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Date: Mon, 4 Feb 2013 11:10:35 +0100
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To: Ard Biesheuvel <ard.biesheuvel@linaro.org>, Felipe Pena <felipe@php.net>, 
	Dmitry Stogov <dmitry@zend.com>, Xinchen Hui <laruence@php.net>
Cc: PHP Internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] more inline assembler noise
From: tyra3l@gmail.com (Ferenc Kovacs)

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On Fri, Jan 18, 2013 at 7:08 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org>wrote:

> Hello,
>
> Again, apologies for prematurely declaring someone else's code 'crap'.
> There are no bugs in the inline x86 assembler in Zend/zend_operators.h, a=
s
> far as I can tell, only two kinds of issues that I still think should be
> addressed.
>
> First of all, from a maintenance pov, having field offsets (like the
> offset of zval.type) and constants (like $0x2 for IS_DOUBLE) hard coded
> inside the instructions is a bad idea.
>
> The other issue is the branching and the floating point instructions. The
> inline assembler addresses the common case, but also adds a bunch of
> instructions that address the corner case, and some branches to jump over
> them. As I indicated in my previous email, branching is relatively costly
> on a modern CPU with deep pipelines and having a bunch of FPU instruction=
s
> in there that hardly ever get executed doesn't help either.
>
> The primary reason for having inline assembler at all is the ability to
> detect overflow. This mainly applies to multiplication, as in that case,
> detecting overflow in C code is much harder compared to reading a conditi=
on
> flag in the CPU (hence the various accelerated implementations in
> zend_signed_multiply.h). However, detecting overflow in
> addition/subtraction implemented in C is much easier, as the code in
> zend_operators.h proves: just a matter of checking the sign bits, or doin=
g
> a simple compare with LONG_MIN/LONG_MAX.
>
> Therefore, I would be interested in finding out which benchmark was used
> to make the case for having these accelerated implementations in the firs=
t
> place. The differences in performance between various implementations are
> very small in the tests I have done.
>
> As for the code style/maintainability, I propose to apply the attached
> patch. The performance is on par, as far as I can tell, but it is arguabl=
y
> better code. I will also hook in the ARM versions once I manage to prove
> that the performance is affected favourably by them.
>
> Regards,
> Ard.
>
>
>
> Before
> -------
>
> $ time php -r 'for ($i =3D 0; $i < 0x7fffffff; $i++);'
>
> real    0m56.910s
> user    0m56.876s
> sys     0m0.008s
>
>
> $ time php -r 'for ($i =3D 0x7fffffff; $i >=3D 0; $i--);'
>
> real    1m34.576s
> user    1m34.518s
> sys     0m0.020s
>
>
> $ time php -r 'for ($i =3D 0; $i < 0x7fffffff; $i +=3D 3);'
>
> real    0m21.494s
> user    0m21.473s
> sys     0m0.008s
>
>
> $ time php -r 'for ($i =3D 0x7fffffff; $i >=3D 0; $i -=3D 3);'
>
> real    0m19.879s
> user    0m19.865s
> sys     0m0.004s
>
>
> After
> -----
>
> $ time php -r 'for ($i =3D 0; $i < 0x7fffffff; $i++);'
>
> real    0m56.687s
> user    0m56.656s
> sys     0m0.004s
>
>
> $ time php -r 'for ($i =3D 0x7fffffff; $i >=3D 0; $i--);'
>
> real    1m28.124s
> user    1m28.082s
> sys     0m0.004s
>
>
> $ time php -r 'for ($i =3D 0; $i < 0x7fffffff; $i +=3D 3);'
>
> real    0m20.561s
> user    0m20.545s
> sys     0m0.004s
>
>
> $ time php -r 'for ($i =3D 0x7fffffff; $i >=3D 0; $i -=3D 3);'
>
> real    0m20.524s
> user    0m20.509s
> sys     0m0.004s
>
>
> --
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>

hi,

any update on this?


--=20
Ferenc Kov=C3=A1cs
@Tyr43l - http://tyrael.hu

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