================
@@ -784,18 +813,33 @@ void
MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
// %xmm2 = copy %xmm0
// ...
// %xmm2 = copy %xmm9
- Tracker.clobberRegister(Def, *TRI, *TII, UseCopyInstr);
+
+ // While we do need to clobber the register here, simply clobbering it
+ // is not sufficient. We also need to remove the COPY record pair for
+ // 'Def' in the tracker. Failing to do so might cause the target to
miss
+ // some opportunities to eliminate redundant copy instructions.
+
+ // 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
----------------
qcolombet wrote:
Rewrite this comment to explain what will happen now.
Not what was happening before this patch.
https://github.com/llvm/llvm-project/pull/70778
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