[llvm-dev] RFC: To add __attribute__((regmask("preserve/clobbered list here"))) in clang

[vivek pandya via llvm-dev]

Hello Clang and LLVM Devs,

I have been working to add support for an attribute in clang and LLVM that
helps
user to guide interprocedural register allocation. But the use case I am
having
is very limited and thus I belieave it is good to have discussion on this
before
sending a patch.

So for IPRA we have a situation where a function is calling a function
which is
written in assembly and it is not defined in current module. IPRA's scope is
limited to a module so for such externally defined function it uses default
calling convention but here as the function is written in assembly user can
provide exact register usage detials. So we dicided to mark declration of
such
function with __attribute__((regmask("clobbered list here"))) so LLVM can
construct regmask out of it and use it with IPRA to improve register
allocation.

For this purpose I added support for this attribute in clang and clang
codegen
this attribute as target dependent attribute and add to declaration. Then
IPRA
constructs regmask from this and use during the optimization.

How ever after disussion on IRC with Joerg Sonnenberger and jroelofs we
think
that this may effect other areas too. So it would be better to discuss this
and
implement this as we all agree upon it.

Sincerely,
Vivek
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