[ConstraintElim] Teach checkAndReplaceCondition about samesign

Author: citymarina

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -1427,7 +1427,7 @@ static std::optional<bool> checkCondition(CmpInst::Predicate Pred, Value *A,
 }
 
 static bool checkAndReplaceCondition(
-    CmpInst *Cmp, ConstraintInfo &Info, unsigned NumIn, unsigned NumOut,
+    ICmpInst *Cmp, ConstraintInfo &Info, unsigned NumIn, unsigned NumOut,
     Instruction *ContextInst, Module *ReproducerModule,
     ArrayRef<ReproducerEntry> ReproducerCondStack, DominatorTree &DT,
     SmallVectorImpl<Instruction *> &ToRemove) {
@@ -1460,6 +1460,15 @@ static bool checkAndReplaceCondition(
           checkCondition(Cmp->getPredicate(), Cmp->getOperand(0),
                          Cmp->getOperand(1), Cmp, Info))
     return ReplaceCmpWithConstant(Cmp, *ImpliedCondition);
+
+  // When the predicate is samesign and unsigned, we can also make use of the
+  // signed predicate information.
+  if (Cmp->hasSameSign() && Cmp->isUnsigned())
+    if (auto ImpliedCondition =
+            checkCondition(Cmp->getSignedPredicate(), Cmp->getOperand(0),
+                           Cmp->getOperand(1), Cmp, Info))
+      return ReplaceCmpWithConstant(Cmp, *ImpliedCondition);
+
   return false;
 }
 

llvm/test/Transforms/ConstraintElimination/transfer-samesign-facts.ll

@@ -309,8 +309,7 @@ define i1 @implied_condition_sgt_ugt(i8 %a, i8 %b)  {
 ; CHECK-NEXT:    [[CMP_SGT:%.*]] = icmp sgt i8 [[A:%.*]], [[B:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP_SGT]], label [[GREATER:%.*]], label [[EXIT:%.*]]
 ; CHECK:       greater:
-; CHECK-NEXT:    [[CMP_UGT:%.*]] = icmp samesign ugt i8 [[A]], [[B]]
-; CHECK-NEXT:    ret i1 [[CMP_UGT]]
+; CHECK-NEXT:    ret i1 true
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret i1 false
 ;
@@ -330,8 +329,7 @@ define i1 @implied_condition_sle_ule(i8 %a) {
 ; CHECK-NEXT:    [[CMP_SLE:%.*]] = icmp sle i8 [[A:%.*]], 42
 ; CHECK-NEXT:    br i1 [[CMP_SLE]], label [[LESS_OR_EQUAL:%.*]], label [[EXIT:%.*]]
 ; CHECK:       less_or_equal:
-; CHECK-NEXT:    [[CMP_ULE:%.*]] = icmp samesign ule i8 [[A]], 42
-; CHECK-NEXT:    ret i1 [[CMP_ULE]]
+; CHECK-NEXT:    ret i1 true
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret i1 false
 ;