Re: Reduce stack usage of _vfiprintf_r()

[Freddie Chopin <freddie_chopin at op dot pl>]

W dniu 2012-10-11 19:29, Corinna Vinschen pisze:
> I tend to prefer this as well.

I don't know why you're all against dynamic allocation - it is basically 
at the core of I/O streams...

Consider this scenario - there are 5 threads that do unbuffered I/O in 
emergency situations (let's say it's stderr). With allocation on stack, 
each thread has to have additional 1kB of stack, even if it will never 
be used. With dynamic allocation, in the worst case scenario (all 
threads doing unbuffered I/O at the exact same time, kernel preempting 
each of them in favor of another) some additional memory will be wasted 
(maybe 20B for thread. In "normal" situation overall usage of RAM will 
be smaller, and the fear of fragmentation is IMHO pointless - the buffer 
is freed right after the function ends, the risk of fragmentation is 
minimal and only in multithreaded environment - fragmentation is not 
possible with single thread.

sbrk() and malloc() will be pulled to executable in both cases, as they 
are used in other parts of stdio.

>  What about this patch?
>
> Index: libc/stdio/vfprintf.c
> ===================================================================
> RCS file: /cvs/src/src/newlib/libc/stdio/vfprintf.c,v
> retrieving revision 1.82
> diff -u -p -r1.82 vfprintf.c
> --- libc/stdio/vfprintf.c	8 Aug 2012 11:04:17 -0000	1.82
> +++ libc/stdio/vfprintf.c	11 Oct 2012 17:28:13 -0000
> @@ -333,8 +333,17 @@ int __sprint_r (struct _reent *, FILE *,
>    * Helper function for `fprintf to unbuffered unix file': creates a
>    * temporary buffer.  We only work on write-only files; this avoids
>    * worries about ungetc buffers and so forth.
> + *
> + * Make sure to avoid inlining when optimizing for size.
>    */
> -static int
> +#ifndef __OPTIMIZE_SIZE__
> +static
> +#else
> +#if defined (__GNUC__) && __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1)
> +__attribute__ ((__noinline__)) static
> +#endif
> +#endif /* __OPTIMIZE_SIZE__ */
> +int
>   _DEFUN(__sbprintf, (rptr, fp, fmt, ap),
>          struct _reent *rptr _AND
>          register FILE *fp   _AND

But it's not optimization for size, but optimization for memory use - in 
all places around newlib __OPTIMIZE_SIZE__ is for reducing code. Anyway 
- why make this fix conditional - there is virtually noone that would 
benefit from current situation...

If you're agains malloc(), then I guess there's nothing more I can do to 
convince you, so this patch is OK but only if unconditional (not only 
when __OPTIMIZE_SIZE__) - there's absolutely no point in leaving current 
behavior in any case.

4\/3!!