Provide no-pic compilation option on android for x86 atom to improve performance

Provide no-pic compilation option on android for x86 atom to improve performance 

TEST=make android_ia32.release atom=true

[Lijuan Xiao, 2013-06-26 07:06:03]

On 2013/06/26 06:37:59, Lijuan Xiao wrote:

No-pic compilation option can improve performance for x86 atom on android. About
7.9% improvement is observed for octane benchmark.

Reviewers, please help have a look. Thanks.

[Jakob Kummerow, 2013-06-26 08:23:01]

High-level comments:
• Why do we need an option to turn off -fpic in the first place? Does it really
default to on in any toolchain?
• Note that this patch as implemented will only affect standalone V8 builds. It
won't have any effect on V8 embedded into Chrome.


As for the patch itself:
I don't think "atom=true" is an intuitive way to turn off PIC, and I don't see
why this option should be implemented only for standalone ia32 Android builds.
PIC is neither Atom nor ia32 nor Android specific.
Let's make the Makefile option's syntax "pic=off", map that to a GYP variable
called 'pic', which defaults to "default" and when set to "off" adds the
-fno-pic flag directly in standalone.gypi, regardless of v8_target_arch and
whether building for Android or not.

[Lijuan Xiao, 2013-06-26 09:03:51]

On 2013/06/26 08:23:01, Jakob wrote:
> High-level comments:
> • Why do we need an option to turn off -fpic in the first place? Does it
really
> default to on in any toolchain?
> • Note that this patch as implemented will only affect standalone V8 builds.
It
> won't have any effect on V8 embedded into Chrome.
> 
> 
> As for the patch itself:
> I don't think "atom=true" is an intuitive way to turn off PIC, and I don't see
> why this option should be implemented only for standalone ia32 Android builds.
> PIC is neither Atom nor ia32 nor Android specific.
> Let's make the Makefile option's syntax "pic=off", map that to a GYP variable
> called 'pic', which defaults to "default" and when set to "off" adds the
> -fno-pic flag directly in standalone.gypi, regardless of v8_target_arch and
> whether building for Android or not.

Hi, thanks for your comments. To my understanding, pic is a default cflag. I
just want to give an option to turn off it for atom, since pic brings overhead
on atom, due to indirectly access to global/static variables and non-static
functions. This is somewhat specific to atom. Yes, we may do this for atom on
other OSs using cflags. Currently I just tested android.

Suggestions and comments are welcome.

BTW, I know this patch only affects standalone V8. Will chrome fetches v8 code
from time to time?

[Jakob Kummerow, 2013-06-26 09:18:00]

On 2013/06/26 09:03:51, Lijuan Xiao wrote:
> Hi, thanks for your comments. To my understanding, pic is a default cflag. 

I find that surprising. FWIW we set -fPIC manually for shared library builds
(grep for it in standalone.gypi).

> I just want to give an option to turn off it for atom, since pic brings
overhead
> on atom, due to indirectly access to global/static variables and non-static
> functions. This is somewhat specific to atom. 

The impact may be bigger on Atom, but AFAIK the way PIC works is not specific to
any platform.

> Yes, we may do this for atom on
> other OSs using cflags. Currently I just tested android.
> 
> Suggestions and comments are welcome.

Well, see my first reply for suggestions.

> BTW, I know this patch only affects standalone V8. Will chrome fetches v8 code
> from time to time?

Sure, we roll V8 into Chrome frequently, but Chrome doesn't use any of the files
this patch is touching. It just depends on V8's tools/gyp/v8.gyp and manages all
compiler flags itself (see Chromium's build/common.gypi).

[Lijuan Xiao, 2013-06-27 08:32:04]

Hi, in fact, I am not using the shared libary building option.-fPIC is preserved
option in GCC. (gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html). I agree with
you that the way PIC works is not specific to any platform. However PIC imposed
overhead for atom, while for other archs,at lease as I know, arm supports
PC-relative addressing, so it will not suffer. V8 atom build should be optimized
for atom. Maybe later we have other options specific to atom, which can also be
controlled with atom=true switch. Do you think it make sense? Thanks. -Lijuan