Re: How can I tune gcc to move up simple common subexpression?

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Nov 8, 2013 at 10:28 AM, Konstantin Vladimirov
<konstantin.vladimirov@gmail.com> wrote:
> Hi,
>
> Consider simple code:
>
> typedef struct
> {
>   unsigned prev;
>   unsigned next;
> } foo_t;
>
> void
> foo( unsigned x, unsigned y)
>   {
>     foo_t *ptr = (foo_t *)((void *)x);
>
>     if (y != 0)
>       {
>          ptr->prev = y;
>          ptr->next = x;
>        }
>      else
>        {
>          ptr->prev = 0; /* or explicitly ptr->prev = y; no difference */
>          ptr->next = 0;
>        }
> }
>
> GCC 4.7.2 and 4.8.1 both on O2 and Os creates code like:
>
> testl %esi, %esi
> movl %edi, %eax
> jne .L5
> movl $0, (%edi)
> movl $0, 4(%rax)
> ret
> .L5:
> movl %esi, (%edi)
> movl %edi, 4(%rax)
> ret
>
> Which can be obviously changed to:
>
> testl %esi, %esi
> movl %edi, %eax
> movl %esi, (%edi)
> jne .L5
> movl $0, 4(%rax)
> ret
> .L5:
> movl %edi, 4(%rax)
> ret
>
> May be there are some options to make it behave so? This is question
> for gcc-help group.
>
> Question for gcc group is trickier:
>
> May be in x86 it is not a big deal, but I am working on my private
> backend, that have predicated instructions and second form is really
> much more prefferable. May be I can somehow tune my backend to achieve
> this effect? I can see that 210r.csa pass (try_optimize_cfg before it,
> not pass itself) can move code upper in  some cases, but only with
> very simple memory addressing and rather unstable, say changing
>
>          ptr->prev = y;
>          ptr->next = x;
>
> to
>
>          ptr->prev = x;
>          ptr->next = y;
>
> may break everything just because next is second member and addressed
> like M[%r+4].
>
> Any ideas?

IIRC there is some code hoisting in RTL GCSE but not very strong.
code-hoisting in GIMPLE via PRE is still in-progress (see PR23286).

Richard.

> ---
> With best regards, Konstantin