[LCSSA] Forget values we create LCSSA phis for

Summary:
Currently we only forget the loop we added LCSSA phis for. But SCEV
expressions in other loops could also depend on the instruction we added
a PHI for and currently we do not invalidate those expressions. This can
happen when we use ScalarEvolution before converting a function to LCSSA
form. The SCEV expressions will refer to the non-LCSSA value. If this
SCEV expression is then used with the expander, we do not preserve LCSSA
form.

This patch properly forgets the values we created PHIs for. Those need
to be recomputed again. This patch fixes PR43458.

Currently SCEV::verify does not catch this mismatch and any test would
need to run multiple passes to trigger the error (e.g. -loop-reduce
-loop-unroll). I will also look into catching this kind of mismatch in
the verifier. Also, we currently forget the whole loop in LCSSA and I'll
check if we can be more surgical.

Reviewers: efriedma, sanjoy.google, reames

Reviewed By: efriedma

Subscribers: zzheng, hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68194

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/LCSSA.cpp b/llvm/lib/Transforms/Utils/LCSSA.cpp
index 29e7c5260f4..b8a9ae2a498 100644
--- a/llvm/lib/Transforms/Utils/LCSSA.cpp
+++ b/llvm/lib/Transforms/Utils/LCSSA.cpp
@@ -74,7 +74,8 @@ static bool isExitBlock(BasicBlock *BB,
 /// that are outside the current loop.  If so, insert LCSSA PHI nodes and
 /// rewrite the uses.
 bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
-                                    DominatorTree &DT, LoopInfo &LI) {
+                                    DominatorTree &DT, LoopInfo &LI,
+                                    ScalarEvolution *SE) {
   SmallVector<Use *, 16> UsesToRewrite;
   SmallSetVector<PHINode *, 16> PHIsToRemove;
   PredIteratorCache PredCache;
@@ -134,6 +135,11 @@ bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
     SSAUpdater SSAUpdate(&InsertedPHIs);
     SSAUpdate.Initialize(I->getType(), I->getName());
 
+    // Force re-computation of I, as some users now need to use the new PHI
+    // node.
+    if (SE)
+      SE->forgetValue(I);
+
     // Insert the LCSSA phi's into all of the exit blocks dominated by the
     // value, and add them to the Phi's map.
     for (BasicBlock *ExitBB : ExitBlocks) {
@@ -368,7 +374,7 @@ bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
       Worklist.push_back(&I);
     }
   }
-  Changed = formLCSSAForInstructions(Worklist, DT, *LI);
+  Changed = formLCSSAForInstructions(Worklist, DT, *LI, SE);
 
   // If we modified the code, remove any caches about the loop from SCEV to
   // avoid dangling entries.