Re: [LAD] Is -ffast-math safe for audio?

On Sat, 24 Nov 2018 11:14:06 +0100
Fons Adriaensen <fons@linuxaudio.org> wrote:
<snip>
>The dangerous thing to do is:
>
>// given float p
>
>int i = (int) floorf (p);
>float f = fmodf (p, 1.0f);
>
>as you could end up with i + f != p.
>
>The safe way is of course:
>
>int i = (int) floorf (p);
>float f = p - i;
>
>
>Ciao,
>

Thanks very much for that Fons :)

I'd been mulling over *exactly* that point for some time. My reasoning being
that in the latter case, if the integer was slightly wrong then using it for
the subtraction should give a remainder that was also slightly wrong, but in a
direction that tends to compensate for the error.

The other thing, is why do we need the floorf? Or in the original example
(which was taken from working code) truncf?

A straight cast to int seems to give exactly the same result, and is at least
twice as fast with -O3 and about 4 times as fast unoptimised.

-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
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