為了除錯方便，有時我們需要在codegen程式碼中新增printf函式，列印除錯資訊
PrintTest.cpp

//clang++ PrintTest.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -o p 
#include <iostream>
#include "llvm/ADT/APFloat.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/LegacyPassManager.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Verifier.h"
#include "llvm/Support/TargetSelect.h"
#include "llvm/Support/raw_os_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/GVN.h"
#include "KaleidoscopeJIT.h"

typedef void (*print_int32_ptr_t)(int32_t);

    llvm::Function *get_printf(llvm::Module *mod)
{
    const char *fun_name = "printf";
    llvm::Function *func = mod->getFunction(fun_name);
    if (func == nullptr)
    {
        llvm::FunctionType *func_type = llvm::FunctionType::get(
            llvm::Type::getInt32Ty(mod->getContext()),
            {llvm::Type::getInt8PtrTy(mod->getContext())},
            true);
        func = llvm::Function::Create(func_type, llvm::GlobalValue::ExternalLinkage, fun_name, mod);
    }
    return func;
}

int main()
{
    llvm::LLVMContext context;

    llvm::InitializeNativeTarget();
    llvm::InitializeNativeTargetAsmPrinter();
    llvm::InitializeNativeTargetAsmParser();

    auto native_jit = llvm::make_unique<llvm::orc::KaleidoscopeJIT>();
    auto module = llvm::make_unique<llvm::Module>("print_module", context);
    module->setDataLayout(native_jit->getTargetMachine().createDataLayout());

    const char *fun_name = "print_test";

    llvm::FunctionType *func_type = llvm::FunctionType::get(
        llvm::Type::getVoidTy(context),
        {llvm::Type::getInt32Ty(context)},
        false);
    auto func_ptr = llvm::Function::Create(func_type, llvm::GlobalValue::ExternalLinkage, fun_name, module.get());

    auto func_arg_iter = func_ptr->arg_begin();
    func_arg_iter->setName("val_");
    llvm::Value *arg_val = &(*func_arg_iter);

    llvm::IRBuilder<> builder(context);

    auto bb_entry = llvm::BasicBlock::Create(context, "entry", func_ptr);
    builder.SetInsertPoint(bb_entry);

    auto printf_ptr = get_printf(module.get());

    builder.CreateCall(printf_ptr,
                       {builder.CreateGlobalStringPtr("in llvm fun, value = %d\n"), arg_val});

    builder.CreateRetVoid();

    llvm::raw_os_ostream os(std::cout);
    module->print(os,nullptr);

    auto jit_mod = native_jit->addModule(std::move(module));
    auto print_sym = native_jit->findSymbol(fun_name);
    if(!print_sym){
        std::cout << "cant't find symbol : " << fun_name << std::endl;
        return -1;
    }

    auto print_ptr = (print_int32_ptr_t)llvm::cantFail(print_sym.getAddress());

    std::cout << "====================" << std::endl;

    (*print_ptr)(12);

    std::cout << "=====================" << std::endl;

    native_jit->removeModule(jit_mod);

    return 0;
}
 

KaleidoscopeJIT.h

//===- KaleidoscopeJIT.h - A simple JIT for Kaleidoscope --------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Contains a simple JIT definition for use in the kaleidoscope tutorials.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_EXECUTIONENGINE_ORC_KALEIDOSCOPEJIT_H
#define LLVM_EXECUTIONENGINE_ORC_KALEIDOSCOPEJIT_H

#include "llvm/ADT/iterator_range.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ExecutionEngine/ExecutionEngine.h"
#include "llvm/ExecutionEngine/JITSymbol.h"
#include "llvm/ExecutionEngine/RTDyldMemoryManager.h"
#include "llvm/ExecutionEngine/SectionMemoryManager.h"
#include "llvm/ExecutionEngine/Orc/CompileUtils.h"
#include "llvm/ExecutionEngine/Orc/IRCompileLayer.h"
#include "llvm/ExecutionEngine/Orc/LambdaResolver.h"
#include "llvm/ExecutionEngine/Orc/RTDyldObjectLinkingLayer.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Mangler.h"
#include "llvm/Support/DynamicLibrary.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include <algorithm>
#include <memory>
#include <string>
#include <vector>

namespace llvm {
namespace orc {

class KaleidoscopeJIT {
public:
  using ObjLayerT = RTDyldObjectLinkingLayer;
  using CompileLayerT = IRCompileLayer<ObjLayerT, SimpleCompiler>;
  using ModuleHandleT = CompileLayerT::ModuleHandleT;

