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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 6 13:30:02 PST 2020

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

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.


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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