It could be lame question, but still... is it possible, that some
implementations (compiler/hardware) will print 1? (even 64bit doesn't
hold 20 decimal digits).?
In Fri, 23 Nov 2018 14:09:02 +0100
Robin Gareus <ro...@gareus.org> wrote:
> 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
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