[MachineCopyPropagation] Recognise and delete no-op moves produced after forwarded uses (#129889)

This change removes 189 static instances of no-op reg-reg moves (i.e.
where src == dest) across llvm-test-suite when compiled for RISC-V
rv64gc and with SPEC included.

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 1105b8c..5102639 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -971,6 +971,19 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
 
     forwardUses(MI);
 
+    // It's possible that the previous transformation has resulted in a no-op
+    // register move (i.e. one where source and destination registers are the
+    // same and are not referring to a reserved register). If so, delete it.
+    CopyOperands = isCopyInstr(MI, *TII, UseCopyInstr);
+    if (CopyOperands &&
+        CopyOperands->Source->getReg() == CopyOperands->Destination->getReg() &&
+        !MRI->isReserved(CopyOperands->Source->getReg())) {
+      MI.eraseFromParent();
+      NumDeletes++;
+      Changed = true;
+      continue;
+    }
+
     // Not a copy.
     SmallVector<Register, 4> Defs;
     const MachineOperand *RegMask = nullptr;