[llvm] [InstCombine] Push freeze through non-recurrence PHIs (PR #157678)

[Cullen Rhodes via llvm-commits]

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@@ -5042,10 +5042,33 @@ InstCombinerImpl::pushFreezeToPreventPoisonFromPropagating(FreezeInst &OrigFI) {
   //   Op1.fr = Freeze(Op1)
   //   ... = Inst(Op1.fr, NonPoisonOps...)
 
-  auto CanPushFreeze = [](Value *V) {
-    if (!isa<Instruction>(V) || isa<PHINode>(V))
+  auto CanPushFreeze = [this](Value *V) {
+    if (!isa<Instruction>(V))
       return false;
 
+    if (auto *PN = dyn_cast<PHINode>(V)) {
+      BasicBlock *BB = PN->getParent();
+      SmallPtrSet<BasicBlock *, 8> VisitedBBs;
+      for (Use &U : PN->incoming_values()) {
+        BasicBlock *InBB = PN->getIncomingBlock(U);
+        // We can't move freeze if the start value is the result of a
+        // terminator (e.g. an invoke).
+        if (auto *OpI = dyn_cast<Instruction>(U)) {
+          if (OpI->isTerminator())
+            return false;
+        }
+
+        // If there's multiple incoming edges from the same predecessor we must
+        // ensure the freeze isn't pushed to a single one of the uses,
+        // invalidating the iterator. We simply don't support this case, but it
+        // could be handled if there's a use case.
+        if (isBackEdge(InBB, BB) || !VisitedBBs.insert(InBB).second ||
+            match(U.get(), m_Undef()))
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c-rhodes wrote:

without this check freeze is pushed to `%x` in this test: https://github.com/llvm/llvm-project/blob/8f905c3e6f022ec6255d1e537eedcdbc029bf99d/llvm/test/Transforms/InstCombine/freeze-phi.ll#L116, which I don't think is right

https://github.com/llvm/llvm-project/pull/157678