  KaleidoscopeJIT()
      : TM(EngineBuilder().selectTarget()), DL(TM->createDataLayout()),
        ObjectLayer([]() { return std::make_shared<SectionMemoryManager>(); }),
        CompileLayer(ObjectLayer, SimpleCompiler(*TM)) {
    llvm::sys::DynamicLibrary::LoadLibraryPermanently(nullptr);
  }

  TargetMachine &getTargetMachine() { return *TM; }

  ModuleHandleT addModule(std::unique_ptr<Module> M) {
    // We need a memory manager to allocate memory and resolve symbols for this
    // new module. Create one that resolves symbols by looking back into the
    // JIT.
    auto Resolver = createLambdaResolver(
        [&](const std::string &Name) {
          if (auto Sym = findMangledSymbol(Name))
            return Sym;
          return JITSymbol(nullptr);
        },
        [](const std::string &S) { return nullptr; });
    auto H = cantFail(CompileLayer.addModule(std::move(M),
                                             std::move(Resolver)));

    ModuleHandles.push_back(H);
    return H;
  }

  void removeModule(ModuleHandleT H) {
    ModuleHandles.erase(find(ModuleHandles, H));
    cantFail(CompileLayer.removeModule(H));
  }

  JITSymbol findSymbol(const std::string Name) {
    return findMangledSymbol(mangle(Name));
  }

private:
  std::string mangle(const std::string &Name) {
    std::string MangledName;
    {
      raw_string_ostream MangledNameStream(MangledName);
      Mangler::getNameWithPrefix(MangledNameStream, Name, DL);
    }
    return MangledName;
  }

  JITSymbol findMangledSymbol(const std::string &Name) {
#ifdef LLVM_ON_WIN32
    // The symbol lookup of ObjectLinkingLayer uses the SymbolRef::SF_Exported
    // flag to decide whether a symbol will be visible or not, when we call
    // IRCompileLayer::findSymbolIn with ExportedSymbolsOnly set to true.
    //
    // But for Windows COFF objects, this flag is currently never set.
    // For a potential solution see: https://reviews.llvm.org/rL258665
    // For now, we allow non-exported symbols on Windows as a workaround.
    const bool ExportedSymbolsOnly = false;
#else
    const bool ExportedSymbolsOnly = true;
#endif

    // Search modules in reverse order: from last added to first added.
    // This is the opposite of the usual search order for dlsym, but makes more
    // sense in a REPL where we want to bind to the newest available definition.
    for (auto H : make_range(ModuleHandles.rbegin(), ModuleHandles.rend()))
      if (auto Sym = CompileLayer.findSymbolIn(H, Name, ExportedSymbolsOnly))
        return Sym;

    // If we can't find the symbol in the JIT, try looking in the host process.
    if (auto SymAddr = RTDyldMemoryManager::getSymbolAddressInProcess(Name))
      return JITSymbol(SymAddr, JITSymbolFlags::Exported);

#ifdef LLVM_ON_WIN32
    // For Windows retry without "_" at beginning, as RTDyldMemoryManager uses
    // GetProcAddress and standard libraries like msvcrt.dll use names
    // with and without "_" (for example "_itoa" but "sin").
    if (Name.length() > 2 && Name[0] == '_')
      if (auto SymAddr =
              RTDyldMemoryManager::getSymbolAddressInProcess(Name.substr(1)))
        return JITSymbol(SymAddr, JITSymbolFlags::Exported);
#endif

    return nullptr;
  }

  std::unique_ptr<TargetMachine> TM;
  const DataLayout DL;
  ObjLayerT ObjectLayer;
  CompileLayerT CompileLayer;
  std::vector<ModuleHandleT> ModuleHandles;
};

} // end namespace orc
} // end namespace llvm

#endif // LLVM_EXECUTIONENGINE_ORC_KALEIDOSCOPEJIT_H
 

使用如下命令編譯:

clang++ PrintTest.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -o p 

輸出結果:

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

@0 = private unnamed_addr constant [25 x i8] c"in llvm fun, value = %d\0A\00"

define void @print_test(i32 %val_) {
entry:
  %0 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([25 x i8], [25 x i8]* @0, i32 0, i32 0), i32 %val_)
  ret void
}

declare i32 @printf(i8*, ...)
====================
in llvm fun, value = 12
=====================