Suboptimal __restrict optimization?

Suboptimal __restrict optimization?

[Ulf Magnusson]

Hi,

Given the code

class C { void f(int *p); int q; };

void C::f(int * __restrict p) __restrict {
    q += 10;
    *p = 7;
    q += 10;
}

g++ 4.5.2 with -O3 generates the following for C::f() (prologue and
epilogue omitted):

mov    0x8(%ebp),%eax // eax = this (= &q)
mov    0xc(%ebp),%ecx // ecx = p
mov    (%eax),%edx    // edx = q
movl   $0x7,(%ecx)    // *p = 7
add    $0x14,%edx     // q += 20
mov    %edx,(%eax)    // save q

If C::f() is rearranged as

void C::f(int * __restrict p) __restrict {
    *p = 7;
    q += 10;
    q += 10;
}

the following is generated instead:

mov    0x8(%ebp),%eax // eax = this (= &q)
mov    0xc(%ebp),%edx // edx = p
movl   $0x7,(%edx)    // *p = 7
addl   $0x14,(%eax)   // q += 20

Is there some reason why GCC couldn't generate this code for the first
version of C::f()? Is this a failure of optimization, or am I missing
something in how __restricted works?

/Ulf

Re: Suboptimal __restrict optimization?

[Ian Lance Taylor]

Ulf Magnusson <ulfalizer@gmail.com> writes:

> Is there some reason why GCC couldn't generate this code for the first
> version of C::f()? Is this a failure of optimization, or am I missing
> something in how __restricted works?

It's a failure of optimization.

Ian

Re: Suboptimal __restrict optimization?

[Ulf Magnusson]

On Mon, Oct 3, 2011 at 10:22 PM, Ian Lance Taylor <iant@google.com> wrote:
> Ulf Magnusson <ulfalizer@gmail.com> writes:
>
>> Is there some reason why GCC couldn't generate this code for the first
>> version of C::f()? Is this a failure of optimization, or am I missing
>> something in how __restricted works?
>
> It's a failure of optimization.
>
> Ian
>

Is this something that has been improved in 4.6.x? (Sorry for the
initial non-reply-all.)

Re: Suboptimal __restrict optimization?

[Richard Guenther]

On Tue, Oct 4, 2011 at 5:07 PM, Ulf Magnusson <ulfalizer@gmail.com> wrote:
> On Mon, Oct 3, 2011 at 10:22 PM, Ian Lance Taylor <iant@google.com> wrote:
>> Ulf Magnusson <ulfalizer@gmail.com> writes:
>>
>>> Is there some reason why GCC couldn't generate this code for the first
>>> version of C::f()? Is this a failure of optimization, or am I missing
>>> something in how __restricted works?
>>
>> It's a failure of optimization.
>>
>> Ian
>>
>
> Is this something that has been improved in 4.6.x? (Sorry for the
> initial non-reply-all.)

With 4.6 we export points-to info to RTL, so in theory it could, but
in reality combine does not work to fuse load, add and store.

Richard.