Re: pthread locking code - C vs. ASM

[Mike Frysinger <vapier at gentoo dot org>]

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

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