[llvm] r212164 - InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN
Sean Silva
chisophugis at gmail.com
Tue Jul 1 23:28:32 PDT 2014
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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