Enable EBX for x86 in 32bits PIC code

[Ilya Enkovich]

2014-08-28 17:01 GMT+04:00 Uros Bizjak <ubizjak@gmail.com>:
> On Fri, Aug 22, 2014 at 2:21 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>> Hi,
>>
>> On Cauldron 2014 we had a couple of talks about relaxation of ebx usage in 32bit PIC mode.  It was decided that the best approach would be to not fix ebx register, use speudo register for GOT base address and let allocator do the rest.  This should be similar to how clang and icc work with GOT base address.  I've been working for some time on such patch and now want to share my results.
>
> +#define PIC_OFFSET_TABLE_REGNUM
>          \
> +  ((TARGET_64BIT && (ix86_cmodel == CM_SMALL_PIC                       \
> +                     || TARGET_PECOFF))
>          \
> +   || !flag_pic ? INVALID_REGNUM                                       \
> +   : X86_TUNE_RELAX_PIC_REG ? (pic_offset_table_rtx ? INVALID_REGNUM   \
> +                              : REAL_PIC_OFFSET_TABLE_REGNUM)          \
> +   : reload_completed ? REGNO (pic_offset_table_rtx)                   \
>     : REAL_PIC_OFFSET_TABLE_REGNUM)
>
> I'd like to avoid X86_TUNE_RELAX_PIC_REG and always treat EBX as an
> allocatable register. This way, we can avoid all mess with implicit
> xchgs in atomic_compare_and_swap<dwi>_doubleword. Also, having
> allocatable EBX would allow us to introduce __builtin_cpuid builtin
> and cleanup cpiud.h.

We should show nice performance to have this feature enabled by
default.  Currently patch causes a set of performance losses. I have a
version of this patch where EBX is relaxed by a compiler flag, not
tune flag.

Ilya