE SGE OGE eq EQ EQ OEQ ne NE NE ONE

%c = udiv i32 %a, %b
%d = sdiv i32 %a, %b
%e = fmul float %a, %b
%f = fdiv float %a, %b

%g = icmp eq i32 %a, %b
%i = icmp slt i32 %a, %b
%j = icmp ult i32 %a, %b
%k = fcmp olt float, %a, %b

Data

i1 1
i32 299792458
float 7.29735257e-3
double 6.62606957e-34

Blocks

Function definitions in LLVM introduce a sequence of labeled basic blocks containing any number of instructions and a final terminator instruction which indicates how control flow yields after the instructions of the basic block are evaluated.

define i1 @foo() {
entry:
  br label %next
next:
  br label %return
return:
  ret i1 0
}

A basic block has either zero (for entry block) or a fixed number of predecessors. A graph with basic blocks as nodes and the predecessors of each basic block as edges constitutes a control flow graph. LLVM's opt command can be used to dump this graph using graphviz.

$ opt -view-cfg module.ll 
$ dot -Tpng module.dot -o module.png

We say a basic block A dominates a different block B in the control flow graph if it's impossible to reach B without passing through "A, equivalently A is the dominator of B.

All logic in LLVM is written in static single assignment (SSA) form. Each variable is assigned precisely once, and every variable is defined before it is used. Updating any existing variable reference creates a new reference with for the resulting output.

Control Flow

• Unconditional Branch
• Conditional Branch
• Switch
• Return
• Phi

\clearpage

Return

The ret function simply exits the current function yielding the current value to the virtual stack.

define i1 @foo() {
  ret i1 0
}

\clearpage

Unconditional Branch

The unconditional branch br simply jumps to any basic block local to the function.

define i1 @foo() {
  entry:
    br label %next
  next:
    br label %return
  return:
    ret i1 0
}

\clearpage

Conditional Branch

The conditional branch br jumps to one of two basic blocks based on whether a test condition is true or false. This corresponds the logic of a traditional "if statement".

define i32 @foo() {
start:
  br i1 true, label %left, label %right
left:
  ret i32 10
right:
  ret i32 20
}

\clearpage

Switch

The switch statement switch jumps to any number of branches based on the equality of value to a jump table matching values to basic blocks.

define i32 @foo(i32 %a) {
entry:
  switch i32 %a, label %default [
    i32 0, label %f
    i32 1, label %g
    i32 2, label %h
  ]
f:
  ret i32 1
g:
  ret i32 2
h:
  ret i32 3
default:
  ret i32 0
}

\clearpage

Phi

A phi node selects a value based on the predecessor of the current block.

define i32 @foo() {
start:
  br i1 true, label %left, label %right
left:
  %plusOne = add i32 0, 1
  br label %merge
right:
  br label %merge
merge:
  %join = phi i32 [ %plusOne, %left ], [ -1, %right ]
  ret i32 %join
}

\clearpage

Loops

The traditional while and for loops can be written in terms of the simpler conditional branching constructrs. For example in C we would write:

int count(int n) 
{
  int i = 0;
  while(i < n) 
  {
    i++;
  }
  return i;
}

Whereas in LLVM we write:

define i32 @count(i32 %n) {
entry:
   br label %loop

loop:
   %i = phi i32 [ 1, %entry ], [ %nextvar, %loop ]
   %nextvar = add i32 %i, 1

   %cmptmp = icmp ult i32 %i, %n
   %booltmp = zext i1 %cmptmp to i32
   %loopcond = icmp ne i32 %booltmp, 0

   br i1 %loopcond, label %loop, label %afterloop

afterloop:
   ret i32 %i
}

\clearpage

Select

Selects the first value if the test value is true, the second if false.

%x = select i1 true, i8 10, i8 20   ; gives 10
%y = select i1 false, i8 10, i8 20  ; gives 20

Calls

• ccc: The C calling convention
• fastcc: The fast calling convention

%result = call i32 @exp(i32 7)

Memory

LLVM uses the traditional load/store model:

• load: Load a typed value from a given reference
• store: Store a typed value in a given reference
• alloca: Allocate a pointer to memory on the virtual stack

%ptr = alloca i32
store i32 3, i32* %ptr
%val = load i32* %ptr

Specific pointer alignment can be specified:

%ptr = alloca i32, align 1024

For allocating in main memory we use an external reference to the C stdlib memory allocator which gives us back a (i8*).

