[llvm] [AArch64] Remove superfluous sxtw in peephole opt (PR #96293)

[Sam Tebbs via llvm-commits]

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@@ -690,6 +691,33 @@ bool AArch64MIPeepholeOpt::visitFMOVDr(MachineInstr &MI) {
   return true;
 }
 
+// Acrocss a basic-block we might have in i32 extract from a value that only
+// operates on upper bits (for example a sxtw). We can replace the COPY with a
+// new version skipping the sxtw.
+bool AArch64MIPeepholeOpt::visitCopy(MachineInstr &MI) {
+  if (MI.getOperand(1).getSubReg() != AArch64::sub_32 ||
+      !MRI->hasOneNonDBGUse(MI.getOperand(1).getReg()))
+    return false;
+
+  MachineInstr *SrcMI = MRI->getUniqueVRegDef(MI.getOperand(1).getReg());
+  MachineInstr *CopyMI = SrcMI;
+  if (SrcMI && SrcMI->isFullCopy() &&
+      MRI->hasOneNonDBGUse(SrcMI->getOperand(1).getReg()))
+    SrcMI = MRI->getUniqueVRegDef(SrcMI->getOperand(1).getReg());
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SamTebbs33 wrote:

Could there be more copies in the chain (i.e. `SrcMI` could be a COPY after line 706). A while loop would eventually get to the end of a long copy chain.

https://github.com/llvm/llvm-project/pull/96293