seq_put_decimal_* optimization

seq_put_decimal_* optimization

[Ulrich Drepper]

If we want to squeeze out the last drop of slack from the /proc text
generation, shouldn't we then avoid code like

+       seq_put_decimal_ull(m, ' ', 0);

which is worse than what was there before? I realize that the code
actually becomes more readable like this.  So how about automatically
optimizing the code by adding something like the following to
seq_file.h:

#define seq_put_decimal_ull(m, delimiter, num) \
  (__builtin_constant_p(num) && \
   __builtin_constant_p(delimiter) && (delimiter) == ' ' \
   ? seq_write(m, " " #num, sizeof(" " #num) - 1) \
   : seq_put_decimal_ull(m, delimiter, num))

Completely untested and it is a minimal change.  It could be written
using an inline function and probably made more general that way
(e.g., handle more delimiters).  But this code should handle the case
that exist today and lead to a (very small) improvement without any
real costs to maintenance.

Re: seq_put_decimal_* optimization

[KAMEZAWA Hiroyuki]

(2012/03/24 21:52), Ulrich Drepper wrote:

> If we want to squeeze out the last drop of slack from the /proc text
> generation, shouldn't we then avoid code like
> 
> +       seq_put_decimal_ull(m, ' ', 0);
> 
> which is worse than what was there before? I realize that the code
> actually becomes more readable like this.  So how about automatically
> optimizing the code by adding something like the following to
> seq_file.h:
> 
> #define seq_put_decimal_ull(m, delimiter, num) \
>   (__builtin_constant_p(num) && \
>    __builtin_constant_p(delimiter) && (delimiter) == ' ' \
>    ? seq_write(m, " " #num, sizeof(" " #num) - 1) \
>    : seq_put_decimal_ull(m, delimiter, num))
> 
> Completely untested and it is a minimal change.  It could be written
> using an inline function and probably made more general that way
> (e.g., handle more delimiters).  But this code should handle the case
> that exist today and lead to a (very small) improvement without any
> real costs to maintenance.
> 

Could you wait a while ?

I'm now considering to add a binary interface /proc/stat.bin and
I would like to use seq_put_decimal_ull() as..
==
int seq_put_decimal_ull(struct seq_file *m, int delimiter, unsigned long long num)
{
	if (m->binstream_format)
		return seq_binstream_attend(num, sizeof(num));
	.....
}
==
Then, we can maintain /proc/stat and /proc/stat.bin in one function
fs/proc/stat.c::show_stat().
Maybe good for applications like top and sa..

Thanks,
-Kame