Teach instcombine to preserve the nsw bit by doing an after-the-fact analysis
when combining add and sub instructions. Patch by Pranav Bhandarkar!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137570 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 5bc7b0b..85091af 100644
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -108,6 +108,44 @@
return true;
}
+// Return true, if No Signed Wrap should be maintained for I.
+// The No Signed Wrap flag can be kept if the operation "B (I.getOpcode) C",
+// where both B and C should be ConstantInts, results in a constant that does
+// not overflow. This function only handles the Add and Sub opcodes. For
+// all other opcodes, the function conservatively returns false.
+static bool MaintainNoSignedWrap(BinaryOperator &I, Value *B, Value *C) {
+ OverflowingBinaryOperator *OBO = dyn_cast<OverflowingBinaryOperator>(&I);
+ if (!OBO || !OBO->hasNoSignedWrap()) {
+ return false;
+ }
+
+ // We reason about Add and Sub Only.
+ Instruction::BinaryOps Opcode = I.getOpcode();
+ if (Opcode != Instruction::Add &&
+ Opcode != Instruction::Sub) {
+ return false;
+ }
+
+ ConstantInt *CB = dyn_cast<ConstantInt>(B);
+ ConstantInt *CC = dyn_cast<ConstantInt>(C);
+
+ if (!CB || !CC) {
+ return false;
+ }
+
+ const APInt &BVal = CB->getValue();
+ const APInt &CVal = CC->getValue();
+ bool Overflow = false;
+
+ if (Opcode == Instruction::Add) {
+ BVal.sadd_ov(CVal, Overflow);
+ } else {
+ BVal.ssub_ov(CVal, Overflow);
+ }
+
+ return !Overflow;
+}
+
/// SimplifyAssociativeOrCommutative - This performs a few simplifications for
/// operators which are associative or commutative:
@@ -159,7 +197,13 @@
I.setOperand(1, V);
// Conservatively clear the optional flags, since they may not be
// preserved by the reassociation.
- I.clearSubclassOptionalData();
+ if (MaintainNoSignedWrap(I, B, C)) {
+ I.clearSubclassOptionalData();
+ I.setHasNoSignedWrap(true);
+ } else {
+ I.clearSubclassOptionalData();
+ }
+
Changed = true;
++NumReassoc;
continue;
@@ -241,14 +285,21 @@
Constant *C2 = cast<Constant>(Op1->getOperand(1));
Constant *Folded = ConstantExpr::get(Opcode, C1, C2);
- Instruction *New = BinaryOperator::Create(Opcode, A, B);
+ BinaryOperator *New = BinaryOperator::Create(Opcode, A, B);
InsertNewInstWith(New, I);
New->takeName(Op1);
I.setOperand(0, New);
I.setOperand(1, Folded);
// Conservatively clear the optional flags, since they may not be
// preserved by the reassociation.
- I.clearSubclassOptionalData();
+ if (MaintainNoSignedWrap(I, C1, C2)) {
+ I.clearSubclassOptionalData();
+ I.setHasNoSignedWrap(true);
+ New->setHasNoSignedWrap(true);
+ } else {
+ I.clearSubclassOptionalData();
+ }
+
Changed = true;
continue;
}