[LoopRotate] Drop loop dispositions when rotating loops. PR56260

This is required because if there is a pure loop-invariant instruction, Loop Rotation
may decide to not clone it and just hoist it instead. If SCEV has previously cached
that it was loop-variant (not being smart enough to prove invariance), we may end
up with inconsistent cache state (which may later trigger false-negative assertion
failures checking that something was invariant).

This is a conservative fix that unconditionally drops the dispositions. We could
only drop it if the hoisting has actually happened, but it should take some time
understanding whether it's safe with all other things this function does.

Differential Revision: https://reviews.llvm.org/D134167
Reviewed By: fhahn

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
index 7bb7cea..da817fb 100644
--- a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
@@ -345,8 +345,12 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
     // all outer loops because insertion and deletion of blocks that happens
     // during the rotation may violate invariants related to backedge taken
     // infos in them.
-    if (SE)
+    if (SE) {
       SE->forgetTopmostLoop(L);
+      // We may hoist some instructions out of loop. In case if they were cached
+      // as "loop variant" or "loop computable", these caches must be dropped.
+      SE->forgetLoopDispositions();
+    }
 
     LLVM_DEBUG(dbgs() << "LoopRotation: rotating "; L->dump());
     if (MSSAU && VerifyMemorySSA)