diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp index 5bc7b0b85391..85091afaf268 100644 --- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp +++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp @@ -108,6 +108,44 @@ bool InstCombiner::ShouldChangeType(Type *From, Type *To) const { 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(&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(B); + ConstantInt *CC = dyn_cast(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 @@ bool InstCombiner::SimplifyAssociativeOrCommutative(BinaryOperator &I) { 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 @@ bool InstCombiner::SimplifyAssociativeOrCommutative(BinaryOperator &I) { Constant *C2 = cast(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; } diff --git a/llvm/test/Transforms/InstCombine/nsw.ll b/llvm/test/Transforms/InstCombine/nsw.ll index 681bdc234b7d..698296384ae2 100644 --- a/llvm/test/Transforms/InstCombine/nsw.ll +++ b/llvm/test/Transforms/InstCombine/nsw.ll @@ -37,3 +37,19 @@ define i64 @shl1(i64 %X, i64* %P) nounwind { %B = shl i64 %A, 8 ret i64 %B } + +; CHECK: @preserve1 +; CHECK: add nsw i32 %x, 5 +define i32 @preserve1(i32 %x) nounwind { + %add = add nsw i32 %x, 2 + %add3 = add nsw i32 %add, 3 + ret i32 %add3 +} + +; CHECK: @nopreserve1 +; CHECK: add i8 %x, -126 +define i8 @nopreserve1(i8 %x) nounwind { + %add = add nsw i8 %x, 127 + %add3 = add nsw i8 %add, 3 + ret i8 %add3 +} \ No newline at end of file