[PATCH] D109782: [SCEV] Stop applying contextual flags in applyLoopGuards

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: mkazantsev, fhahn, efriedma.
Herald added subscribers: bollu, hiraditya, mcrosier.
reames requested review of this revision.
Herald added a project: LLVM.

This fixes a violation of the wrap flag rules introduced in c4048d8f <https://reviews.llvm.org/rGc4048d8f50aaf2c4c13b8d3e138abc34a22da754>.  As noted in the original review, the NUW is legal to infer from the structure of the replacee, but a) there's no test coverage, and b) this should be done generically for all multiplies.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D109782

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13495,6 +13495,11 @@
 const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
   auto CollectCondition = [&](ICmpInst::Predicate Predicate, const SCEV *LHS,
                               const SCEV *RHS, ValueToSCEVMapTy &RewriteMap) {
+    // WARNING: It is generally unsound to apply any wrap flags to the proposed
+    // replacement SCEV which isn't directly implied by the replacee SCEV.  In
+    // particular, using contextual facts to imply flags is *NOT* legal.  See
+    // the scoping rules for flags in the header to understand why.
+
     // If we have LHS == 0, check if LHS is computing a property of some unknown
     // SCEV %v which we can rewrite %v to express explicitly.
     const SCEVConstant *RHSC = dyn_cast<SCEVConstant>(RHS);
@@ -13507,10 +13512,7 @@
       if (matchURem(LHS, URemLHS, URemRHS)) {
         if (const SCEVUnknown *LHSUnknown = dyn_cast<SCEVUnknown>(URemLHS)) {
           Value *V = LHSUnknown->getValue();
-          auto Multiple =
-              getMulExpr(getUDivExpr(URemLHS, URemRHS), URemRHS,
-                         (SCEV::NoWrapFlags)(SCEV::FlagNUW | SCEV::FlagNSW));
-          RewriteMap[V] = Multiple;
+          RewriteMap[V] = getMulExpr(getUDivExpr(URemLHS, URemRHS), URemRHS);
           return;
         }
       }


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