[RFC] bb-wide errno optimization

[Rich Felker]

while reading ash.c i noticed an interesting hack to save some space:

----------------------------------------------------------------------
#ifdef __GLIBC__
/* glibc sucks */
static int *dash_errno;
#undef errno
#define errno (*dash_errno)
#endif

...

ash_main(...)
{
...
#ifdef __GLIBC__
        dash_errno = __errno_location();
#endif
----------------------------------------------------------------------

while this looks ugly and nonportable i think it's possible to do a
portable version of this that would work with any libc and prevent the
overhead of accessing errno through a function call or tls variable.
(admittedly the latter is not as expensive but not free either). the
idea is something like:

----------------------------------------------------------------------
/* in libbb.h */

#ifndef IN_BB_MAIN
extern int *bb_errno;
#undef errno
#define errno (*bb_errno)
#endif

/* in main() */

bb_errno = &errno;
----------------------------------------------------------------------

comments?

rich