[ConstraintElimination] Move logic for replacing ssub overflow users (NFC)

[fhahn]

LGTM, thanks!

[fhahn]

This needs to be rebased, current main contains an update to return a bool to indicate whether IR has been changed.

[zjaffal]

rebase on top of main to fix failing tests.

[zjaffal]

Rebase on top of main

[fhahn]

LGTM, thanks!

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

if (R.IsEq) {
       CSToUse.addVariableRowFill(R.Coefficients);
 
       DFSInStack.emplace_back(NumIn, NumOut, R.IsSigned,
                               SmallVector<Value *, 2>());
     }
   }
 }
 
-static bool
-tryToSimplifyOverflowMath(IntrinsicInst *II, ConstraintInfo &Info,
+static bool replaceSubOverflowUses(IntrinsicInst *II, Value *A, Value *B,
                                    SmallVectorImpl<Instruction *> &ToRemove) {
-  auto DoesConditionHold = [](CmpInst::Predicate Pred, Value *A, Value *B,
-                              ConstraintInfo &Info) {
-    auto R = Info.getConstraintForSolving(Pred, A, B);
-    if (R.size() < 2 || !R.isValid(Info))
-      return false;
-
-    auto &CSToUse = Info.getCS(R.IsSigned);
-    return CSToUse.isConditionImplied(R.Coefficients);
-  };
-
   bool Changed = false;
-  if (II->getIntrinsicID() == Intrinsic::ssub_with_overflow) {
-    // If A s>= B && B s>= 0, ssub.with.overflow(a, b) should not overflow and
-    // can be simplified to a regular sub.
-    Value *A = II->getArgOperand(0);
-    Value *B = II->getArgOperand(1);
-    if (!DoesConditionHold(CmpInst::ICMP_SGE, A, B, Info) ||
-        !DoesConditionHold(CmpInst::ICMP_SGE, B,
-                           ConstantInt::get(A->getType(), 0), Info))
-      return false;
-
-    IRBuilder<> Builder(II->getParent(), II->getIterator());
-    Value *Sub = nullptr;
-    for (User *U : make_early_inc_range(II->users())) {
-      if (match(U, m_ExtractValue<0>(m_Value()))) {
-        if (!Sub)
-          Sub = Builder.CreateSub(A, B);
-        U->replaceAllUsesWith(Sub);
-        Changed = true;
-      } else if (match(U, m_ExtractValue<1>(m_Value()))) {
-        U->replaceAllUsesWith(Builder.getFalse());
-        Changed = true;
-      } else
-        continue;
 
-      if (U->use_empty()) {
-        auto *I = cast<Instruction>(U);
-        ToRemove.push_back(I);
-        I->setOperand(0, PoisonValue::get(II->getType()));
-        Changed = true;
-      }
-    }
 
-    if (II->use_empty()) {
-      II->eraseFromParent();
-      Changed = true;
-    }
+  return Changed;
+}
+
+static bool
+tryToSimplifyOverflowMath(IntrinsicInst *II, ConstraintInfo &Info,
+                          SmallVectorImpl<Instruction *> &ToRemove) {
+  auto DoesConditionHold = [](CmpInst::Predicate Pred, Value *A, Value *B,
+                              ConstraintInfo &Info) {
+    auto R = Info.getConstraintForSolving(Pred, A, B);
+    if (R.size() < 2 || !R.isValid(Info))
+      return false;
+
+    auto &CSToUse = Info.getCS(R.IsSigned);
+    return CSToUse.isConditionImplied(R.Coefficients);
+  };
+
+  bool Changed = false;
+  if (II->getIntrinsicID() == Intrinsic::ssub_with_overflow) {
+    // If A s>= B && B s>= 0, ssub.with.overflow(a, b) should not overflow and
+    // can be simplified to a regular sub.
+    Value *A = II->getArgOperand(0);
+    Value *B = II->getArgOperand(1);
+    if (!DoesConditionHold(CmpInst::ICMP_SGE, A, B, Info) ||
+        !DoesConditionHold(CmpInst::ICMP_SGE, B,
+                           ConstantInt::get(A->getType(), 0), Info))
+      return false;
+    Changed = replaceSubOverflowUses(II, A, B, ToRemove);
   }
   return Changed;
 }
 
 static bool eliminateConstraints(Function &F, DominatorTree &DT) {
   bool Changed = false;
   DT.updateDFSNumbers();