[llvm] [LLVM] Make s_getpc_b64 rematerializable (PR #71823)

[Jay Foad via llvm-commits]

jayfoad wrote:

> I found two places that can code-generate an S_GETPC_B64, one in function SIInstrInfo::insertIndirectBranch() and one in function SIInstrInfo::expandPostRAPseudo(). I think that both of these are happening after register allocation, so at that point rematerialization can not be done.

Right. Rematerializing getpc is a weird case, because the rematerialized instruction will definitely _not_ have the same result as the original instruction. But I guess the only reasonable use of getpc (pre-RA) is calling the intrinsic llvm.amdgc.s.getpc and then extracting the high 32 bits of the result, which should be safe.

https://github.com/llvm/llvm-project/pull/71823