* Question on GIMPLE shifts
@ 2023-11-01 9:52 Daniil Frolov
2023-11-01 16:00 ` Andrew Pinski
0 siblings, 1 reply; 3+ messages in thread
From: Daniil Frolov @ 2023-11-01 9:52 UTC (permalink / raw)
To: gcc
Hi!
When investigating bit shifts I got an incomprehensible moment with
the following example:
int f(int x, int k)
{
int tmp = x >> k;
return (tmp & 1) << 10;
}
If we would like to take a look into GIMPLE then we'll get:
int f (int x, int k)
{
int tmp;
int D.2746;
int _1;
int _5;
<bb 2> :
tmp_4 = x_2(D) >> k_3(D);
_1 = tmp_4 << 10;
_5 = _1 & 1024;
<bb 3> :
<L0>:
return _5;
}
Is the expression '_1 = tmp_4 << 10' considered legal in GIMPLE? Given
the
semantics of C bit shifts, this statement could modify the sign bit,
potentially leading to overflow.
---
With best regards,
Daniil
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: Question on GIMPLE shifts
2023-11-01 9:52 Question on GIMPLE shifts Daniil Frolov
@ 2023-11-01 16:00 ` Andrew Pinski
2023-11-02 8:00 ` Richard Biener
0 siblings, 1 reply; 3+ messages in thread
From: Andrew Pinski @ 2023-11-01 16:00 UTC (permalink / raw)
To: Daniil Frolov; +Cc: gcc
On Wed, Nov 1, 2023 at 3:56 AM Daniil Frolov <exactlywb@ispras.ru> wrote:
>
> Hi!
>
> When investigating bit shifts I got an incomprehensible moment with
> the following example:
>
> int f(int x, int k)
> {
> int tmp = x >> k;
> return (tmp & 1) << 10;
> }
>
> If we would like to take a look into GIMPLE then we'll get:
>
> int f (int x, int k)
> {
> int tmp;
> int D.2746;
> int _1;
> int _5;
>
> <bb 2> :
> tmp_4 = x_2(D) >> k_3(D);
> _1 = tmp_4 << 10;
> _5 = _1 & 1024;
>
> <bb 3> :
> <L0>:
> return _5;
>
> }
>
> Is the expression '_1 = tmp_4 << 10' considered legal in GIMPLE? Given
> the
> semantics of C bit shifts, this statement could modify the sign bit,
> potentially leading to overflow.
Except it was not undefined in C90.
Thanks,
Andrew
>
> ---
> With best regards,
> Daniil
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: Question on GIMPLE shifts
2023-11-01 16:00 ` Andrew Pinski
@ 2023-11-02 8:00 ` Richard Biener
0 siblings, 0 replies; 3+ messages in thread
From: Richard Biener @ 2023-11-02 8:00 UTC (permalink / raw)
To: Andrew Pinski; +Cc: Daniil Frolov, gcc
On Wed, Nov 1, 2023 at 5:01 PM Andrew Pinski via Gcc <gcc@gcc.gnu.org> wrote:
>
> On Wed, Nov 1, 2023 at 3:56 AM Daniil Frolov <exactlywb@ispras.ru> wrote:
> >
> > Hi!
> >
> > When investigating bit shifts I got an incomprehensible moment with
> > the following example:
> >
> > int f(int x, int k)
> > {
> > int tmp = x >> k;
> > return (tmp & 1) << 10;
> > }
> >
> > If we would like to take a look into GIMPLE then we'll get:
> >
> > int f (int x, int k)
> > {
> > int tmp;
> > int D.2746;
> > int _1;
> > int _5;
> >
> > <bb 2> :
> > tmp_4 = x_2(D) >> k_3(D);
> > _1 = tmp_4 << 10;
> > _5 = _1 & 1024;
> >
> > <bb 3> :
> > <L0>:
> > return _5;
> >
> > }
> >
> > Is the expression '_1 = tmp_4 << 10' considered legal in GIMPLE? Given
> > the
> > semantics of C bit shifts, this statement could modify the sign bit,
> > potentially leading to overflow.
>
> Except it was not undefined in C90.
Also in GIMPLE/GENERIC left-shifts are always logical and the result is
modulo-reduced to the target type. There's no (undefined) arithmetic overflow
involved for any shift operation (but there is for multiply). Only the shift
argument magnitude is constrained.
Richard.
> Thanks,
> Andrew
>
> >
> > ---
> > With best regards,
> > Daniil
^ permalink raw reply [flat|nested] 3+ messages in thread
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2023-11-01 16:00 ` Andrew Pinski
2023-11-02 8:00 ` Richard Biener
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