Automerge: [RISCV] Handle implicit defs when ensuring pseudo dominates in peephole (#148181)

Previously we just assumed that no instruction that needed to be moved
would have an implicit def, but vnclip pseudos will.

We can still try to move them but we just need to check that no
instructions between have any reads or writes to the physical register.

Fixes #147986

Author: lukel97

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -521,16 +521,23 @@ bool RISCVVectorPeephole::convertToUnmasked(MachineInstr &MI) const {
 /// Check if it's safe to move From down to To, checking that no physical
 /// registers are clobbered.
 static bool isSafeToMove(const MachineInstr &From, const MachineInstr &To) {
-  assert(From.getParent() == To.getParent() && !From.hasImplicitDef());
-  SmallVector<Register> PhysUses;
+  assert(From.getParent() == To.getParent());
+  SmallVector<Register> PhysUses, PhysDefs;
   for (const MachineOperand &MO : From.all_uses())
     if (MO.getReg().isPhysical())
       PhysUses.push_back(MO.getReg());
+  for (const MachineOperand &MO : From.all_defs())
+    if (MO.getReg().isPhysical())
+      PhysDefs.push_back(MO.getReg());
   bool SawStore = false;
-  for (auto II = From.getIterator(); II != To.getIterator(); II++) {
+  for (auto II = std::next(From.getIterator()); II != To.getIterator(); II++) {
     for (Register PhysReg : PhysUses)
       if (II->definesRegister(PhysReg, nullptr))
         return false;
+    for (Register PhysReg : PhysDefs)
+      if (II->definesRegister(PhysReg, nullptr) ||
+          II->readsRegister(PhysReg, nullptr))
+        return false;
     if (II->mayStore()) {
       SawStore = true;
       break;

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

@@ -55,3 +55,42 @@ body: |
     %mask:vmv0 = COPY $v0
     %y:vrnov0 = PseudoVMERGE_VVM_M1 %passthru, %passthru, %x, %mask, %avl, 5 /* e32 */
 ...
+---
+name: vnclip_move_past_passthru
+body: |
+  bb.0:
+    liveins: $x8, $v0, $v8
+    ; CHECK-LABEL: name: vnclip_move_past_passthru
+    ; CHECK: liveins: $x8, $v0, $v8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x8
+    ; CHECK-NEXT: %passthru:vrnov0 = COPY $v8
+    ; CHECK-NEXT: %mask:vmv0 = COPY $v0
+    ; CHECK-NEXT: %y:vrnov0 = PseudoVNCLIPU_WV_MF2_MASK %passthru, $noreg, $noreg, %mask, 0, %avl, 5 /* e32 */, 0 /* tu, mu */, implicit-def $vxsat
+    %avl:gprnox0 = COPY $x8
+    %x:vr = PseudoVNCLIPU_WV_MF2 $noreg, $noreg, $noreg, 0, -1, 5, 3, implicit-def $vxsat
+    %passthru:vrnov0 = COPY $v8
+    %mask:vmv0 = COPY $v0
+    %y:vrnov0 = PseudoVMERGE_VVM_M1 %passthru, %passthru, %x, %mask, %avl, 5 /* e32 */
+...
+---
+name: vnclip_cant_move_past_passthru
+body: |
+  bb.0:
+    liveins: $x8, $v0, $v8
+    ; CHECK-LABEL: name: vnclip_cant_move_past_passthru
+    ; CHECK: liveins: $x8, $v0, $v8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x8
+    ; CHECK-NEXT: %x:vr = PseudoVNCLIPU_WV_MF2 $noreg, $noreg, $noreg, 0, -1, 5 /* e32 */, 3 /* ta, ma */, implicit-def $vxsat
+    ; CHECK-NEXT: %vxsat:gpr = COPY $vxsat
+    ; CHECK-NEXT: %passthru:vrnov0 = COPY $v8
+    ; CHECK-NEXT: %mask:vmv0 = COPY $v0
+    ; CHECK-NEXT: %y:vrnov0 = PseudoVMERGE_VVM_M1 %passthru, %passthru, %x, %mask, %avl, 5 /* e32 */
+    %avl:gprnox0 = COPY $x8
+    %x:vr = PseudoVNCLIPU_WV_MF2 $noreg, $noreg, $noreg, 0, -1, 5, 3, implicit-def $vxsat
+    %vxsat:gpr = COPY $vxsat
+    %passthru:vrnov0 = COPY $v8
+    %mask:vmv0 = COPY $v0
+    %y:vrnov0 = PseudoVMERGE_VVM_M1 %passthru, %passthru, %x, %mask, %avl, 5 /* e32 */
+...