Is this correct behaviour?

Is this correct behaviour?

[Bingfeng Mei]

Hi, 
I compile the following code with arm gcc 4.6 (x86 is the similar with one of 4.7 snapshot).
I noticed "a" is written to memory three times instead of being added by 3 and written at the
end. Doesn't restrict guarantee "a" won't be aliased to "p" so 3 "a++" can be optimized?

Thanks,
Bingfeng Mei

int a;
int P[100];
void foo (int * restrict p)
{
  P[0] = *p;
  a++;
  P[1] = *p;
  a++;
  P[2] = *p;
  a++;
}

~/work/install-arm/bin/arm-elf-gcc tst.c -O2 -S -std=c99

foo:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        ldr     r3, .L2
        ldr     r1, [r3, #0]
        ldr     ip, [r0, #0]
        ldr     r2, .L2+4
        str     r4, [sp, #-4]!
        add     r4, r1, #1
        str     r4, [r3, #0]
        str     ip, [r2, #0]
        ldr     ip, [r0, #0]
        add     r4, r1, #2
        str     r4, [r3, #0]
        str     ip, [r2, #4]
        ldr     r0, [r0, #0]
        add     r1, r1, #3
        str     r0, [r2, #8]
        str     r1, [r3, #0]
        ldmfd   sp!, {r4}
        bx      lr

Re: Is this correct behaviour?

[Richard Guenther]

On Tue, Sep 6, 2011 at 5:30 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
> Hi,
> I compile the following code with arm gcc 4.6 (x86 is the similar with one of 4.7 snapshot).
> I noticed "a" is written to memory three times instead of being added by 3 and written at the
> end. Doesn't restrict guarantee "a" won't be aliased to "p" so 3 "a++" can be optimized?

No it does not.

> Thanks,
> Bingfeng Mei
>
> int a;
> int P[100];
> void foo (int * restrict p)
> {
>  P[0] = *p;
>  a++;
>  P[1] = *p;
>  a++;
>  P[2] = *p;
>  a++;
> }
>
> ~/work/install-arm/bin/arm-elf-gcc tst.c -O2 -S -std=c99
>
> foo:
>        @ args = 0, pretend = 0, frame = 0
>        @ frame_needed = 0, uses_anonymous_args = 0
>        @ link register save eliminated.
>        ldr     r3, .L2
>        ldr     r1, [r3, #0]
>        ldr     ip, [r0, #0]
>        ldr     r2, .L2+4
>        str     r4, [sp, #-4]!
>        add     r4, r1, #1
>        str     r4, [r3, #0]
>        str     ip, [r2, #0]
>        ldr     ip, [r0, #0]
>        add     r4, r1, #2
>        str     r4, [r3, #0]
>        str     ip, [r2, #4]
>        ldr     r0, [r0, #0]
>        add     r1, r1, #3
>        str     r0, [r2, #8]
>        str     r1, [r3, #0]
>        ldmfd   sp!, {r4}
>        bx      lr
>
>

RE: Is this correct behaviour?

[Bingfeng Mei]

> -----Original Message-----
> From: Richard Guenther [mailto:richard.guenther@gmail.com]
> Sent: 06 September 2011 16:42
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org
> Subject: Re: Is this correct behaviour?
> 
> On Tue, Sep 6, 2011 at 5:30 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
> > Hi,
> > I compile the following code with arm gcc 4.6 (x86 is the similar
> with one of 4.7 snapshot).
> > I noticed "a" is written to memory three times instead of being added
> by 3 and written at the
> > end. Doesn't restrict guarantee "a" won't be aliased to "p" so 3
> "a++" can be optimized?
> 
> No it does not.

Then how do I tell compiler that "a" is not aliased if I have to use global variable? 

> 
> > Thanks,
> > Bingfeng Mei
> >
> > int a;
> > int P[100];
> > void foo (int * restrict p)
> > {
> >  P[0] = *p;
> >  a++;
> >  P[1] = *p;
> >  a++;
> >  P[2] = *p;
> >  a++;
> > }
> >
> > ~/work/install-arm/bin/arm-elf-gcc tst.c -O2 -S -std=c99
> >
> > foo:
> >        @ args = 0, pretend = 0, frame = 0
> >        @ frame_needed = 0, uses_anonymous_args = 0
> >        @ link register save eliminated.
> >        ldr     r3, .L2
> >        ldr     r1, [r3, #0]
> >        ldr     ip, [r0, #0]
> >        ldr     r2, .L2+4
> >        str     r4, [sp, #-4]!
> >        add     r4, r1, #1
> >        str     r4, [r3, #0]
> >        str     ip, [r2, #0]
> >        ldr     ip, [r0, #0]
> >        add     r4, r1, #2
> >        str     r4, [r3, #0]
> >        str     ip, [r2, #4]
> >        ldr     r0, [r0, #0]
> >        add     r1, r1, #3
> >        str     r0, [r2, #8]
> >        str     r1, [r3, #0]
> >        ldmfd   sp!, {r4}
> >        bx      lr
> >
> >

Re: Is this correct behaviour?

[Ian Lance Taylor]

"Bingfeng Mei" <bmei@broadcom.com> writes:

> Then how do I tell compiler that "a" is not aliased if I have to use global variable? 
>
>> 
>> > Thanks,
>> > Bingfeng Mei
>> >
>> > int a;
>> > int P[100];
>> > void foo (int * restrict p)
>> > {
>> >  P[0] = *p;
>> >  a++;
>> >  P[1] = *p;
>> >  a++;
>> >  P[2] = *p;
>> >  a++;
>> > }

How about

int a;
int P[100];
void foo (int * restrict p)
{
  foo1 (p, P, &a);
}
void foo1 (int * restrict p, int * restrict pp, int * restrict pa)
{
  pp[0] = *p;
  a++;
  pp[1] = *p;
  a++;
  pp[2] = *p;
  a++;
}

Ian