[llvm] 0dd755f - [SCEV] Stop applying contextual flags in applyLoopGuards

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2021-09-14T14:14:52-07:00
New Revision: 0dd755f0274245e320cd3eb0882ffe57542af252

URL: https://github.com/llvm/llvm-project/commit/0dd755f0274245e320cd3eb0882ffe57542af252
DIFF: https://github.com/llvm/llvm-project/commit/0dd755f0274245e320cd3eb0882ffe57542af252.diff

LOG: [SCEV] Stop applying contextual flags in applyLoopGuards

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.

Differential Revision: https://reviews.llvm.org/D109782

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp

Removed: 
    


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diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 800b9ee1a7cf..60912adfd5a2 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13495,6 +13495,11 @@ class SCEVLoopGuardRewriter : public SCEVRewriteVisitor<SCEVLoopGuardRewriter> {
 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);
@@ -13507,10 +13512,7 @@ const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
       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;
         }
       }