Delete the merge operand instead of the false operand, and add tests for it

Author: lukel97

llvm/lib/Target/RISCV/RISCVFoldMasks.cpp

@@ -130,8 +130,9 @@ bool RISCVFoldMasks::convertVMergeToVMv(MachineInstr &MI, MachineInstr *V0Def) {
     return false;
 
   MI.setDesc(TII->get(NewOpc));
-  MI.removeOperand(2); // False operand
-  MI.removeOperand(3); // Mask operand
+  MI.removeOperand(1);  // Merge operand
+  MI.tieOperands(0, 1); // Tie false to dest
+  MI.removeOperand(3);  // Mask operand
   MI.addOperand(
       MachineOperand::CreateImm(RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED));
 

llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-to-vmv.mir

@@ -0,0 +1,71 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 3
+# RUN: llc %s -o - -mtriple=riscv64 -mattr=+v -run-pass=riscv-fold-masks \
+# RUN:     -verify-machineinstrs | FileCheck %s
+
+---
+name: undef_passthru
+body: |
+  bb.0:
+    liveins: $x1, $v8, $v9
+    ; CHECK-LABEL: name: undef_passthru
+    ; CHECK: liveins: $x1, $v8, $v9
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %false:vr = COPY $v8
+    ; CHECK-NEXT: %true:vr = COPY $v9
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x1
+    ; CHECK-NEXT: %mask:vmv0 = PseudoVMSET_M_B8 %avl, 5 /* e32 */
+    ; CHECK-NEXT: $v0 = COPY %mask
+    ; CHECK-NEXT: %x:vr = PseudoVMV_V_V_M1 %false, %true, %avl, 5 /* e32 */, 0 /* tu, mu */
+    %false:vr = COPY $v8
+    %true:vr = COPY $v9
+    %avl:gprnox0 = COPY $x1
+    %mask:vmv0 = PseudoVMSET_M_B8 %avl, 5
+    $v0 = COPY %mask
+    %x:vrnov0 = PseudoVMERGE_VVM_M1 $noreg, %false, %true, $v0, %avl, 5
+...
+---
+name: undef_false
+body: |
+  bb.0:
+    liveins: $x1, $v8, $v9
+    ; CHECK-LABEL: name: undef_false
+    ; CHECK: liveins: $x1, $v8, $v9
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %pt:vrnov0 = COPY $v8
+    ; CHECK-NEXT: %false:vr = COPY $noreg
+    ; CHECK-NEXT: %true:vr = COPY $v9
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x1
+    ; CHECK-NEXT: %mask:vmv0 = PseudoVMSET_M_B8 %avl, 5 /* e32 */
+    ; CHECK-NEXT: $v0 = COPY %mask
+    ; CHECK-NEXT: %x:vrnov0 = PseudoVMERGE_VVM_M1 %pt, %false, %true, $v0, %avl, 5 /* e32 */
+    %pt:vrnov0 = COPY $v8
+    %false:vr = COPY $noreg
+    %true:vr = COPY $v9
+    %avl:gprnox0 = COPY $x1
+    %mask:vmv0 = PseudoVMSET_M_B8 %avl, 5
+    $v0 = COPY %mask
+    %x:vrnov0 = PseudoVMERGE_VVM_M1 %pt, %false, %true, $v0, %avl, 5
+...
+---
+name: equal_passthru_false
+body: |
+  bb.0:
+    liveins: $x1, $v8, $v9
+    ; CHECK-LABEL: name: equal_passthru_false
+    ; CHECK: liveins: $x1, $v8, $v9
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %false:vr = COPY $v8
+    ; CHECK-NEXT: %pt:vrnov0 = COPY $v8
+    ; CHECK-NEXT: %true:vr = COPY $v9
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x1
+    ; CHECK-NEXT: %mask:vmv0 = PseudoVMSET_M_B8 %avl, 5 /* e32 */
+    ; CHECK-NEXT: $v0 = COPY %mask
+    ; CHECK-NEXT: %x:vr = PseudoVMV_V_V_M1 %false, %true, %avl, 5 /* e32 */, 0 /* tu, mu */
+    %false:vr = COPY $v8
+    %pt:vrnov0 = COPY $v8
+    %true:vr = COPY $v9
+    %avl:gprnox0 = COPY $x1
+    %mask:vmv0 = PseudoVMSET_M_B8 %avl, 5
+    $v0 = COPY %mask
+    %x:vrnov0 = PseudoVMERGE_VVM_M1 %pt, %false, %true, $v0, %avl, 5
+...