[SCEV] Stop applying contextual flags in applyLoopGuards

[reames]

Summary

This fixes a violation of the wrap flag rules introduced in c4048d8f. 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.

[efriedma]

LGTM

[fhahn]

Thanks for fixing this! LGTM

llvm/lib/Analysis/ScalarEvolution.cpp

13499

nit: replacee -> replaced?

llvm/lib/Analysis/ScalarEvolution.cpp

if (I == Map.end())
       return Expr;
     return I->second;
   }
 };
 
 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 structure of that
+    // 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);
     if (Predicate == CmpInst::ICMP_EQ && RHSC &&
         RHSC->getValue()->isNullValue()) {
       // If LHS is A % B, i.e. A % B == 0, rewrite A to (A /u B) * B to
       // explicitly express that.
       const SCEV *URemLHS = nullptr;
       const SCEV *URemRHS = nullptr;
       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;
         }
       }
     }
 
     if (!isa<SCEVUnknown>(LHS) && isa<SCEVUnknown>(RHS)) {
       std::swap(LHS, RHS);
       Predicate = CmpInst::getSwappedPredicate(Predicate);