[PowerPC] support inline asm Y constraint

[nickdesaulniers]

The PPC port of the Linux kernel is looking to use an undocumented constraint Y in inline asm.

https://lore.kernel.org/linuxppc-dev/20240521123919.245886-1-mpe@ellerman.id.au/

We should add support for this to LLVM.

[llvmbot]

@llvm/issue-subscribers-backend-powerpc

Author: Nick Desaulniers (paternity leave) (nickdesaulniers)

The PPC port of the Linux kernel is looking to use an undocumented constraint `Y` in inline asm.

https://lore.kernel.org/linuxppc-dev/20240521123919.245886-1-mpe@ellerman.id.au/

We should add support for this to LLVM.

[chenzheng1030]

Where can I get an explanation for constraint Y? Is Y a PPC specific constraint. I can not find it in GCC manual or IBM XLC manual. Thanks.

[mpe]

It's only documented internally in GCC as A memory operand for a DS-form instruction.

There's a GCC bug to document it publicly, which has some more commentary.