[PATCH] D128252: [AMDGPU] Lowering VGPR to SGPR copies to v_readfirstlane_b32 if profitable.

[Jay Foad via Phabricator via llvm-commits]

foad added a comment.

In D128252#3648385 <https://reviews.llvm.org/D128252#3648385>, @alex-t wrote:

> In D128252#3644778 <https://reviews.llvm.org/D128252#3644778>, @foad wrote:
>
>> In future I would like VGPR-to-SGPR copies to be legal
>
> They are already legal.

Currently `SIInstrInfo::copyPhysReg` does this:

  if (RI.isSGPRClass(RC)) {
    if (!RI.isSGPRClass(SrcRC)) {
      reportIllegalCopy(this, MBB, MI, DL, DestReg, SrcReg, KillSrc);
      return;
    }

That is why I say they are illegal. I would like to change this, so that copyPhysReg will allow them and implement them by emitting readfirstlane.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D128252/new/

https://reviews.llvm.org/D128252