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

[Shilei Tian 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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shiltian 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.

This fix can only fix one case IIUC so the comment needs to be adapted.

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