atomic_*_barrier macros

[Matt Turner]

The Xorg Xserver currently includes a lot of custom barrier macros for
MMIO. I have doubts about the correctness of some of them, and this
type of thing seems like something that would be provided by a central
library. So I checked to see if glibc had anything.

I found the atomic_{full,read,write}_barrier macros in atomic.h, but
(1) atomic.h isn't installed, and (2) not all platforms have all the
macros. I grepped through glibc.git and glibc-ports.git and determined
that:

None of the macros are provided -
- /sysdeps/i386/i486/bits/atomic.h
- /sysdeps/x86_64/bits/atomic.h
- /sysdeps/s390/bits/atomic.h
- Port: /sysdeps/m68k/m680x0/m68020/bits/atomic.h
- Port: /sysdeps/m68k/coldfire/bits/atomic.h
- Port: /sysdeps/arm/bits/atomic.h
- Port: /sysdeps/unix/sysv/linux/hppa/bits/atomic.h
- Port: /sysdeps/unix/sysv/linux/arm/nptl/bits/atomic.h

Only atomic_full_barrier -
- /sysdeps/ia64/bits/atomic.h
- Port: /sysdeps/unix/sysv/linux/arm/nptl/bits/atomic.h
- Port: /sysdeps/mips/bits/atomic.h

All barrier macros provided -
PowerPC{32,64}
Port: Alpha
SPARCv9-32
SPARC64

The Xserver currently has barrier macros for alpha, x86, amd64, ia64,
sparc, mips, arm, and powerpc -- so these are the most important.

What's the correct thing to do here? Could we add atomic macros for
needed platforms and export atomic.h?

Suggestions wanted.

Matt Turner