[SCEV] Check correct value for UB (#124302)

This is a followup to #117152. That patch introduced a check for
UB/poison on BEValue. However, the SCEV we're actually going to use is
Shifted. In some cases, it's possible for Shifted to contain UB, while
BEValue doesn't.

In the test case the values are:

BEValue: (-1 * (zext i8 (-83 + ((-83 /u {1,+,1}<%loop>) *
{-1,+,-1}<%loop>)) to i32))<nuw><nsw>
Shifted: (-173 + (-1 * (zext i8 ((-83 /u {0,+,1}<%loop>) *
{0,+,-1}<%loop>) to i32))<nuw><nsw>)<nuw><nsw>

Fixes https://github.com/llvm/llvm-project/issues/123550.

1 files changed, 12 insertions, 14 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 7d7d37b..2ce4087 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5917,20 +5917,18 @@ const SCEV *ScalarEvolution::createAddRecFromPHI(PHINode *PN) {
     //   PHI(f(0), f({1,+,1})) --> f({0,+,1})
 
     // Do not allow refinement in rewriting of BEValue.
-    if (isGuaranteedNotToCauseUB(BEValue)) {
-      const SCEV *Shifted = SCEVShiftRewriter::rewrite(BEValue, L, *this);
-      const SCEV *Start = SCEVInitRewriter::rewrite(Shifted, L, *this, false);
-      if (Shifted != getCouldNotCompute() && Start != getCouldNotCompute() &&
-          ::impliesPoison(BEValue, Start)) {
-        const SCEV *StartVal = getSCEV(StartValueV);
-        if (Start == StartVal) {
-          // Okay, for the entire analysis of this edge we assumed the PHI
-          // to be symbolic.  We now need to go back and purge all of the
-          // entries for the scalars that use the symbolic expression.
-          forgetMemoizedResults(SymbolicName);
-          insertValueToMap(PN, Shifted);
-          return Shifted;
-        }
+    const SCEV *Shifted = SCEVShiftRewriter::rewrite(BEValue, L, *this);
+    const SCEV *Start = SCEVInitRewriter::rewrite(Shifted, L, *this, false);
+    if (Shifted != getCouldNotCompute() && Start != getCouldNotCompute() &&
+        isGuaranteedNotToCauseUB(Shifted) && ::impliesPoison(Shifted, Start)) {
+      const SCEV *StartVal = getSCEV(StartValueV);
+      if (Start == StartVal) {
+        // Okay, for the entire analysis of this edge we assumed the PHI
+        // to be symbolic.  We now need to go back and purge all of the
+        // entries for the scalars that use the symbolic expression.
+        forgetMemoizedResults(SymbolicName);
+        insertValueToMap(PN, Shifted);
+        return Shifted;
       }
     }
   }