Re: [patch][RFC] 160-bits bitmap_element

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

On Fri, Aug 17, 2012 at 2:12 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Fri, Aug 17, 2012 at 2:04 PM, Steven Bosscher <stevenb.gcc@gmail.com> wrote:
>> On Fri, Aug 17, 2012 at 1:54 PM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> Well, another effect of reducing the size of BITMAP_WORD is that
>>> operations are not performed in a mode optimally using CPU regs
>>> (did you check code generation differences on a 64bit host?).
>>
>> I did, on x86_64 and on powerpc64. The effect is not dramatic, most of
>> these machines can perform 32 bits operations just fine (I think the
>> only exception would be alpha, maybe?).
>
> I wonder how bad code gets when we unconditionally use GCCs generic
> vector support to do
>
> Index: gcc/bitmap.c
> ===================================================================
> --- gcc/bitmap.c        (revision 190469)
> +++ gcc/bitmap.c        (working copy)
> @@ -1446,6 +1446,17 @@ bitmap_compl_and_into (bitmap a, const_b
>           unsigned ix;
>           BITMAP_WORD ior = 0;
>
> +#if BITMAP_ELEMENT_WORDS == 5
> +         typedef v4si unsigned int __attribute__((vector_size((16))));
> +         v4si cleared4 = *(v4si *)&a_elt->bits[0] & *(v4si *)&b_elt->bits[0];
> +         BITMAP_WORD cleared5 = a_elt->bits[4] & b_elt->bits[4];
> +         v4si r4 = *(v4si *)&b_elt->bits[0] ^ cleared4;
> +         BITMAP_WORD r5 = b_elt->bits[4] ^ cleared5;
> +         *(v4si *)&a_elt->bits[0] = r4;
> +         a_elt->bits[4] = r5;
> +         ior4 |= r4;
> +         ior5 |= r5;
> +#else
>           for (ix = 0; ix < BITMAP_ELEMENT_WORDS; ix++)
>
> of course with a proper #if GCC_VERSION.  The theory is we should be
> able to lower it to the same scalar code or even v2si operations where
> only those are available ...
>
> Just an idea and eventually an opportunity to improve generic vector
> lowering if the above really does not work out.

Or figure out if or why not the vectorizer does catch this (of course we do
not enable that with -O2 which we eventually should in a very conservative
mode).

Richard.

> Richard.
>
>> Ciao!
>> Steven