[PATCH] D46044: [LoopUnrollPeel] Fix potentially incorrect invalidation of SCEV in peelLoop

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: chandlerc, hfinkel, sanjoy, reames.
Herald added subscribers: javed.absar, zzheng.

Current runtime unrolling invalidates parent loop saying that it might have changed
after the inner loop has changed, but it doesn't bother to do the same to its parents.
With patch https://reviews.llvm.org/rL329047, SCEV becomes much smarter about calculation of exit counts for
outer loops. We might need to invalidate not only the immediate parent, but also
any of its parents as well.

There is no clear evidence that there is some miscompile happening because of this
, but the common sense says that the current code may be wrong.


https://reviews.llvm.org/D46044

Files:
  lib/Transforms/Utils/LoopUnrollPeel.cpp


Index: lib/Transforms/Utils/LoopUnrollPeel.cpp
===================================================================
--- lib/Transforms/Utils/LoopUnrollPeel.cpp
+++ lib/Transforms/Utils/LoopUnrollPeel.cpp
@@ -656,9 +656,10 @@
     LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
   }
 
-  // If the loop is nested, we changed the parent loop, update SE.
+  // If the loop is nested, we could have changed any of its parent loops,
+  // update SE.
   if (Loop *ParentLoop = L->getParentLoop()) {
-    SE->forgetLoop(ParentLoop);
+    SE->forgetTopmostLoop(ParentLoop);
 
     // FIXME: Incrementally update loop-simplify
     simplifyLoop(ParentLoop, DT, LI, SE, AC, PreserveLCSSA);


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