[AlignmentFromAssumptions] Don't divide by zero for unknown starting alignment

The routine that determines an alignment given some SCEV returns zero if the
answer is unknown. In a case where we could determine the increment of an
AddRec but not the starting alignment, we would compute the integer modulus by
zero (which is illegal and traps). Prevent this by returning early if either
the start or increment alignment is unknown (zero).

llvm-svn: 217544

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp b/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
index a662accf69f..7ab16f13811 100644
--- a/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
+++ b/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
@@ -179,7 +179,9 @@ static unsigned getNewAlignment(const SCEV *AASCEV, const SCEV *AlignSCEV,
     DEBUG(dbgs() << "\tnew start alignment: " << NewAlignment << "\n");
     DEBUG(dbgs() << "\tnew inc alignment: " << NewIncAlignment << "\n");
 
-    if (NewAlignment > NewIncAlignment) {
+    if (!NewAlignment || !NewIncAlignment) {
+      return 0;
+    } else if (NewAlignment > NewIncAlignment) {
       if (NewAlignment % NewIncAlignment == 0) {
         DEBUG(dbgs() << "\tnew start/inc alignment: " <<
                         NewIncAlignment << "\n");
@@ -191,7 +193,7 @@ static unsigned getNewAlignment(const SCEV *AASCEV, const SCEV *AlignSCEV,
                         NewAlignment << "\n");
         return NewAlignment;
       }
-    } else if (NewIncAlignment == NewAlignment && NewIncAlignment) {
+    } else if (NewIncAlignment == NewAlignment) {
       DEBUG(dbgs() << "\tnew start/inc alignment: " <<
                       NewAlignment << "\n");
       return NewAlignment;