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

[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
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qcolombet wrote:

Shouldn't we forget about L1 when we get to L2?

I think the underlying issue is that when we reach L4, L2 shouldn't be in the tracker copy anyway.

Now, regarding the fix itself, just glancing at the code, I believe it is not general enough. For instance, what happens if I have an unrelated `COPY` between L1 and L2?

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