[Mesa3d-dev] gl_sqrt in mmath.c

[Klaus N. <kl...@ma...>]

Hi,

I have been thinking about making a faster 'log_2' function by programming
it myself instead of using the one in libc.
I started by looking at the gl_sqrt function that is being used in the
Mesa-library in the file 'src/mmath.c'.

I made a small program to test both the speed and the precision and at
home on a K6-2 with Debian-2.1 I had a rather terrible result: If I did
not use the compiler-option '-fstrict-aliasing' the libc-sqrt is almost
double as fast as the 'gl_sqrt', while the precision is o.k. 1% error or
so.

But with '-fstrict-aliasing', which was used in my auto-generated Makefile
for Mesa, the results for sqrt are for some numbers up to 10 times greater
than the once generated by gl_sqrt, while the speed for normal sqrt is
still higher.

So I wanted  to write you and tell this but then I just tried my program
here on a PIII whith RH7.0 I got complete different results: gl_sqrt is a
bit faster than sqrt, and 'strict-aliasing' has no effect on speed or
results. The error in the results is always up to 20%. If I use
'-mcpu=pentium-pro -march=pentium-pro' the precision is completely wrong
(20 or 30 digit number instead of 1.000).

Maybe you want to try yourself and find out if I did something wrong or
what could be happening.

To compile put the program below into Mesa/src and I compiled at home with

gcc -I../include -ffast-math -fexpensive-optimizations -fstrict-aliasing
-mcpu=k6 -march=k6 -DUSE_X86_ASM -DFAST_MATH -O3 test.c mmath.c -o test

The program is:


#include "mmath.h"
#include <time.h>

#define NEW_SQRT(X) gl_sqrt(X)
#define OLD_SQRT(X) ((float)sqrt(X))

int main()
{
float x,y;
clock_t start, end;

_mesa_init_math();

x = 0.0f;
y = 0.0f;

start = clock();
while (x<1000000.0f)
{
        y *= NEW_SQRT(x);
        x += 0.1f;
}
end = clock();

printf ("Time for mesa sqrt: %f\n", (float)(end-start)/CLOCKS_PER_SEC);

x = 0.0f;
y = 0.0f;

start = clock ();
while (x<1000000.0f)
{
        y *= OLD_SQRT(x);
        x += 0.1f;
}
end = clock();

printf ("Time for normal sqrt: %f\n", (float)(end-start)/CLOCKS_PER_SEC);

x= 1.0f;
while (x<100.0f)
{
printf("x = %f\tMesa_square(x)/standard_square(x) = %f\n", x,
OLD_SQRT(x)/NEW_SQ
RT(x));
x+=2.4;
}
}