[llvm] [WIP][AMDGPU] Fix emitting illegal COPY (PR #131752)

[Pankaj Dwivedi via llvm-commits]

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@@ -899,6 +899,55 @@ bool SIFixSGPRCopies::lowerSpecialCase(MachineInstr &MI,
                                       MI, MI.getDebugLoc())) {
       I = std::next(I);
       MI.eraseFromParent();
+    } else {
+      // At this point, if we still have a VGPR → SGPR copy, it is completely
+      // illegal. We assume that it was intentionally introduced and should be
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PankajDwivedi-25 wrote:

> Is it safe to assume? For example, https://github.com/llvm/llvm-project/issues/130443 can also introduce VGPR to SGPR copy but that doesn't seem to be the case here

I think it is similar case here as llvm/test/CodeGen/AMDGPU/fix-illegal-copy.ll?
Is it wrong to replace those COPY emitted with readfirstlane here in  llvm/test/CodeGen/AMDGPU/issue130443.ll?

https://github.com/llvm/llvm-project/pull/131752