[llvm-dev] Adding a clang commandline option to change backend behaviour

[Aaron Smith via llvm-dev]

Define your option in the backend and declare it extern in the other
files you want to use it in.

// X86TargetMachine.cpp
cl::opt<bool>
    MyOption("my new option", cl::init(true), cl::Hidden,
                    cl::desc("Enable my new option"));

// X86SomeOtherFile.cpp
extern cl::opt<bool> MyOption;
if (MyOption) {
 // do something
}

On Mon, Jan 6, 2020 at 1:30 PM Craig Topper via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> CommandFlags.inc is only included by llc and opt. I think it mostly just sets things on TargetMachine and TargetOptions and connects them to command line options. Clang has its own code for setting up TargetMachine and TargetOptions.
>
> I think a lot of configuration things these days tend to be done with function attributes in IR. You can just query the function attribute wherever without any need to have a centralized option. They also probably works better with LTO since the option is stored for each function so different source files can have different settings.
>
> ~Craig
>
>
> On Mon, Jan 6, 2020 at 1:12 PM David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> Mostly I've only worked with debug info - so anything that I need passed down to the backend goes into MCOptions, MCAsmInfo, and things like that.
>>
>> If you want to affect the way LLVM middle end optimization passes behave, that may require a different channel (might find some options on the LLVMContext? Maybe, but maybe not - perhaps those sort of parameters get passed straight to the passes in some way)
>>
>> On Mon, Jan 6, 2020 at 12:20 PM Oskar Szakinnis via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>
>>> Hello everyone,
>>>
>>> I'm currently working on a project that required me to fundamentally modify certain mechanisms of the x86 backend, as well as certain functions within PrologEpilogInserter (so my changes are not confined to a single pass - e.g. they include changes to the x86RegisterInfo constructor).
>>> In my current solution, I have added an option within llvm/include/llvm/CodeGen/CommandFlags.inc to enable my modifications (the modifications are placed within if-conditions in multiple segments of the code).
>>> This requires me to compile test code in two steps (first run clang with -emit-llvm, then run llc with my option enabled in the command line).
>>>
>>> Now I would like to be able to enable my modification through a clang command line option. What would be the canonical way to do so and which steps do I have to take? I already looked into the dev guide, but perhaps I missed something.
>>>
>>> I already tried using -mllvm as a workaround, but for some reason my option is not being recognized. Ultimately, I'd prefer not to rely on -mllvm anyway.
>>>
>>> To sum up my question: I want to be able to pass a boolean command line parameter to clang and use it within certain conditional statements within multiple files of the backend.
>>>
>>> Thank you for your help and let me know if my intent is unclear.
>>>
>>> Oskar
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
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