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Correctly fold divide-by-constant, even when faced with overflow.

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llvm-svn: 47287
This commit is contained in:
Nick Lewycky 2008-02-18 22:48:05 +00:00
parent 0d0f16ca85
commit 1f3c58df08
3 changed files with 50 additions and 2 deletions
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29
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -605,6 +605,28 @@ static ConstantInt *Subtract(ConstantInt *C1, ConstantInt *C2) {
static ConstantInt *Multiply(ConstantInt *C1, ConstantInt *C2) {
return ConstantInt::get(C1->getValue() * C2->getValue());
}
/// MultiplyOverflows - True if the multiply can not be expressed in an int
/// this size.
static bool MultiplyOverflows(ConstantInt *C1, ConstantInt *C2, bool sign) {
uint32_t W = C1->getBitWidth();
APInt LHSExt = C1->getValue(), RHSExt = C2->getValue();
if (sign) {
LHSExt.sext(W * 2);
RHSExt.sext(W * 2);
} else {
LHSExt.zext(W * 2);
RHSExt.zext(W * 2);
}
APInt MulExt = LHSExt * RHSExt;
if (sign) {
APInt Min = APInt::getSignedMinValue(W).sext(W * 2);
APInt Max = APInt::getSignedMaxValue(W).sext(W * 2);
return MulExt.slt(Min) || MulExt.sgt(Max);
} else
return MulExt.ugt(APInt::getLowBitsSet(W * 2, W));
}
/// ComputeMaskedBits - Determine which of the bits specified in Mask are
/// known to be either zero or one and return them in the KnownZero/KnownOne
@ -2632,8 +2654,11 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
if (Instruction *LHS = dyn_cast<Instruction>(Op0))
if (Instruction::BinaryOps(LHS->getOpcode()) == I.getOpcode())
if (ConstantInt *LHSRHS = dyn_cast<ConstantInt>(LHS->getOperand(1))) {
return BinaryOperator::create(I.getOpcode(), LHS->getOperand(0),
Multiply(RHS, LHSRHS));
if (MultiplyOverflows(RHS, LHSRHS, I.getOpcode()==Instruction::SDiv))
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
else
return BinaryOperator::create(I.getOpcode(), LHS->getOperand(0),
Multiply(RHS, LHSRHS));
}
if (!RHS->isZero()) { // avoid X udiv 0

14
test/Transforms/InstCombine/2008-02-16-SDivOverflow.ll Normal file
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@ -0,0 +1,14 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {ret i.* 0} | count 2
; PR2048
define i32 @i(i32 %a) {
%tmp1 = sdiv i32 %a, -1431655765
%tmp2 = sdiv i32 %tmp1, 3
ret i32 %tmp2
}
define i8 @j(i8 %a) {
%tmp1 = sdiv i8 %a, 64
%tmp2 = sdiv i8 %tmp1, 3
ret i8 %tmp2
}

9
test/Transforms/InstCombine/2008-02-16-SDivOverflow2.ll Normal file
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@ -0,0 +1,9 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {sdiv i8 \%a, 9}
; PR2048
define i8 @i(i8 %a) {
%tmp1 = sdiv i8 %a, -3
%tmp2 = sdiv i8 %tmp1, -3
ret i8 %tmp2
}