[SCEV] Do not preserve samesign after truncation

dtcxzyw · dtcxzyw · commit 1387a685962d · 2025-03-31T20:26:23.000+08:00
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11804,8 +11804,10 @@ bool ScalarEvolution::isImpliedCond(CmpPredicate Pred, const SCEV *LHS,
                                           MaxValue)) {
         const SCEV *TruncFoundLHS = getTruncateExpr(FoundLHS, NarrowType);
         const SCEV *TruncFoundRHS = getTruncateExpr(FoundRHS, NarrowType);
-        if (isImpliedCondBalancedTypes(Pred, LHS, RHS, FoundPred, TruncFoundLHS,
-                                       TruncFoundRHS, CtxI))
+        // We cannot preserve samesign after truncation.
+        if (isImpliedCondBalancedTypes(
+                Pred, LHS, RHS, static_cast<ICmpInst::Predicate>(FoundPred),
+                TruncFoundLHS, TruncFoundRHS, CtxI))
           return true;
       }
     }
diff --git a/llvm/test/Transforms/IndVarSimplify/pr126409.ll b/llvm/test/Transforms/IndVarSimplify/pr126409.ll
@@ -10,7 +10,7 @@ define void @pr126409() {
 ; CHECK:       [[FOR_BODY_PREHEADER]]:
 ; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
 ; CHECK:       [[FOR_BODY]]:
-; CHECK-NEXT:    br i1 true, label %[[FOR_BODY]], label %[[IF_END_LOOPEXIT:.*]]
+; CHECK-NEXT:    br i1 false, label %[[FOR_BODY]], label %[[IF_END_LOOPEXIT:.*]]
 ; CHECK:       [[IF_END_LOOPEXIT]]:
 ; CHECK-NEXT:    br label %[[IF_END]]
 ; CHECK:       [[IF_END]]:

Original file line number	Diff line number	Diff line change
`@@ -11804,8 +11804,10 @@ bool ScalarEvolution::isImpliedCond(CmpPredicate Pred, const SCEV *LHS,`
`11804`	`11804`	`MaxValue)) {`
`11805`	`11805`	`const SCEV *TruncFoundLHS = getTruncateExpr(FoundLHS, NarrowType);`
`11806`	`11806`	`const SCEV *TruncFoundRHS = getTruncateExpr(FoundRHS, NarrowType);`
`11807`		`- if (isImpliedCondBalancedTypes(Pred, LHS, RHS, FoundPred, TruncFoundLHS,`
`11808`		`- TruncFoundRHS, CtxI))`
	`11807`	`+ // We cannot preserve samesign after truncation.`
	`11808`	`+ if (isImpliedCondBalancedTypes(`
	`11809`	`+ Pred, LHS, RHS, static_cast<ICmpInst::Predicate>(FoundPred),`
	`11810`	`+ TruncFoundLHS, TruncFoundRHS, CtxI))`
`11809`	`11811`	`return true;`
`11810`	`11812`	`}`
`11811`	`11813`	`}`