teach SCEV analysis of PHI nodes that PHI recurences formed

with GEP instructions are always NUW, because PHIs cannot wrap
the end of the address space.

llvm-svn: 123105

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index fa828acc5be..b675e286bad 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2719,6 +2719,11 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
                   HasNUW = true;
                 if (OBO->hasNoSignedWrap())
                   HasNSW = true;
+              } else if (isa<GEPOperator>(BEValueV)) {
+                // If the increment is a GEP, then we know it won't perform an
+                // unsigned overflow, because the address space cannot be
+                // wrapped around.
+                HasNUW = true;
               }
 
               const SCEV *StartVal = getSCEV(StartValueV);