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Re: ffs16

Cross-posted to 2 groups.

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Joe keane wrote:
> In article<9kmj69-hhi2.ln1@ntp6.tmsw.no>,
> Terje Mathisen<"terje.mathisen at tmsw.no">  wrote:
>> When did you last time the uint16->float->exponent trick?
>
> just now
>
> time in us
>
> 'server'
> 239	testfx
> 270	testfy
>
> 'PC'
> 211	testfx
> 186	testfy

OK, so the 'x' version is 64K tests of the table lookup, while 'y' is 
the same number of fp conversions.

Problem one: The table lookups will have 100% cache hits since you are 
doing nothing else, so you don't measure the misses you'll get in a real 
program.

Problem two: The tests have a really funky way to randomize the tests 
(multiply by 31), which means that the table lookup version will have 
very close to 100% branch prediction hits for the pattern of switching 
between the top and low byte.

Problem three: How common will zero be?

This affects the branch hit ratio for the fp version.

All that said, you are getting effectively the same speed for both 
versions, and the fp code has zero table space so better cache behavior.

Terje

>
> -- fhsx.c
> static const char fhstab[256] =
> {
>       99, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
>       4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
>       5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
>       5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
> [...]
> };
>
>
> extern int kk_fhsx(int mask)
> {
>      int ret;
>
>      ret = (mask&  0xFF00) != 0 ? fhstab[mask>>  8&  0xFF] + 8 :
> 	fhstab[mask&  0xFF];
>      return ret;
> }
>
> -- fhsy.c
> union u
> {
>      float f;
>      unsigned int u;
> };
>
>
> extern int kk_fhsy(int mask)
> {
>      union u t;
>      int bex;
>      int ret;
>
>      t.f = (float) mask;
>      bex = t.u>>  23;
>      ret = bex - 127;
>      return ret;
> }
>
> -- testfx.c
> #include<stdio.h>
>
> static void testfx(int *sump);
> extern int kk_fhsx(int mask);
>
>
> [main]
>
>
> static void testfx(int *sump)
> {
>      int m;
>
>      for (m = 0x1; m<= 0xFFFF; m++)
>      {
> 	int mask;
> 	int ret;
>
> 	mask = 31 * m&  0xFFFF;
> 	ret = kk_fhsx(mask);
> 	/* *sump += ret; */
>      }
> }
>
> -- testfy.c
> #include<stdio.h>
>
> static void testfy(int *sump);
> extern int kk_fhsy(int mask);
>
>
> [main]
>
>
> static void testfy(int *sump)
> {
>      int m;
>
>      for (m = 0x1; m<= 0xFFFF; m++)
>      {
> 	int mask;
> 	int ret;
>
> 	mask = 31 * m&  0xFFFF;
> 	ret = kk_fhsy(mask);
> 	/* *sump += ret; */
>      }
> }


-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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ffs16 jgk@panix.com (Joe keane) - 2012-04-22 21:36 +0000
  Re: ffs16 Keith Thompson <kst-u@mib.org> - 2012-04-22 15:23 -0700
  Re: ffs16 Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-22 20:37 -0400
    Re: ffs16 Philip Lantz <prl@canterey.us> - 2012-04-24 06:36 -0700
      Re: ffs16 Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-24 20:32 -0400
        Re: ffs16 Philip Lantz <prl@canterey.us> - 2012-04-25 00:30 -0700
  Re: ffs16 Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-23 16:22 +0200
    Re: ffs16 Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-23 21:46 -0400
      Re: ffs16 Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-24 14:16 +0200
        Re: ffs16 Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-04-24 20:40 -0400
    Re: ffs16 jgk@panix.com (Joe keane) - 2012-04-24 19:06 +0000
      Re: ffs16 Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-25 00:38 +0200
        Re: ffs16 jgk@panix.com (Joe keane) - 2012-04-25 20:08 +0000
          Re: ffs16 "James Van Buskirk" <not_valid@comcast.net> - 2012-04-25 15:06 -0600
            Re: ffs16 "James Van Buskirk" <not_valid@comcast.net> - 2012-04-25 16:30 -0600
        Re: ffs16 jgk@panix.com (Joe keane) - 2012-04-25 21:25 +0000
          Re: ffs16 Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-26 01:38 +0200
            Re: ffs16 jgk@panix.com (Joe keane) - 2012-04-26 19:34 +0000
            Re: ffs16 jgk@panix.com (Joe keane) - 2012-04-26 20:41 +0000
              Re: ffs16 Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-27 02:47 +0200
  Re: ffs16 MitchAlsup <MitchAlsup@aol.com> - 2012-04-23 13:01 -0700
    Re: ffs16 Terje Mathisen <"terje.mathisen at tmsw.no"> - 2012-04-24 03:28 +0200
      Re: ffs16 nmm1@cam.ac.uk - 2012-04-24 09:15 +0100
      Re: ffs16 Chris Gray <cg@GraySage.com> - 2012-04-24 13:37 -0600

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