[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.

Author: asb

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;

llvm/test/CodeGen/RISCV/GlobalISel/constbarrier-rv32.ll

@@ -38,7 +38,6 @@ define void @constant_fold_barrier_i128(ptr %p) {
 ; RV32-NEXT:    seqz a7, a6
 ; RV32-NEXT:    and a1, a7, a1
 ; RV32-NEXT:    add a7, a4, zero
-; RV32-NEXT:    add a5, a5, zero
 ; RV32-NEXT:    sltu a4, a4, a4
 ; RV32-NEXT:    or a1, a3, a1
 ; RV32-NEXT:    add a7, a7, a1

llvm/test/CodeGen/RISCV/GlobalISel/constbarrier-rv64.ll

@@ -26,7 +26,6 @@ define i128 @constant_fold_barrier_i128(i128 %x) {
 ; RV64-NEXT:    and a0, a0, a2
 ; RV64-NEXT:    add a0, a0, a2
 ; RV64-NEXT:    sltu a2, a0, a2
-; RV64-NEXT:    add a1, a1, zero
 ; RV64-NEXT:    add a1, a1, a2
 ; RV64-NEXT:    ret
 entry:

llvm/test/CodeGen/RISCV/machine-copyprop-noop-removal.mir

@@ -3,7 +3,6 @@
 
 ## This test was added to capture a case where MachineCopyPropagation risks
 ## leaving a no-op register move (add, x0, reg).
-## FIXME: No-op register move is left behind after machine-cp.
 
 ---
 name: ham
@@ -22,7 +21,6 @@ body: |
   ; CHECK-NEXT:   liveins: $x10
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   $x11 = ADDI $x0, 0
-  ; CHECK-NEXT:   renamable $x10 = ADD $x0, killed renamable $x10
   ; CHECK-NEXT:   BEQ renamable $x10, $x0, %bb.4
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.2:

llvm/test/CodeGen/RISCV/sextw-removal.ll

@@ -1352,7 +1352,6 @@ define signext i32 @sextw_sh2add(i1 zeroext %0, ptr %1, i32 signext %2, i32 sign
 ; NOREMOVAL-LABEL: sextw_sh2add:
 ; NOREMOVAL:       # %bb.0:
 ; NOREMOVAL-NEXT:    sh2add a2, a2, a3
-; NOREMOVAL-NEXT:    mv a2, a2
 ; NOREMOVAL-NEXT:    beqz a0, .LBB22_2
 ; NOREMOVAL-NEXT:  # %bb.1:
 ; NOREMOVAL-NEXT:    sw a2, 0(a1)