%ptr = call i8* @malloc(i32 %objectsize)

For structures:

extractvalue {i32, float} %a, 0              ; gives i32
extractvalue {i32, {float, double}} %a, 0, 1 ; gives double
extractvalue [2 x i32] %a, 0                 ; yields i32

%x = insertvalue {i32, float} %b, float %val, 1         ; gives {i32 1, float %b}
%y = insertvalue {i32, float} zeroinitializer, i32 1, 0 ; gives {i32 1, float 0}

GetElementPtr

Casts

• trunc
• zext
• sext
• fptoui
• fptosi
• uitofp
• sitofp
• fptrunc
• fpext
• ptrtoint
• inttoptr
• bitcast

trunc i32 257 to i8         ; yields i8  1
zext i32 257 to i64         ; yields i64 257
sext i8 -1 to i16           ; yields i16 65535
bitcast <2 x i32> %a to i64 ; yields i64 %a

Toolchain

$ llc example.ll -o example.s             # compile
$ lli example.ll                          # execute
$ opt -S example.bc -o example.ll         # to assembly
$ opt example.ll -o example.bc            # to bitcode
$ opt -O3 example.ll -o example.opt.ll -S # run optimizer

Individual modules can be linked together.

$ llvm-link a.ll b.ll -o c.ll -S

Link time optimization.

$ clang -O4 -emit-llvm a.c -c -o a.bc 
$ clang -O4 -emit-llvm a.c -c -o a.bc 
$ llvm-link a.bc b.bc -o all.bc
$ opt -std-compile-opts -std-link-opts -O3 all.bc -o optimized.bc

The clang project is a C compiler that targets LLVM as it's intermediate representation. In the case where we'd like to know how some specific C construct maps into LLVM IR we can ask clang to dump its internal IR using the -emit-llvm flag.

# clang -emit-llvm -S add.c -o -
int add(int x)
{
  return x+1;
}

; ModuleID = 'add.c'
define i32 @add(i32 %x) nounwind uwtable {
entry:
  %x.addr = alloca i32, align 4
  store i32 %x, i32* %x.addr, align 4
  %0 = load i32* %x.addr, align 4
  %add = add nsw i32 %0, 1
  ret i32 %add
}

LLVM is using a C++ API underneath the hood of all these tools. If you need to work directly with the API it can be useful to be able to expand out the LLVM IR into the equivalent C++ code.

$ llc example.ll -march=cpp -o -

Function* func_test1 = mod->getFunction("test1");
if (!func_test1) {
func_test1 = Function::Create(
 /*Type=*/FuncTy_0,
 /*Linkage=*/GlobalValue::ExternalLinkage,
 /*Name=*/"test1", mod); 
func_test1->setCallingConv(CallingConv::C);
}
AttrListPtr func_test1_PAL;
func_test1->setAttributes(func_test1_PAL);

{
 Function::arg_iterator args = func_test1->arg_begin();
 Value* int32_x = args++;
 int32_x->setName("x");
 Value* int32_y = args++;
 int32_y->setName("y");
 Value* int32_z = args++;
 int32_z->setName("z");
 
 BasicBlock* label_1 = BasicBlock::Create(mod->getContext(), "",func_test1,0);
 
 BinaryOperator* int32_a = BinaryOperator::Create(
   Instruction::And, int32_z, int32_x, "a", label_1);
 BinaryOperator* int32_b = BinaryOperator::Create(
   Instruction::And, int32_z, int32_y, "b", label_1);
 BinaryOperator* int32_c = BinaryOperator::Create(
   Instruction::Xor, int32_a, int32_b, "c", label_1);
 ReturnInst::Create(mod->getContext(), int32_c, label_1);
 
}

llvm-general

The LLVM bindings for Haskell are split across two packages:

• llvm-general-pure is a pure Haskell representation of the LLVM IR.

• llvm-general is the FFI bindings to LLVM required for constructing the C representation of the LLVM IR and performing optimization and compilation.

llvm-general-pure does not require the LLVM libraries be available on the system.

GHCi can have issues with the FFI and can lead to errors when working with llvm-general. If you end up with errors like the following, then you are likely trying to use GHCi or runhaskell and it is unable to link against your LLVM library. Instead compile with standalone ghc.

Loading package llvm-general-3.3.8.2 
... linking 
... ghc: /usr/lib/llvm-3.3/lib/libLLVMSupport.a: unknown symbol `_ZTVN4llvm14error_categoryE'
ghc: unable to load package `llvm-general-3.3.8.2'

Code Generation (LLVM)

Resources

• LLVM Language Reference
• Implementing a JIT Compiled Language with Haskell and LLVM

\clearpage