[llvm] [MCP] Enhance MCP copy Instruction removal for special case (PR #70778)

[Matt Arsenault via llvm-commits]

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@@ -735,6 +736,27 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
         MCRegister Def = RegDef.asMCReg();
         MCRegister Src = RegSrc.asMCReg();
 
+        // Target may lost some opportunity to further remove the redundant
+        // copy instruction, consider the following sequence:
+        // L1: r0 = COPY r9     <- TrackMI
+        // L2: r0 = COPY r8     <- TrackMI
+        // L3: use r0           <- Remove L2 from MaybeDeadCopies
+        // L4: early-clobber r9 <- Invalid L2 from Tracker
+        // L5: r0 = COPY r8     <- Miss remove chance
+        // L6: use r0           <- Miss remove L5 chance
+        if (LastMI) {
+          std::optional<DestSourcePair> PrevCopyOperands =
+              isCopyInstr(*LastMI, *TII, UseCopyInstr);
+          if (PrevCopyOperands) {
+            Register PrevRegDef = PrevCopyOperands->Destination->getReg();
+            // We could remove the previous copy from tracker directly.
+            if (TRI->isSubRegisterEq(RegDef, PrevRegDef)) {
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arsenm wrote:

This sub register check is suspicious. The underlying tracker is already operating in terms of regunits, so I don't expect you to need to look for overlapping defs 

https://github.com/llvm/llvm-project/pull/70778