Re: pthread locking code - C vs. ASM

[Clemens Kolbitsch <clemens dot kol at gmx dot at>]

On Saturday 19 January 2008 21:33:13 Mike Frysinger wrote:
> On Saturday 19 January 2008, Clemens Kolbitsch wrote:
> > Why are some (e.g. locking) functions implemented twice -- once in C and
> > once in assembler?
>
> why is anything written in assembler ?  for optimization.
>
> > Is the C code used to get an asselber starting point
> > which is then optimized and saved into a second file?
>
> that is one way of looking at it.  you write the generic C version for
> everyone to use and if there's enough interest, re-implement it for a
> specific operating system/architecture in assembly.  if an appropriate
> optimized version exists in the source tree, glibc will use that, otherwise
> it'll default to the generic C version.
>
> > For example pthread_rwlock_unlock is implemented in
> > source/nptl/sysdeps/unix/sysv/linux/i386/i486/pthread_rwlock_unlock.S
> > and
> > source/nptl/pthread_rwlock_unlock.c
> >
> > If I modify the asm code only, is there a chance that some script (e.g.
> > during make clean && make) will overwrite my changes or is the ASM code
> > the correct place to edit things (otherwise I don't see my modifications
> > being linked into the binary...)?
>
> no, that is not how the build system works.  pretty much all assembly code
> in glibc is hand written and not autogenerated.  glibc uses search paths
> depending on your operating system, target architecture, and specific cpu
> to figure out which implementation to use.
>
> if your target operating system is linux, the target architecture is i386,
> and the specific cpu implementation is i486, then
> nptl/sysdeps/unix/sysv/linux/i386/i486/pthread_rwlock_unlock.S is the file
> to modify.  if your target is {linux,i386,i686}, then you should actually
> look at nptl/sysdeps/unix/sysv/linux/i386/i686/pthread_rwlock_unlock.S.
> -mike

perfect ... that's what I thought.. but just wasn't sure!
Thanks!!