1. How do you ensure that the compiler did not optimize away the
assignment to q or the call to sqrt() due to recognition that it has no
side-effects (in 80x86 processors, there is a FP sqrt instruction so a
serious compiler knows to inline the function call and that it leaves no
side effects)?
2. Why not use q += sqrt(i) in the benchmark (solves the above problems
without special compiler options or obscure pragmas)?
On Mon, 2005-08-08 at 16:02 +0300, Marc A. Volovic wrote:
> 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.
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