c - 如何使用 LLVM-C 对浮点恒等函数进行 JIT 编译

Question

到目前为止，我已经编写了以下代码:

#include <stdlib.h>
#include <stdio.h>
#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>


int main (int argc, char const *argv[])
{ 
  char *error = NULL;
  LLVMLinkInMCJIT();
  LLVMInitializeNativeTarget();
  LLVMInitializeNativeAsmPrinter();
  LLVMInitializeNativeAsmParser();

  LLVMModuleRef mod = LLVMModuleCreateWithName("minimal_module");
  LLVMTypeRef identity_args[] = { LLVMDoubleType(), NULL };
  LLVMValueRef identity = LLVMAddFunction(mod, "identity", LLVMFunctionType(LLVMDoubleType(), identity_args, 1, 0));
  LLVMSetFunctionCallConv(identity, LLVMCCallConv);
  LLVMValueRef n = LLVMGetParam(identity, 0);

  LLVMBasicBlockRef entry = LLVMAppendBasicBlock(identity, "entry");
  LLVMBuilderRef builder = LLVMCreateBuilder();
  LLVMPositionBuilderAtEnd(builder, entry);
  LLVMBuildRet(builder, n);

  LLVMVerifyModule(mod, LLVMAbortProcessAction, &error);
  LLVMDisposeMessage(error);

  LLVMExecutionEngineRef engine;
  error = NULL;
  if(LLVMCreateJITCompilerForModule(&engine, mod, 2, &error) != 0) { 
    fprintf(stderr, "%s\n", error);
    LLVMDisposeMessage(error);
    abort();
  } 

  LLVMDumpModule(mod);

  LLVMGenericValueRef exec_args[] = {LLVMCreateGenericValueOfFloat(LLVMDoubleType(), 1.25)};
  LLVMGenericValueRef exec_res = LLVMRunFunction(engine, identity, 1, exec_args);
  fprintf(stderr, "\n");
  fprintf(stderr, "; Running identity(%f) with JIT...\n", 1.25);
  fprintf(stderr, "; Result: %f\n", LLVMGenericValueToFloat(LLVMDoubleType(), exec_res));

  LLVMRemoveModule(engine, mod, &mod, &error);
  LLVMDisposeModule(mod);
  LLVMDisposeExecutionEngine(engine);
  LLVMDisposeBuilder(builder);
  return 0;
} 

使用 32 位整数的相应实现有效，即 identity(42) 将返回 42。然而，上面的浮点版本返回 0.0。上面的程序生成以下输出:

; ModuleID = 'minimal_module'
source_filename = "minimal_module"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

define double @identity(double) {
entry:
  ret double %0
}

; Running identity(1.250000) with JIT...
; Result: 0.000000

我在 AMD 计算机上使用 Debian Jessie 下的 LLVM-3.9。我的问题是:我做错了什么？

Answer 1

我找到了解决问题的方法。除了使用 LLVMRunFunction 之外，还可以使用 LLVMGetFunctionAddress 来获取函数的地址并使用 C(或使用 FFI 库)直接调用它。

#include <stdlib.h>
#include <stdio.h>
#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>


int main (int argc, char const *argv[])
{
  char *error = NULL;
  LLVMLinkInMCJIT();
  LLVMInitializeNativeTarget();
  LLVMInitializeNativeAsmPrinter();
  LLVMInitializeNativeAsmParser();

  LLVMModuleRef mod = LLVMModuleCreateWithName("minimal_module");
  LLVMTypeRef identity_args[] = { LLVMInt32Type() };
  LLVMValueRef identity = LLVMAddFunction(mod, "identity", LLVMFunctionType(LLVMInt32Type(), identity_args, 1, 0));
  LLVMSetFunctionCallConv(identity, LLVMCCallConv);
  LLVMValueRef n = LLVMGetParam(identity, 0);

  LLVMBasicBlockRef entry = LLVMAppendBasicBlock(identity, "entry");
  LLVMBuilderRef builder = LLVMCreateBuilder();
  LLVMPositionBuilderAtEnd(builder, entry);
  LLVMBuildRet(builder, n);

  LLVMVerifyModule(mod, LLVMAbortProcessAction, &error);
  LLVMDisposeMessage(error);

  LLVMExecutionEngineRef engine;
  error = NULL;
  if(LLVMCreateJITCompilerForModule(&engine, mod, 2, &error) != 0) {
    fprintf(stderr, "%s\n", error);
    LLVMDisposeMessage(error);
    abort();
  }
  int (*fun)(int) = (int (*)(int))LLVMGetFunctionAddress(engine, "identity");

  LLVMDumpModule(mod);

  fprintf(stderr, "\n");
  fprintf(stderr, "; Running identity(42) with JIT...\n");
  fprintf(stderr, "; Result: %d\n", (*fun)(42));

  LLVMRemoveModule(engine, mod, &mod, &error);
  LLVMDisposeModule(mod);
  LLVMDisposeExecutionEngine(engine);
  LLVMDisposeBuilder(builder);
  return 0;
}