[CGP] add another bailout for degenerate code (PR41064)

This is almost the same as:
rL355345
...and should prevent any potential crashing from examples like:
https://bugs.llvm.org/show_bug.cgi?id=41064
...although the bug was masked by:
rL355823
...and I'm not sure how to repro the problem after that change.

llvm-svn: 356218

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index e70e4ee0308..e8498eb375e 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -1267,8 +1267,12 @@ static bool combineToUAddWithOverflow(CmpInst *Cmp, const TargetLowering &TLI,
 static bool combineToUSubWithOverflow(CmpInst *Cmp, const TargetLowering &TLI,
                                       const DataLayout &DL, DominatorTree &DT,
                                       bool &ModifiedDT) {
-  // Convert (A u> B) to (A u< B) to simplify pattern matching.
+  // We are not expecting non-canonical/degenerate code. Just bail out.
   Value *A = Cmp->getOperand(0), *B = Cmp->getOperand(1);
+  if (isa<Constant>(A) && isa<Constant>(B))
+    return false;
+
+  // Convert (A u> B) to (A u< B) to simplify pattern matching.
   ICmpInst::Predicate Pred = Cmp->getPredicate();
   if (Pred == ICmpInst::ICMP_UGT) {
     std::swap(A, B);