[RISCV] Fold vmv.v.v into vmerge

We support folding vmv.v.v into its source via the foldVMV_V_V peephole, but currently it requires that the source has a policy operand.

PseudoVMERGEs (as well as add-with-carry/subtract-with-borrow pseudos) don't have a policy operand, and instead just seem to derive TA/TU from whether or not the passthru is undef, since the mask policy doesn't affect them.

This patch allows pseudos without policy operands, given that if there's no policy operand then it will default to TU when the passthru is undefined which should be conservatively correct.

I previously tried adding a policy operand to vmerge/vadc etc., but this ended up being a lot of churn.

This removes a bunch of redundant vmv.v.vs that I noticed with EVL tail folding on llvm-test-suite.

Author: lukel97

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -646,8 +646,7 @@ bool RISCVVectorPeephole::foldVMV_V_V(MachineInstr &MI) {
   if (!Src || Src->hasUnmodeledSideEffects() ||
       Src->getParent() != MI.getParent() ||
       !RISCVII::isFirstDefTiedToFirstUse(Src->getDesc()) ||
-      !RISCVII::hasVLOp(Src->getDesc().TSFlags) ||
-      !RISCVII::hasVecPolicyOp(Src->getDesc().TSFlags))
+      !RISCVII::hasVLOp(Src->getDesc().TSFlags))
     return false;
 
   // Src's dest needs to have the same EEW as MI's input.
@@ -681,12 +680,14 @@ bool RISCVVectorPeephole::foldVMV_V_V(MachineInstr &MI) {
                                               *Src->getParent()->getParent()));
   }
 
-  // If MI was tail agnostic and the VL didn't increase, preserve it.
-  int64_t Policy = RISCVVType::TAIL_UNDISTURBED_MASK_UNDISTURBED;
-  if ((MI.getOperand(5).getImm() & RISCVVType::TAIL_AGNOSTIC) &&
-      RISCV::isVLKnownLE(MI.getOperand(3), SrcVL))
-    Policy |= RISCVVType::TAIL_AGNOSTIC;
-  Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc())).setImm(Policy);
+  if (RISCVII::hasVecPolicyOp(Src->getDesc().TSFlags)) {
+    // If MI was tail agnostic and the VL didn't increase, preserve it.
+    int64_t Policy = RISCVVType::TAIL_UNDISTURBED_MASK_UNDISTURBED;
+    if ((MI.getOperand(5).getImm() & RISCVVType::TAIL_AGNOSTIC) &&
+        RISCV::isVLKnownLE(MI.getOperand(3), SrcVL))
+      Policy |= RISCVVType::TAIL_AGNOSTIC;
+    Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc())).setImm(Policy);
+  }
 
   MRI->constrainRegClass(Src->getOperand(0).getReg(),
                          MRI->getRegClass(MI.getOperand(0).getReg()));

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-insert-subvector.ll

@@ -554,9 +554,8 @@ define <vscale x 2 x i1> @insert_nxv2i1_v4i1_0(<vscale x 2 x i1> %v, ptr %svp) {
 ; VLA-NEXT:    vsetivli zero, 4, e8, mf4, ta, ma
 ; VLA-NEXT:    vmv.v.i v10, 0
 ; VLA-NEXT:    vmv1r.v v0, v8
-; VLA-NEXT:    vmerge.vim v8, v10, 1, v0
 ; VLA-NEXT:    vsetvli zero, zero, e8, mf4, tu, ma
-; VLA-NEXT:    vmv.v.v v9, v8
+; VLA-NEXT:    vmerge.vim v9, v10, 1, v0
 ; VLA-NEXT:    vsetvli a0, zero, e8, mf4, ta, ma
 ; VLA-NEXT:    vmsne.vi v0, v9, 0
 ; VLA-NEXT:    ret
@@ -568,9 +567,8 @@ define <vscale x 2 x i1> @insert_nxv2i1_v4i1_0(<vscale x 2 x i1> %v, ptr %svp) {
 ; VLS-NEXT:    vmv.v.i v9, 0
 ; VLS-NEXT:    vmerge.vim v10, v9, 1, v0
 ; VLS-NEXT:    vmv1r.v v0, v8
-; VLS-NEXT:    vmerge.vim v8, v9, 1, v0
 ; VLS-NEXT:    vsetvli zero, zero, e8, mf4, tu, ma
-; VLS-NEXT:    vmv.v.v v10, v8
+; VLS-NEXT:    vmerge.vim v10, v9, 1, v0
 ; VLS-NEXT:    vsetvli zero, zero, e8, mf4, ta, ma
 ; VLS-NEXT:    vmsne.vi v0, v10, 0
 ; VLS-NEXT:    ret

llvm/test/CodeGen/RISCV/rvv/vmv.v.v-peephole.ll

@@ -227,9 +227,7 @@ define <vscale x 1 x i64> @vmerge(<vscale x 1 x i64> %passthru, <vscale x 1 x i6
 ; CHECK-LABEL: vmerge:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vsetvli zero, a0, e64, m1, tu, ma
-; CHECK-NEXT:    vmv1r.v v11, v8
-; CHECK-NEXT:    vmerge.vvm v11, v9, v10, v0
-; CHECK-NEXT:    vmv.v.v v8, v11
+; CHECK-NEXT:    vmerge.vvm v8, v9, v10, v0
 ; CHECK-NEXT:    ret
   %a = call <vscale x 1 x i64> @llvm.riscv.vmerge.nxv1i64.nxv1i64(<vscale x 1 x i64> %passthru, <vscale x 1 x i64> %x, <vscale x 1 x i64> %y, <vscale x 1 x i1> %m, iXLen %avl)
   %b = call <vscale x 1 x i64> @llvm.riscv.vmv.v.v.nxv1i64(<vscale x 1 x i64> %passthru, <vscale x 1 x i64> %a, iXLen %avl)

llvm/test/CodeGen/RISCV/rvv/vmv.v.v-peephole.mir

@@ -160,10 +160,9 @@ body: |
     ; CHECK-LABEL: name: vmerge_vvm
     ; CHECK: liveins: $v8, $v0
     ; CHECK-NEXT: {{  $}}
-    ; CHECK-NEXT: %passthru:vr = COPY $v8
+    ; CHECK-NEXT: %passthru:vrnov0 = COPY $v8
     ; CHECK-NEXT: %mask:vmv0 = COPY $v0
-    ; CHECK-NEXT: %x:vrnov0 = PseudoVMERGE_VVM_M1 $noreg, %passthru, $noreg, %mask, 4, 5 /* e32 */
-    ; CHECK-NEXT: %z:vr = PseudoVMV_V_V_M1 %passthru, %x, 4, 5 /* e32 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %x:vrnov0 = PseudoVMERGE_VVM_M1 %passthru, %passthru, $noreg, %mask, 4, 5 /* e32 */
     %passthru:vr = COPY $v8
     %mask:vmv0 = COPY $v0
     %x:vrnov0 = PseudoVMERGE_VVM_M1 $noreg, %passthru, $noreg, %mask, 4, 5 /* e32 */