[llvm] ff218cb - [InstSimplify] Fold degenerate abs of abs form
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 10 13:38:06 PDT 2020
On Tue, Sep 8, 2020 at 5:59 PM Philip Reames <listmail at philipreames.com>
wrote:
> Given:
>
> x == 0 ? -abs(x) : Y
>
> We should really be producing:
>
> x == 0 ? 0 : Y
>
> This seems like something CVP might be able to handle, or even an
> alternate instcombine rule.
>
> And once we have
>
> x == 0 ? 0 : abs(x)
>
> We should definitely fold that to abs(x).
>
> Not suggesting that your change needs removed; just that there's an
> alternate, slightly more general way of achieving the same thing.
>
> Philip
>
I agree that handling the pattern in this way would be ideal, but it's not
entirely simple. I've put up https://reviews.llvm.org/D87480 as a first
step. This is sufficient to fold x == 0 ? abs(x) : Y to x == 0 ? 0 : Y at
least (but not negative abs).
Nikita
On 9/6/20 12:45 AM, Nikita Popov via llvm-commits wrote:
> > Author: Nikita Popov
> > Date: 2020-09-06T09:43:08+02:00
> > New Revision: ff218cbc84ff3783cb5ad030397adef8c9e8d444
> >
> > URL:
> https://github.com/llvm/llvm-project/commit/ff218cbc84ff3783cb5ad030397adef8c9e8d444
> > DIFF:
> https://github.com/llvm/llvm-project/commit/ff218cbc84ff3783cb5ad030397adef8c9e8d444.diff
> >
> > LOG: [InstSimplify] Fold degenerate abs of abs form
> >
> > This addresses the remaining issue from D87188. Due to a series of
> > folds, we may end up with abs-of-abs represented as
> > x == 0 ? -abs(x) : abs(x). Rather than recognizing this as a special
> > abs pattern and doing an abs-of-abs fold on it afterwards,
> > I'm directly folding this to one of the select operands in InstSimplify.
> >
> > The general pattern falls into the "select with operand replaced"
> > category, but that fold is not powerful enough to recognize that
> > both hands of the select are the same for value zero.
> >
> > Differential Revision: https://reviews.llvm.org/D87197
> >
> > Added:
> >
> >
> > Modified:
> > llvm/lib/Analysis/InstructionSimplify.cpp
> > llvm/test/Transforms/InstSimplify/abs_intrinsic.ll
> >
> > Removed:
> >
> >
> >
> >
> ################################################################################
> > diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp
> b/llvm/lib/Analysis/InstructionSimplify.cpp
> > index 3139b5a96b27..7c13b41bc7e6 100644
> > --- a/llvm/lib/Analysis/InstructionSimplify.cpp
> > +++ b/llvm/lib/Analysis/InstructionSimplify.cpp
> > @@ -3965,6 +3965,15 @@ static Value *simplifySelectWithICmpCond(Value
> *CondVal, Value *TrueVal,
> > if (match(FalseVal, isRotate) && TrueVal == X && CmpLHS == ShAmt &&
> > Pred == ICmpInst::ICMP_EQ)
> > return FalseVal;
> > +
> > + // X == 0 ? abs(X) : -abs(X) --> -abs(X)
> > + // X == 0 ? -abs(X) : abs(X) --> abs(X)
> > + if (match(TrueVal, m_Intrinsic<Intrinsic::abs>(m_Value(X))) &&
> > + match(FalseVal,
> m_Neg(m_Intrinsic<Intrinsic::abs>(m_Specific(X)))))
> > + return FalseVal;
> > + if (match(TrueVal, m_Neg(m_Intrinsic<Intrinsic::abs>(m_Value(X))))
> &&
> > + match(FalseVal, m_Intrinsic<Intrinsic::abs>(m_Specific(X))))
> > + return FalseVal;
> > }
> >
> > // Check for other compares that behave like bit test.
> >
> > diff --git a/llvm/test/Transforms/InstSimplify/abs_intrinsic.ll
> b/llvm/test/Transforms/InstSimplify/abs_intrinsic.ll
> > index e9305a927c42..70b50da9f041 100644
> > --- a/llvm/test/Transforms/InstSimplify/abs_intrinsic.ll
> > +++ b/llvm/test/Transforms/InstSimplify/abs_intrinsic.ll
> > @@ -205,10 +205,7 @@ define i1 @abs_ule_int_min(i8 %x) {
> > define i32 @select_abs_of_abs_eq(i32 %x) {
> > ; CHECK-LABEL: @select_abs_of_abs_eq(
> > ; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[X:%.*]],
> i1 false)
> > -; CHECK-NEXT: [[NEG:%.*]] = sub i32 0, [[ABS]]
> > -; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 0
> > -; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 [[NEG]], i32
> [[ABS]]
> > -; CHECK-NEXT: ret i32 [[SEL]]
> > +; CHECK-NEXT: ret i32 [[ABS]]
> > ;
> > %abs = call i32 @llvm.abs.i32(i32 %x, i1 false)
> > %neg = sub i32 0, %abs
> > @@ -220,10 +217,7 @@ define i32 @select_abs_of_abs_eq(i32 %x) {
> > define i32 @select_abs_of_abs_ne(i32 %x) {
> > ; CHECK-LABEL: @select_abs_of_abs_ne(
> > ; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[X:%.*]],
> i1 false)
> > -; CHECK-NEXT: [[NEG:%.*]] = sub i32 0, [[ABS]]
> > -; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[X]], 0
> > -; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 [[ABS]], i32
> [[NEG]]
> > -; CHECK-NEXT: ret i32 [[SEL]]
> > +; CHECK-NEXT: ret i32 [[ABS]]
> > ;
> > %abs = call i32 @llvm.abs.i32(i32 %x, i1 false)
> > %neg = sub i32 0, %abs
> > @@ -236,9 +230,7 @@ define i32 @select_nabs_of_abs_eq(i32 %x) {
> > ; CHECK-LABEL: @select_nabs_of_abs_eq(
> > ; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[X:%.*]],
> i1 false)
> > ; CHECK-NEXT: [[NEG:%.*]] = sub i32 0, [[ABS]]
> > -; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 0
> > -; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 [[ABS]], i32
> [[NEG]]
> > -; CHECK-NEXT: ret i32 [[SEL]]
> > +; CHECK-NEXT: ret i32 [[NEG]]
> > ;
> > %abs = call i32 @llvm.abs.i32(i32 %x, i1 false)
> > %neg = sub i32 0, %abs
> > @@ -251,9 +243,7 @@ define i32 @select_nabs_of_abs_ne(i32 %x) {
> > ; CHECK-LABEL: @select_nabs_of_abs_ne(
> > ; CHECK-NEXT: [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[X:%.*]],
> i1 false)
> > ; CHECK-NEXT: [[NEG:%.*]] = sub i32 0, [[ABS]]
> > -; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[X]], 0
> > -; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 [[NEG]], i32
> [[ABS]]
> > -; CHECK-NEXT: ret i32 [[SEL]]
> > +; CHECK-NEXT: ret i32 [[NEG]]
> > ;
> > %abs = call i32 @llvm.abs.i32(i32 %x, i1 false)
> > %neg = sub i32 0, %abs
> >
> >
> >
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> > llvm-commits at lists.llvm.org
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>
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