On Wed, 28 Jun 2006 14:24:37 -0700 Junio C Hamano wrote: > Marco Roeland <marco.roeland@xxxxxxxxx> 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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