llvm dev - Jan 2020 - Adding a clang commandline option to change backend behaviour

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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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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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
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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