[LLVMdev] Disabling optimizations when using llvm::createPrintModulePass

[Daniel Albuschat]

Nice, the output is now:

; ModuleID = 'test'

define i16 @foo() {
__entry__:
  %0 = add i16 40, 2
  ret i16 %0
}

Thanks, Richard.

2013/11/28 Richard Osborne <richard at xmos.com>:
> 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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>