[LoopPred] Fix two subtle issues found by inspection

This patch fixes two issues noticed by inspection when going to enable the loop predication code in IndVarSimplify.

Issue 1 - Both the LoopPredication transform, and the already on by default optimizeLoopExits transform, modify the exit count of the exits they modify. (either to 0 or Infinity) Looking at the code more closely, this was not reflected into SCEV and we were instead running later transforms with incorrect SCEVs. Fixing this requires forgetting the loop, weakening a too strong assert, and updating SCEV to not pessimize results when a loop is provable untaken. I haven't been able to find a test case to demonstrate the miscompile.

Issue 2 - For modules without a data layout, we can end up with unsized pointer typed exit counts. Just bail out of this case.

I think these are the last two issues which need addressed before we enable this by default. The code has already survived a decent amount of fuzzing without revealing either of the above.

Differential Revision: https://reviews.llvm.org/D69695

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index e4295f64dca..ebdaf4eda07 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -7075,6 +7075,17 @@ ScalarEvolution::computeBackedgeTakenCount(const Loop *L,
   // Do a union of all the predicates here.
   for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
     BasicBlock *ExitBB = ExitingBlocks[i];
+
+    // We canonicalize untaken exits to br (constant), ignore them so that
+    // proving an exit untaken doesn't negatively impact our ability to reason
+    // about the loop as whole.
+    if (auto *BI = dyn_cast<BranchInst>(ExitBB->getTerminator()))
+      if (auto *CI = dyn_cast<ConstantInt>(BI->getCondition())) {
+        bool ExitIfTrue = !L->contains(BI->getSuccessor(0));
+        if ((ExitIfTrue && CI->isZero()) || (!ExitIfTrue && CI->isOne()))
+          continue;
+      }
+
     ExitLimit EL = computeExitLimit(L, ExitBB, AllowPredicates);
 
     assert((AllowPredicates || EL.Predicates.empty()) &&