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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

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


---
Full diff: https://github.com/llvm/llvm-project/pull/148181.diff


2 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp (+10-3) 
- (modified) llvm/test/CodeGen/RISCV/rvv/vmerge-peephole.mir (+39) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp b/llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp
index 07907298386c3..84ef53985484f 100644
--- a/llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp
+++ b/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;
diff --git a/llvm/test/CodeGen/RISCV/rvv/vmerge-peephole.mir b/llvm/test/CodeGen/RISCV/rvv/vmerge-peephole.mir
index 03204468dc14c..0b95e558d8236 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vmerge-peephole.mir
+++ b/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 */
+...

``````````

</details>


https://github.com/llvm/llvm-project/pull/148181