InstCombine: sub nsw %x, C -> add nsw %x, -C if C isn't INT_MIN

We can preserve nsw during this transform if -C won't overflow.

llvm-svn: 216269

include/llvm/IR/Constant.h

@@ -69,6 +69,9 @@ public:
   /// Return true if the value is negative zero or null value.
   bool isZeroValue() const;
 
+  /// \brief Return true if the value is not the smallest signed value.
+  bool isNotMinSignedValue() const;
+
   /// \brief Return true if the value is the smallest signed value.
   bool isMinSignedValue() const;
 

lib/IR/Constants.cpp

@@ -151,6 +151,29 @@ bool Constant::isMinSignedValue() const {
   return false;
 }
 
+bool Constant::isNotMinSignedValue() const {
+  // Check for INT_MIN integers
+  if (const ConstantInt *CI = dyn_cast<ConstantInt>(this))
+    return !CI->isMinValue(/*isSigned=*/true);
+
+  // Check for FP which are bitcasted from INT_MIN integers
+  if (const ConstantFP *CFP = dyn_cast<ConstantFP>(this))
+    return !CFP->getValueAPF().bitcastToAPInt().isMinSignedValue();
+
+  // Check for constant vectors which are splats of INT_MIN values.
+  if (const ConstantVector *CV = dyn_cast<ConstantVector>(this))
+    if (Constant *Splat = CV->getSplatValue())
+      return Splat->isNotMinSignedValue();
+
+  // Check for constant vectors which are splats of INT_MIN values.
+  if (const ConstantDataVector *CV = dyn_cast<ConstantDataVector>(this))
+    if (Constant *Splat = CV->getSplatValue())
+      return Splat->isNotMinSignedValue();
+
+  // It *may* contain INT_MIN, we can't tell.
+  return false;
+}
+
 // Constructor to create a '0' constant of arbitrary type...
 Constant *Constant::getNullValue(Type *Ty) {
   switch (Ty->getTypeID()) {

lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1539,6 +1539,9 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
              "Expected a subtraction operator!");
       if (BO->hasNoSignedWrap() && I.hasNoSignedWrap())
         Res->setHasNoSignedWrap(true);
+    } else {
+      if (cast<Constant>(Op1)->isNotMinSignedValue() && I.hasNoSignedWrap())
+        Res->setHasNoSignedWrap(true);
     }
 
     return Res;
@@ -1629,7 +1632,7 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
 
     // 0 - (X sdiv C)  -> (X sdiv -C)  provided the negation doesn't overflow.
     if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) && match(Op0, m_Zero()) &&
-        !C->isMinSignedValue() && !C->isOneValue())
+        C->isNotMinSignedValue() && !C->isOneValue())
       return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));
 
     // 0 - (X << Y)  -> (-X << Y)   when X is freely negatable.

test/Transforms/InstCombine/sub.ll

@@ -521,3 +521,11 @@ define i4 @test43(i4 %x, i4 %y) {
 ; CHECK-NEXT: [[RET:%.*]] = sub nuw i4 [[OR]], [[AND]]
 ; CHECK: ret i4 [[RET]]
 }
+
+define i32 @test44(i32 %x) {
+  %sub = sub nsw i32 %x, 32768
+  ret i32 %sub
+; CHECK-LABEL: @test44(
+; CHECK-NEXT: [[ADD:%.*]] = add nsw i32 %x, -32768
+; CHECK: ret i32 [[ADD]]
+}