[llvm] [AMDGPU] Do not forward reserved aperture registers into wider copies (PR #220517)

[Matt Arsenault via llvm-commits]

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@@ -879,6 +893,14 @@ void MachineCopyPropagation::forwardUses(MachineInstr &MI) {
     if (MRI->isReserved(CopySrc) && !MRI->isConstantPhysReg(CopySrc))
       continue;
 
+    // A reserved source may lack subregs the copy is later split into.
+    if (MRI->isReserved(CopySrc) && isCopyInstr(MI, *TII, UseCopyInstr) &&
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arsenm wrote:

The reserved part shouldn't matter, it just happens this AMDGPU case involves a reserved register. You may be able to craft a similar case using SGPRs with true16?

https://github.com/llvm/llvm-project/pull/220517