On Monday 08 August 2005 16:02, Marc A. Volovic wrote:
gcc -O2 -fno-math-errno -o /tmp/bnch1 /tmp/1.c -lm
See gcc(1) man page:
-fno-math-errno
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions.
YMMV, but the difference between running with gcc 3.4.3 was huge:
1). No optimization:
gcc -o /tmp/bnch-noop /tmp/drek.c -lm
real 0m5.067s
user 0m5.048s
sys 0m0.018s
2). With -O2 (or -O3):
gcc -O2 /tmp/bnch-O2 /tmp/drek.c -lm
real 0m4.440s
user 0m4.358s
sys 0m0.001s
3). With -O2(or -O3) and -fno-math-errno:
gcc -O2 -fno-math-errno -o /tmp/bnch-O2-no-math-errno /tmp/drek.c -lm
real 0m0.228s
user 0m0.226s
sys 0m0.002s
--Ariel
> Example of an idiotic benchmark:
>
> int
> main()
> {
> long long i;
> double q;
>
> for (i=0; i<10000000; i++) {
> q = sqrt(i);
> }
> }
>
> Under gcc 3.3.5 (Debian Sarge) this pile of drek executes in 1.4-1.8
> seconds (depending on -O level). Under icc 9.0 it executes between in
> 3.4 seconds for -O0 and -O1, and in 0.015 seconds for -O2.
>
> And don't tell me this is not a valid benchmark. I know. This is as
> artificial as I can get without using a wooden leg.
>
> M
>
>
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--
--
Ariel Biener
e-mail: [EMAIL PROTECTED]
PGP: http://www.tau.ac.il/~ariel/pgp.html
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