[llvm] AMDGPU: Handle rewriting VGPR MFMA fed from AGPR copy (PR #153022)

[Jeffrey Byrnes via llvm-commits]

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@@ -154,6 +186,88 @@ bool AMDGPURewriteAGPRCopyMFMAImpl::recomputeRegClassExceptRewritable(
   return true;
 }
 
+bool AMDGPURewriteAGPRCopyMFMAImpl::tryReassigningMFMAChain(
+    MachineInstr &MFMA, unsigned HintOpIdx, MCPhysReg PhysRegHint) const {
+  // src2 and dst have the same physical class constraint; try to preserve
+  // the original src2 subclass if one were to exist.
+  SmallVector<MachineInstr *, 4> RewriteCandidates = {&MFMA};
+  SmallSetVector<Register, 4> RewriteRegs;
+
+  Register MFMAHintReg = MFMA.getOperand(HintOpIdx).getReg();
+  // Make sure we reassign the MFMA we found the copy from first. We want
+  // to ensure dst ends up in the physreg we were originally copying to.
+  RewriteRegs.insert(MFMAHintReg);
+
+  // We've found av = COPY (MFMA), and need to verify that we can trivially
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jrbyrnes wrote:

Stale comment?

https://github.com/llvm/llvm-project/pull/153022