From: Sergey Vlasov <vsu@altlinux•ru>
To: Junio C Hamano <junkio@cox•net>
Cc: Marco Roeland <marco.roeland@xs4all•nl>,
git@vger•kernel.org, pasky@suse•cz
Subject: Re: [PATCH] Makefile: set USE_PIC on Linux x86_64 for linking with Git.pm
Date: Thu, 29 Jun 2006 13:04:00 +0400 [thread overview]
Message-ID: <20060629130400.c280de67.vsu@altlinux.ru> (raw)
In-Reply-To: <7vbqsdynvu.fsf@assigned-by-dhcp.cox.net>
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On Wed, 28 Jun 2006 14:24:37 -0700 Junio C Hamano wrote:
> Marco Roeland <marco.roeland@xs4all•nl> writes:
>
> > Even for Linux someone mentioned that probably i386 is the exception in
> > _not_ needing the -fPIC linkage. It might even be specific to the Perl
> > "xs" implementation specifics?
In general, -fPIC is required when building shared libraries. On some
systems (e.g., Linux/i386) you can get away without -fPIC, but with a
penalty on memory use and load time: non-PIC code will need relocations,
therefore its pages will no longer be shared between different
processes, and relocations will be performed immediately after loading
the shared library.
> USE_PIC is for pleasing Perly git and nothing else right now.
>
> > So I should have added "Works for me (TM)"! ;-)
>
> That would have been more explicit way to tell me that this is a
> partial solution and I should solicit help from people on other
> platforms.
>
> By the way, I had an impression that compiling things with -fPIC
> when not necessary was generally a bad idea from performance
> point of view. If that is the case we might want to compile,
> under USE_PIC, everything with -fPIC in a separate area to
> compile and link with Git.xs, without affecting the C-only core
> code.
This is exactly what libtool does (if both static and shared libraries
are compiled, each file is compiled twice - once with -fPIC -DPIC, and
once without these options).
But I suspect that even libtool won't help with Perl anyway, unless we
create a proper libgit.so and then link our Perl extension with it.
> I suspect this would largely depend on the architecture. I ran
> git-fsck-objects compiled with and without -fPIC (after "make
> clean" to rebuild everything) on a fully packed copy of the
> linux-2.6 repository on my x86_64 box, and did not see
> meaningful differences:
>
> : gitster; /usr/bin/time ../git.junio/git-fsck-objects-no-pic --full
> 109.71user 5.01system 1:54.89elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (14major+1834967minor)pagefaults 0swaps
> : gitster; /usr/bin/time ../git.junio/git-fsck-objects-with-pic --full
> 109.05user 4.97system 1:54.08elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (0major+1834981minor)pagefaults 0swaps
> : gitster;
This is because most of time is spent inside SHA-1 and zlib routines,
which are the same in these cases. Using mozilla-sha1 code might show
some difference.
And the effect of -fPIC on x86_64 is smaller than on i386, because
x86_64 has 2x more registers than i386, therefore loss of one register
is less noticeable.
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next prev parent reply other threads:[~2006-06-29 9:04 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-06-28 18:35 [PATCH] Makefile: set USE_PIC on Linux x86_64 for linking with Git.pm Marco Roeland
2006-06-28 18:53 ` Junio C Hamano
2006-06-28 18:59 ` Junio C Hamano
2006-06-28 19:21 ` Marco Roeland
2006-06-28 19:55 ` [PATCH] Makefile: set USE_PIC on Linux x86_64 for linking with Git.xs Marco Roeland
2006-06-28 20:52 ` [PATCH] Makefile: set USE_PIC on Linux x86_64 for linking with Git.pm Pavel Roskin
2006-06-29 22:22 ` Jakub Narebski
2006-06-30 0:03 ` Pavel Roskin
2006-06-30 0:29 ` Jakub Narebski
2006-06-28 19:08 ` Marco Roeland
2006-06-28 21:24 ` Junio C Hamano
2006-06-29 9:04 ` Sergey Vlasov [this message]
2006-06-29 9:58 ` Josef Weidendorfer
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