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[原创]初探LLVM JIT: 一个最小的JIT的例子（llvm 8）
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发表于: 2020-10-26 10:22
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我接触llvm的主要目的是写pass，但llvm ir以及ir对应的api让我很头疼。为了快速熟悉这套api，我需要一种快速编写、快速测试的方法，最后了解到llvm jit的存在。
但是在网上找了一些教材后，发现都不能顺利的跑起来，就连llvm源码中提供的example我也没能顺利的跑起来，一度让我怀疑自己是不是该放弃。在垂死挣扎之后，终于让我跑通了。
下面用一个简单例子来演示怎么使用llvm api新建module以及function，并且用jit去执行函数。
我们的目的是生成下面两个函数，并且调用foo，最后能打出执行结果11。
我把整个代码分成两部分，一部分是使用llvm ir指令生成上面函数，第二部分是使用llvm jit去执行上面函数。
下面是第一部分代码，
下面是第二部分代码
最后使用clang++进行编译生成可执行文件
下面是执行的效果，可以看到生成的中间代码以及运行的结果。通过这种方式，感觉可以很方便的学习llvm ir指令以及对应的api。
代码仓库地址
int add1(int x) {
return x+1;
}
int foo() {
return add1(10);
}
int add1(int x) {
return x+1;
}
int foo() {
return add1(10);
}
int initModule(LLVMContext &Context, Module *M, Function *FooF) {
// Create the add1 function entry and insert this entry into module M. The
// function will have a return type of "int" and take an argument of "int".
Function *Add1F =
cast<Function>(M->getOrInsertFunction("add1", Type::getInt32Ty(Context),
Type::getInt32Ty(Context)));
// Add a basic block to the function. As before, it automatically inserts
// because of the last argument.
BasicBlock *BB = BasicBlock::Create(Context, "EntryBlock", Add1F);
// Create a basic block builder with default parameters. The builder will
// automatically append instructions to the basic block `BB'.
IRBuilder<> builder(BB);
// Get pointers to the constant `1'.
Value *One = builder.getInt32(1);
// Get pointers to the integer argument of the add1 function...
assert(Add1F->arg_begin() != Add1F->arg_end()); // Make sure there's an arg
Argument *ArgX = &*Add1F->arg_begin(); // Get the arg
ArgX->setName("AnArg"); // Give it a nice symbolic name for fun.
// Create the add instruction, inserting it into the end of BB.
Value *Add = builder.CreateAdd(One, ArgX);
// Create the return instruction and add it to the basic block
builder.CreateRet(Add);
// Now, function add1 is ready.
// Now we're going to create function `foo', which returns an int and takes no
// arguments.
// Add a basic block to the FooF function.
BB = BasicBlock::Create(Context, "EntryBlock", FooF);
// Tell the basic block builder to attach itself to the new basic block
builder.SetInsertPoint(BB);
// Get pointer to the constant `10'.
Value *Ten = builder.getInt32(10);
// Pass Ten to the call to Add1F
CallInst *Add1CallRes = builder.CreateCall(Add1F, Ten);
Add1CallRes->setTailCall(true);
// Create the return instruction and add it to the basic block.
builder.CreateRet(Add1CallRes);
}int initModule(LLVMContext &Context, Module *M, Function *FooF) {
// Create the add1 function entry and insert this entry into module M. The
// function will have a return type of "int" and take an argument of "int".
Function *Add1F =
cast<Function>(M->getOrInsertFunction("add1", Type::getInt32Ty(Context),
Type::getInt32Ty(Context)));
// Add a basic block to the function. As before, it automatically inserts
// because of the last argument.
BasicBlock *BB = BasicBlock::Create(Context, "EntryBlock", Add1F);
// Create a basic block builder with default parameters. The builder will
// automatically append instructions to the basic block `BB'.
IRBuilder<> builder(BB);
// Get pointers to the constant `1'.
Value *One = builder.getInt32(1);
// Get pointers to the integer argument of the add1 function...
assert(Add1F->arg_begin() != Add1F->arg_end()); // Make sure there's an arg
Argument *ArgX = &*Add1F->arg_begin(); // Get the arg
ArgX->setName("AnArg"); // Give it a nice symbolic name for fun.
// Create the add instruction, inserting it into the end of BB.
Value *Add = builder.CreateAdd(One, ArgX);
// Create the return instruction and add it to the basic block
builder.CreateRet(Add);
// Now, function add1 is ready.
// Now we're going to create function `foo', which returns an int and takes no
// arguments.
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