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Re: Enable EBX for x86 in 32bits PIC code


On Mon, Jul 7, 2014 at 12:00 PM, Evgeny Stupachenko <evstupac@gmail.com> wrote:
> Hi All,
>
> Currently GCC permanently reserves EBX as the GOT register.
>
>     (config/i386/i386.c:4289)
>
>       /* The PIC register, if it exists, is fixed.  */
>       j = PIC_OFFSET_TABLE_REGNUM;
>       if (j != INVALID_REGNUM)
>         fixed_regs[j] = call_used_regs[j] = 1;
>
> This leads to significant performance losses in PIC mode:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54232
> According to my measurements ~3% generally and up to 20% in inner loops.
>
> CLANG uses all registers for allocation and therefore now has
> competitive advantage in 32bits PIC mode comparing to GCC.
> This mode is used in all Android applications and therefore is
> important  for many compiler customers.
>
> There are at least 2 possible solutions.
>
> 1.
>
> While call expand emit SET_GOT -> EBX and MOV EBX -> some local register: LGOT
> Prior to each call emit MOV LGOT -> EBX
> Use LGOT as new GOT register for globals.
>
> 2.
>
> Set EBX as each CALL parameter.
> Emit MOV EBX->LGOT in each call.
> Use LGOT as new GOT register for globals.
>
> Do you have any comments, ideas?

Use some LCM algorithm for placing %ebx loads, similar to
how we treat vzeroupper?

Compute some simple IPA info on whether %ebx is provided/needed
by callers/callees?

Richard.

> Thanks,
> Evgeny


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