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InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN

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It is not safe to negate the smallest signed integer, doing so yields
the same number back.

This fixes PR20186.

llvm-svn: 212164
This commit is contained in:
David Majnemer 2014-07-02 06:07:09 +00:00
parent 220bba5c5d
commit 5449bfbb6f
4 changed files with 47 additions and 3 deletions
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3
include/llvm/IR/Constant.h
View File

@ -64,6 +64,9 @@ public:
/// Return true if the value is negative zero or null value.
bool isZeroValue() const;
/// \brief Return true if the value is the smallest signed value.
bool isMinSignedValue() const;
/// canTrap - Return true if evaluation of this constant could trap. This is
/// true for things like constant expressions that could divide by zero.
bool canTrap() const;

22
lib/IR/Constants.cpp
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@ -107,6 +107,28 @@ bool Constant::isAllOnesValue() const {
return false;
}
bool Constant::isMinSignedValue() 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->isMinSignedValue();
// 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->isMinSignedValue();
return false;
}
// Constructor to create a '0' constant of arbitrary type...
Constant *Constant::getNullValue(Type *Ty) {
switch (Ty->getTypeID()) {

6
lib/Transforms/InstCombine/InstCombineAddSub.cpp
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@ -1553,9 +1553,9 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
return BinaryOperator::CreateAnd(Op0,
Builder->CreateNot(Y, Y->getName() + ".not"));
// 0 - (X sdiv C) -> (X sdiv -C)
if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) &&
match(Op0, m_Zero()))
// 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())
return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));
// 0 - (X << Y) -> (-X << Y) when X is freely negatable.

19
test/Transforms/InstCombine/sub.ll
View File

@ -444,3 +444,22 @@ define <2 x i64> @test36(<2 x i64> %A) {
; CHECK-NEXT: %sub = mul <2 x i64> %A, <i64 7, i64 15>
; CHECK-NEXT: ret <2 x i64> %sub
}
define <2 x i64> @test37(<2 x i64> %A) {
%shl = shl <2 x i64> %A, <i64 3, i64 4>
%sub = sub <2 x i64> %shl, %A
ret <2 x i64> %sub
; CHECK-LABEL: @test37(
; CHECK-NEXT: %sub = mul <2 x i64> %A, <i64 7, i64 15>
; CHECK-NEXT: ret <2 x i64> %sub
}
define i32 @test38(i32 %A) {
%div = sdiv i32 %A, -2147483648
%sub = sub nsw i32 0, %div
ret i32 %sub
; CHECK-LABEL: @test38(
; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 %A, -2147483648
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 0, [[DIV]]
; CHECK-NEXT: ret i32 [[SUB]]
}