[llvm] [InstCombine] Support well-defined recurrences in isGuaranteedNotToBeUndefOrPoison (PR #150420)

[David Sherwood via llvm-commits]

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@@ -7569,6 +7569,66 @@ bool llvm::impliesPoison(const Value *ValAssumedPoison, const Value *V) {
 
 static bool programUndefinedIfUndefOrPoison(const Value *V, bool PoisonOnly);
 
+Use *llvm::canFoldFreezeIntoRecurrence(
+    PHINode *PN, DominatorTree *DT, bool &StartNeedsFreeze,
+    SmallVectorImpl<Instruction *> *DropFlags) {
+  // Detect whether this is a recurrence with a start value and some number of
+  // backedge values. We'll check whether we can push the freeze through the
+  // backedge values (possibly dropping poison flags along the way) until we
+  // reach the phi again. In that case, we can move the freeze to the start
+  // value.
+  Use *StartU = nullptr;
+  SmallVector<Value *> Worklist;
+  for (Use &U : PN->incoming_values()) {
+    if (DT && DT->dominates(PN->getParent(), PN->getIncomingBlock(U))) {
+      // Add backedge value to worklist.
+      Worklist.push_back(U.get());
+      continue;
+    }
+
+    // Don't bother handling multiple start values.
+    if (StartU)
+      return nullptr;
+    StartU = &U;
+  }
+
+  if (!StartU || Worklist.empty())
+    return nullptr; // Not a recurrence.
+
+  Value *StartV = StartU->get();
+  BasicBlock *StartBB = PN->getIncomingBlock(*StartU);
+  StartNeedsFreeze = !isGuaranteedNotToBeUndefOrPoison(StartV);
+  // We can't insert freeze if the start value is the result of the
+  // terminator (e.g. an invoke).
+  if (StartNeedsFreeze && StartBB->getTerminator() == StartV)
+    return nullptr;
+
+  SmallPtrSet<Value *, 32> Visited;
+  while (!Worklist.empty()) {
+    Value *V = Worklist.pop_back_val();
+    if (!Visited.insert(V).second)
+      continue;
+
+    if (Visited.size() > 32)
+      return nullptr; // Limit the total number of values we inspect.
+
+    // Assume that PN is non-poison, because it will be after the transform.
+    if (V == PN || isGuaranteedNotToBeUndefOrPoison(V))
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david-arm wrote:

Given you have access to the DominatorTree you could also pass that in `isGuaranteedNotToBeUndefOrPoison`, right?

https://github.com/llvm/llvm-project/pull/150420