Restrict pointers

[Jan Hoogerbrugge]

Hi,

As far as I know, memory references via two pointers where one of them is a 
restrict pointer and the other pointer is not restricted and is not derived 
from the first one are independent. GCC sees it differently: both pointers 
have to be restricted. I can demonstrate it by means of the following code:

int restrict_test0(int *p, int *q) /* no restrict pointers */
{
        *p = 0;
        return *q;
}

int restrict_test1a(int *restrict p, int *q) /* first pointer is restricted 
*/
{
        *p = 0;
        return *q;
}

int restrict_test1b(int *p, int *restrict q) /* second pointer is restricted 
*/
{
        *p = 0;
        return *q;
}

int restrict_test2(int *restrict p, int *restrict q) /* both pointers are 
restricted */
{
        *p = 0;
        return *q;
}

The scheduler would like the schedule the long latency load of *p before the 
store of *q. Dependences allow this only in the case of restrict_test2() 
where two restrict pointers are involved. This is the resulting Alpha code 
for 3.4.0 20040303:

$restrict_test0..ng:
restrict_test0:
        .frame $30,0,$26,0
        .prologue 0
        stl $31,0($16)
        ldl $0,0($17)
        ret $31,($26),1
        .end restrict_test0
        .align 2
        .align 4
        .globl restrict_test1a
        .ent restrict_test1a
$restrict_test1a..ng:
restrict_test1a:
        .frame $30,0,$26,0
        .prologue 0
        stl $31,0($16)
        ldl $0,0($17)
        ret $31,($26),1
        .end restrict_test1a
        .align 2
        .align 4
        .globl restrict_test1b
        .ent restrict_test1b
$restrict_test1b..ng:
restrict_test1b:
        .frame $30,0,$26,0
        .prologue 0
        stl $31,0($16)
        ldl $0,0($17)
        ret $31,($26),1
        .end restrict_test1b
        .align 2
        .align 4
        .globl restrict_test2
        .ent restrict_test2
$restrict_test2..ng:
restrict_test2:
        .frame $30,0,$26,0
        .prologue 0
        ldl $0,0($17)
        stl $31,0($16)
        ret $31,($26),1
        .end restrict_test2
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.0 20040303 (prerelease)"

As you can see, only in the case of restrict_rest2, the load is scheduled 
above the store. GCC should also do this in the case of restrict_test1a and 
restrict_test1b.

I already filed this as a bug (14192) but the status remains unconfirmed for 
several weeks. Could somebody tell whether this is ideed a performance bug?

Regards,
Jan

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