[RISCV] Preserve tail agnostic policy in foldVMV_V_V

This patch helps avoid regressions in an upcoming patch by making sure we don't accidentally lose a tail agnostic policy when folding a vmv.v.v into its source.

The previous comment about RISCVInsertVSETVLI relaxing the policy didn't take into account the fact that there's a policy operand on vmv.v.v, which can be tail agnostic.

If the tail is agnostic (via either the policy operand or the passthru being undef) and vmv.v.v's VL <= Src's VL, then Src's tail can be made agnostic.

Author: lukel97

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -529,10 +529,13 @@ bool RISCVVectorPeephole::foldVMV_V_V(MachineInstr &MI) {
                                               *Src->getParent()->getParent()));
   }
 
-  // Use a conservative tu,mu policy, RISCVInsertVSETVLI will relax it if
-  // passthru is undef.
-  Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc()))
-      .setImm(RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED);
+  // If MI was tail agnostic and the VL didn't increase, preserve it.
+  int64_t Policy = RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED;
+  bool TailAgnostic = (MI.getOperand(5).getImm() & RISCVII::TAIL_AGNOSTIC) ||
+                      Passthru.getReg() == RISCV::NoRegister;
+  if (TailAgnostic && isVLKnownLE(MI.getOperand(3), SrcVL))
+    Policy |= RISCVII::TAIL_AGNOSTIC;
+  Src->getOperand(RISCVII::getVecPolicyOpNum(Src->getDesc())).setImm(Policy);
 
   MRI->replaceRegWith(MI.getOperand(0).getReg(), Src->getOperand(0).getReg());
   MI.eraseFromParent();

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

@@ -27,7 +27,7 @@ body: |
     ; CHECK: liveins: $v8
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: %passthru:vr = COPY $v8
-    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 1 /* ta, mu */
     %passthru:vr = COPY $v8
     %x:vr = PseudoVADD_VV_M1 %passthru, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
     %y:vr = PseudoVMV_V_V_M1 %passthru, %x, 4, 5 /* e32 */, 1 /* ta, mu */
@@ -55,7 +55,7 @@ body: |
     ; CHECK: liveins: $v8
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: %passthru:vr = COPY $v8
-    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 4, 5 /* e32 */, 1 /* ta, mu */
     %passthru:vr = COPY $v8
     %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 4, 5 /* e32 */, 0 /* tu, mu */
     %y:vr = PseudoVMV_V_V_M1 $noreg, %x, 4, 5 /* e32 */, 0 /* tu, mu */