[llvm] r212164 - InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN
David Majnemer
david.majnemer at gmail.com
Tue Jul 1 23:54:47 PDT 2014
I doubt this sort of code shows up often in the wild but sure, why not.
X/INT_MIN now canonicalizes to an icmp instead of an sdiv (as of r212167).
We now generate the following x86:
cmpl $-2147483648, %edi
sete %al
movzbl %al, %eax
On Tue, Jul 1, 2014 at 11:28 PM, Sean Silva <chisophugis at gmail.com> wrote:
> Did you see Stephen Canon's reply in "[LLVMdev] Probable error in
> InstCombine" about an alternative instcombine for this case? Quoted here
> for convenience:
>
>
> On Tue, Jul 1, 2014 at 4:59 PM, Stephen Canon <scanon at apple.com> wrote:
>
>> Worth noting that if C is INTTYPE_MIN, (X/C) is equivalent to (X ==
>> INTTYPE_MIN), and CMP + SETcc/CMOV is generally cheaper than integer
>> division.
>>
>> – Steve
>>
>
> -- Sean Silva
>
>
>
> On Wed, Jul 2, 2014 at 12:07 AM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>
>> Author: majnemer
>> Date: Wed Jul 2 01:07:09 2014
>> New Revision: 212164
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=212164&view=rev
>> Log:
>> InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN
>>
>> It is not safe to negate the smallest signed integer, doing so yields
>> the same number back.
>>
>> This fixes PR20186.
>>
>> Modified:
>> llvm/trunk/include/llvm/IR/Constant.h
>> llvm/trunk/lib/IR/Constants.cpp
>> llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
>> llvm/trunk/test/Transforms/InstCombine/sub.ll
>>
>> Modified: llvm/trunk/include/llvm/IR/Constant.h
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Constant.h?rev=212164&r1=212163&r2=212164&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/include/llvm/IR/Constant.h (original)
>> +++ llvm/trunk/include/llvm/IR/Constant.h Wed Jul 2 01:07:09 2014
>> @@ -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;
>>
>> Modified: llvm/trunk/lib/IR/Constants.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Constants.cpp?rev=212164&r1=212163&r2=212164&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/IR/Constants.cpp (original)
>> +++ llvm/trunk/lib/IR/Constants.cpp Wed Jul 2 01:07:09 2014
>> @@ -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()) {
>>
>> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=212164&r1=212163&r2=212164&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
>> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Wed Jul
>> 2 01:07:09 2014
>> @@ -1553,9 +1553,9 @@ Instruction *InstCombiner::visitSub(Bina
>> 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.
>>
>> Modified: llvm/trunk/test/Transforms/InstCombine/sub.ll
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub.ll?rev=212164&r1=212163&r2=212164&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/InstCombine/sub.ll (original)
>> +++ llvm/trunk/test/Transforms/InstCombine/sub.ll Wed Jul 2 01:07:09 2014
>> @@ -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]]
>> +}
>>
>>
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>
>
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