[InstCombine] Fix infinite loop due to bitcast <-> phi transforms

Fix for https://bugs.llvm.org/show_bug.cgi?id=44245.

The optimizeBitCastFromPhi() and FoldPHIArgOpIntoPHI() end up
fighting against each other, because optimizeBitCastFromPhi()
assumes that bitcasts of loads will get folded. This doesn't
happen here, because a dangling phi node prevents the one-use
fold in https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp#L620-L628 from triggering.

This patch fixes the issue by explicitly performing the load
combine as part of the bitcast of phi transform. Other attempts
to force the load to be combined first were ultimately too
unreliable.

Differential Revision: https://reviews.llvm.org/D71164

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index f6c7bb5d447..71b7f279e5f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -2319,9 +2319,14 @@ Instruction *InstCombiner::optimizeBitCastFromPhi(CastInst &CI, PHINode *PN) {
       if (auto *C = dyn_cast<Constant>(V)) {
         NewV = ConstantExpr::getBitCast(C, DestTy);
       } else if (auto *LI = dyn_cast<LoadInst>(V)) {
-        Builder.SetInsertPoint(LI->getNextNode());
-        NewV = Builder.CreateBitCast(LI, DestTy);
-        Worklist.Add(LI);
+        // Explicitly perform load combine to make sure no opposing transform
+        // can remove the bitcast in the meantime and trigger an infinite loop.
+        Builder.SetInsertPoint(LI);
+        NewV = combineLoadToNewType(*LI, DestTy);
+        // Remove the old load and its use in the old phi, which itself becomes
+        // dead once the whole transform finishes.
+        replaceInstUsesWith(*LI, UndefValue::get(LI->getType()));
+        eraseInstFromFunction(*LI);
       } else if (auto *BCI = dyn_cast<BitCastInst>(V)) {
         NewV = BCI->getOperand(0);
       } else if (auto *PrevPN = dyn_cast<PHINode>(V)) {