Be wary of abnormal exits from loop when exploiting UB

We can safely rely on a NoWrap add recurrence causing UB down the road
only if we know the loop does not have a exit expressed in a way that is
opaque to ScalarEvolution (e.g. by a function call that conditionally
calls exit(0)).

I believe with this change PR28012 is fixed.

Note: I had to change some llvm-lit tests in LoopReroll, since it looks
like they were depending on this incorrect behavior.

llvm-svn: 272237

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 9278aa05dcb..867b0515003 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -7199,7 +7199,8 @@ ScalarEvolution::howFarToZero(const SCEV *V, const Loop *L, bool ControlsExit,
   // compute the backedge count.  In this case, the step may not divide the
   // distance, but we don't care because if the condition is "missed" the loop
   // will have undefined behavior due to wrapping.
-  if (ControlsExit && AddRec->hasNoSelfWrap()) {
+  if (ControlsExit && AddRec->hasNoSelfWrap() &&
+      loopHasNoAbnormalExits(AddRec->getLoop())) {
     const SCEV *Exact =
         getUDivExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
     return ExitLimit(Exact, Exact, P);