Subject: Re: [PATCH] Makefile: set USE_PIC on Linux x86_64 for linking with Git.pm

From: Sergey Vlasov <[email-masked]>
Date: Thu, 29 Jun 2006 13:04:00 +0400

On Wed, 28 Jun 2006 14:24:37 -0700 Junio C Hamano wrote:

> Marco Roeland <[email-masked]> 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.