[LLVMdev] Disabling optimizations when using llvm::createPrintModulePass

[Richard Osborne]

IRBuilder is a templated class, and one of the template arguments is the constant folder to use. By default it uses the ConstantFolder class which does target-independant constant folding. If you want to disable constant folding you can specify the NoFolder class instead, i.e. declare the builder as follows:

IRBuilder<true, llvm::NoFolder> builder(body)

On 26 Nov 2013, at 19:23, Daniel Albuschat <d.albuschat at gmail.com> wrote:

> Hello,
> 
> using the LLVM API, I've build one very simple function that adds two
> ConstantInts and returns the result.
> 
> I noticed that, when I emit IR code, it is optimized to a simple "ret
> i16 42" when I add 40 and 2. I'd like to see the operations that are
> necessary to compute the result, though.
> Can I somehow disable this optimization in the pass, leading to more
> verbose IR code?
> 
> Here is the code I use to create the IR:
> 
>    llvm::LLVMContext c;
>    llvm::Module module("test", c);
>    llvm::Type * functionType = llvm::IntegerType::get(c, 16);
>    llvm::Function * llvmFunction = llvm::cast
> <llvm::Function>(module.getOrInsertFunction("foo", functionType,
> nullptr));
>    llvmFunction->setCallingConv(llvm::CallingConv::C);
>    llvm::BasicBlock * body = llvm::BasicBlock::Create(c, "__entry__",
> llvmFunction);
>    llvm::IRBuilder <> builder(body);
>    llvm::Value * result =
> builder.CreateBinOp(llvm::Instruction::BinaryOps::Add,
> llvm::ConstantInt::getSigned(functionType, 40),
> llvm::ConstantInt::getSigned(functionType, 2));
>    builder.CreateRet(result);
> 
>    llvm::verifyModule(module, llvm::PrintMessageAction);
> 
>    std::string errorInfo;
>    llvm::raw_fd_ostream fileStream("test.ll", errorInfo);
> 
>    llvm::PassManager pm;
>    pm.add(llvm::createPrintModulePass(& fileStream));
>    pm.run(module);
> 
> And here is the result:
> 
>    ; ModuleID = 'test'
> 
>    define i16 @foo() {
>    __entry__:
>      ret i16 42
>    }
> 
> 
> (Somehow I am beginning to get the feeling that operations on literals
> are always evaluated at compile-time and that this can not be
> prevented, except for first storing the values in memory.)
> 
> Greetings,
> 
> Daniel Albuschat
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