23 Nov
2018
23 Nov
'18
3:09 p.m.
On 11/23/2018 01:00 PM, Will Godfrey wrote:
[...]
Thanks for going into this in such detail Robin. I
never realised fp stuff
could be *quite* so, umm, approximate!
Depending on context and the maths, the difference may not matter at
all, or may be off completely..
float a = (1 + 1e20) - 1e20;
float b = 1 + 1e20; b -= 1e20;
printf ("%f %f\n", a, b);
Any guesses what this code prints without compiling it? :)
I was using O3. Following on from this I found that if
I removed either O3 or
ffast-math the problem disappeared.
OK, so SSE/AVX/compiler-intrinsics are the issue at hand.
It's quite possible that I'm over-thinking
this as there are actually only a
few iterations before the initial figures are re-calculated, but I don't like
mysteries :(
You could also disable this for a specific function only
__attribute__((optimize("-fno-fast-math")))
void different_mysteries_here (float val)
{
...
}
ciao,
robin