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

[Craig Topper via llvm-dev]

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
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>> llvm-dev at lists.llvm.org
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>>
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