[llvm] [GISel][CombinerHelper] Push freeze through non-poison-producing operands (PR #90618)

[Matt Arsenault via llvm-commits]

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@@ -223,6 +223,64 @@ void CombinerHelper::applyCombineCopy(MachineInstr &MI) {
   replaceRegWith(MRI, DstReg, SrcReg);
 }
 
+bool CombinerHelper::matchFreezeOfSingleMaybePoisonOperand(
+    MachineInstr &MI, BuildFnTy &MatchInfo) {
+  // Ported from InstCombinerImpl::pushFreezeToPreventPoisonFromPropagating
+  Register DstOp = MI.getOperand(0).getReg();
+  Register OrigOp = MI.getOperand(1).getReg();
+
+  if (OrigOp.isPhysical() || !MRI.hasOneNonDBGUse(OrigOp))
+    return false;
+
+  MachineInstr *OrigDef = MRI.getUniqueVRegDef(OrigOp);
+  // Avoid trying to fold G_PHI, G_UNMERGE_VALUES, G_FREEZE (the latter is
+  // handled by idempotent_prop).
+  if (!OrigDef || OrigDef->isPHI() || isa<GUnmerge>(OrigDef) ||
+      isa<GFreeze>(OrigDef))
+    return false;
+
+  if (canCreateUndefOrPoison(OrigOp, MRI,
+                             /*ConsiderFlagsAndMetadata*/ false))
+    return false;
+
+  std::optional<MachineOperand> MaybePoisonOperand;
+  for (MachineOperand &Operand : OrigDef->uses()) {
+    // Avoid working on non-register operands.
+    if (!Operand.isReg())
+      return false;
+
+    if (isGuaranteedNotToBeUndefOrPoison(Operand.getReg(), MRI))
+      continue;
+
+    if (!MaybePoisonOperand)
+      MaybePoisonOperand = Operand;
+    // We have more than one maybe-poison operand. Moving the freeze is unsafe.
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arsenm wrote:

Weird comment placement, use braces and put after the else? 

https://github.com/llvm/llvm-project/pull/90618