[SCEV] Fix the movement of insertion point in expander. PR35406.

We cannot move the insertion point to header if SCEV contains div/rem
operations due to they may go over check for zero denominator.

Reviewers: sanjoy, mkazantsev, sebpop
Reviewed By: sebpop
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D41229

llvm-svn: 320789

1 files changed, 19 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
index 0caff18264d..86f714b930d 100644
--- a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -1732,10 +1732,28 @@ Value *SCEVExpander::expand(const SCEV *S) {
         InsertPt = &*L->getHeader()->getFirstInsertionPt();
       }
     } else {
+      // We can move insertion point only if there is no div or rem operations
+      // otherwise we are risky to move it over the check for zero denominator.
+      auto SafeToHoist = [](const SCEV *S) {
+        return !SCEVExprContains(S, [](const SCEV *S) {
+                  if (const auto *D = dyn_cast<SCEVUDivExpr>(S)) {
+                    if (const auto *SC = dyn_cast<SCEVConstant>(D->getRHS()))
+                      // Division by non-zero constants can be hoisted.
+                      return SC->getValue()->isZero();
+                    // All other divisions should not be moved as they may be
+                    // divisions by zero and should be kept within the
+                    // conditions of the surrounding loops that guard their
+                    // execution (see PR35406).
+                    return true;
+                  }
+                  return false;
+                });
+      };
       // If the SCEV is computable at this level, insert it into the header
       // after the PHIs (and after any other instructions that we've inserted
       // there) so that it is guaranteed to dominate any user inside the loop.
-      if (L && SE.hasComputableLoopEvolution(S, L) && !PostIncLoops.count(L))
+      if (L && SE.hasComputableLoopEvolution(S, L) && !PostIncLoops.count(L) &&
+          SafeToHoist(S))
         InsertPt = &*L->getHeader()->getFirstInsertionPt();
       while (InsertPt->getIterator() != Builder.GetInsertPoint() &&
              (isInsertedInstruction(InsertPt) ||