Re: Patch to add __builtin_printf -> fputs transformations

["Kaveh R. Ghazi" <ghazi at caip dot rutgers dot edu>]

 > From: Bruce Korb <bkorb@cruzio.com>
 > 
 > > Nevertheless, my testing on solaris2.7 indicates that
 > > fputs("hello-world\n",stdout) in isolation is about 40% faster than
 > > printf("hello-world\n").
 > 
 > I think this speaks of libc problems.  *SOME* additional overhead
 > is certainly understandable.  40% (in unrolled loops, I assume)?
 > Naw.  I suppose I should read code before spouting off, but really?

Really.

Of course the libc matters but I see improvement everywhere.  Here's a
contrived program written to maximally expose stdio times.  (Note
fputs->fwrite is a future builtin opt I plan to submit, its not
currently done but is provided to show that further improvements are
possible.)  Compile this with gcc -O2 -funroll-loops:

#include <stdio.h>
#define MAX_ITER 5000000
#if defined(FWRITE)
#define STMT(string) fwrite(string,1,__builtin_strlen(string),stdout)
#elif defined(FPUTS)
#define STMT(string) fputs(string, stdout)
#else
#define STMT(string) printf(string)
#endif
int main()
{
  int i;
  for (i=0; i < MAX_ITER; i++)
    STMT ("hello world\n");
  return 0;
}

I tuned MAX_ITER on each platform so that the program ran for about
5-15 user+sys seconds using printf.  That gives me a good S/N ratio
while not filling up my disk.  So the numbers are only relatively
meaningful across, not down between arches.

The numbers below were generated using "time a.out > foo".

arch		printf	fputs	fwrite	fputs/printf	fwrite/printf
----		------	-----	------	------------	-------------
x86-linux	15.11	13.29	7.93	.88		.525
solaris2.7	12.94	8.39	6.98	.64		.539
irix6.2		7.822	5.68	4.879	.73		.624

Solaris seems to benefit the most from the printf->fputs conversion,
it got a 36% drop in this particular test.  But look closely at the
glibc-linux drop from printf->fwrite, it almost cut in half.

		--Kaveh
--
Kaveh R. Ghazi			Engagement Manager / Project Services
ghazi@caip.rutgers.edu		Qwest Internet Solutions