Friday, June 13, 2008

The secret of the LLVM C bindings

Ever wanted to use LLVM from C? Can't find any documentation? Welcome.

Since I'm considering retargeting CLISP'S JIT Compiler I've been experimenting with LLVM. LLVM is an optimizing compiler for a virtual instruction set. Technically, it is very interesting. And this year, with Apple and CLANG in the game, it seems to be here to stay.

A Factorial in C with LLVM
Let's make a factorial function using the C bindings of LLVM 2.3+.
The function we will describe in LLVM instructions is illustrated below.

I inserted the phi instruction manually to make things more interesting.
Paste this in your favorite editor and save it as "fac.c":

// Headers required by LLVM
#include <llvm-c/Core.h>
#include <llvm-c/Analysis.h>
#include <llvm-c/ExecutionEngine.h>
#include <llvm-c/Target.h>
#include <llvm-c/Transforms/Scalar.h>

// General stuff
#include <stdlib.h>
#include <stdio.h>

int main (int argc, char const *argv[])
char *error = NULL; // Used to retrieve messages from functions
LLVMModuleRef mod = LLVMModuleCreateWithName("fac_module");
LLVMTypeRef fac_args[] = { LLVMInt32Type() };
LLVMValueRef fac = LLVMAddFunction(mod, "fac", LLVMFunctionType(LLVMInt32Type(), fac_args, 1, 0));
LLVMSetFunctionCallConv(fac, LLVMCCallConv);
LLVMValueRef n = LLVMGetParam(fac, 0);

LLVMBasicBlockRef entry = LLVMAppendBasicBlock(fac, "entry");
LLVMBasicBlockRef iftrue = LLVMAppendBasicBlock(fac, "iftrue");
LLVMBasicBlockRef iffalse = LLVMAppendBasicBlock(fac, "iffalse");
LLVMBasicBlockRef end = LLVMAppendBasicBlock(fac, "end");
LLVMBuilderRef builder = LLVMCreateBuilder();

LLVMPositionBuilderAtEnd(builder, entry);
LLVMValueRef If = LLVMBuildICmp(builder, LLVMIntEQ, n, LLVMConstInt(LLVMInt32Type(), 0, 0), "n == 0");
LLVMBuildCondBr(builder, If, iftrue, iffalse);

LLVMPositionBuilderAtEnd(builder, iftrue);
LLVMValueRef res_iftrue = LLVMConstInt(LLVMInt32Type(), 1, 0);
LLVMBuildBr(builder, end);

LLVMPositionBuilderAtEnd(builder, iffalse);
LLVMValueRef n_minus = LLVMBuildSub(builder, n, LLVMConstInt(LLVMInt32Type(), 1, 0), "n - 1");
LLVMValueRef call_fac_args[] = {n_minus};
LLVMValueRef call_fac = LLVMBuildCall(builder, fac, call_fac_args, 1, "fac(n - 1)");
LLVMValueRef res_iffalse = LLVMBuildMul(builder, n, call_fac, "n * fac(n - 1)");
LLVMBuildBr(builder, end);

LLVMPositionBuilderAtEnd(builder, end);
LLVMValueRef res = LLVMBuildPhi(builder, LLVMInt32Type(), "result");
LLVMValueRef phi_vals[] = {res_iftrue, res_iffalse};
LLVMBasicBlockRef phi_blocks[] = {iftrue, iffalse};
LLVMAddIncoming(res, phi_vals, phi_blocks, 2);
LLVMBuildRet(builder, res);

LLVMVerifyModule(mod, LLVMAbortProcessAction, &error);
LLVMDisposeMessage(error); // Handler == LLVMAbortProcessAction -> No need to check errors

LLVMExecutionEngineRef engine;
LLVMModuleProviderRef provider = LLVMCreateModuleProviderForExistingModule(mod);
error = NULL;
LLVMCreateJITCompiler(&engine, provider, &error);
if(error) {
fprintf(stderr, "%s\n", error);

LLVMPassManagerRef pass = LLVMCreatePassManager();
LLVMAddTargetData(LLVMGetExecutionEngineTargetData(engine), pass);
// LLVMAddDemoteMemoryToRegisterPass(pass); // Demotes every possible value to memory
LLVMRunPassManager(pass, mod);

LLVMGenericValueRef exec_args[] = {LLVMCreateGenericValueOfInt(LLVMInt32Type(), 10, 0)};
LLVMGenericValueRef exec_res = LLVMRunFunction(engine, fac, 1, exec_args);
fprintf(stderr, "\n");
fprintf(stderr, "; Running fac(10) with JIT...\n");
fprintf(stderr, "; Result: %d\n", LLVMGenericValueToInt(exec_res, 0));

return 0;

Compiling the code
Generating the object file is a no-brainer:

gcc `llvm-config --cflags` -c fac.c
Linking is a little trickier. Even though you are writing C code, you have to use a C++ linker.

g++ `llvm-config --libs --cflags --ldflags core analysis executionengine jit interpreter native` fac.o -o fac
All set!


Marc-André Lureau said...

You might need to call "LLVMLinkInJIT" nowadays.

thanks for this introduction to LLVM

Anonymous said...

For LLVM 2.6, I needed to make a couple changes:
At the start of main, add:

2) LLVMCreateJITCompiler has an extra argument (Optimization level), and has a return value of whether an error occurred. So replace lines 58-59 with:

if(LLVMCreateJITCompiler(&engine, provider, 2, &error) != 0) {

Anonymous said...

Not sure why, but on Ubuntu 10.04, this only compiles for me if I put fac.o *before* the `llvm-config`. It's weird; this is the only time I can think of that the linker's behaviour has been influenced by the order of the object/library files.

g++ fac.o `llvm-config --libs --cflags --ldflags core analysis executionengine jit interpreter native` -o fac

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