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

[Craig Topper via llvm-dev]

I don't think I can help make the attribute work without know what you're
changing in the X86RegisterInfo constructor?

~Craig


On Thu, Jan 9, 2020 at 1:19 PM Oskar Szakinnis <oskar at szakinnis.de> wrote:

> Thank you all for your suggestions!
>
> @David, thanks for the advice, I'll check MCOptions again and look into
> LLVMContext.
> @Craig, @Aaron, great suggestion, moving the option away from
> CommandFlags.inc to a more appropriate file and adding an extern
> declaration to the corresponding header gives me more flexibility and
> allows me to invoke the option by using -mllvm. From my current
> understanding, adding a function attribute would not be sufficient, since I
> need to query the option unrelated to functions, e.g. as early as when the
> X86RegisterInfo constructor is called. As of now, I'm not aware of any
> mechanism within the codebase that would allow me to do so, but perhaps you
> can point out something.
>
> Thanks again
> Oskar
>
> > Aaron Smith <aaron.lee.smith at gmail.com> hat am 6. Januar 2020 um 23:06
> geschrieben:
> >
> >
> > 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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> > >
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>
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