[LAU] building a debian system for audio

[Karl Hammar]

fons at kokkinizita.net:
> On Wed, May 23, 2007 at 10:33:12PM +0200, Karl Hammar wrote:
> > On Wed, May 23, 2007 at 10:00:38PM +0200, Fons Adriaensen wrote:
...
> > > float  a, b, x;
> > > int    y;
> > > 
> > >    y = (int)((x - a) * b + 0.5f);
> > > 
> > > I have this calculation twice, in the same source file. In one
> > > case it's within a for loop, in the second case it's just a single
> > > calculation. Both use exactly the same values for a, b, and x, yet
> > > the result is different (by 1). The value before the cast its something
> > > like 115.50424, and that gets rounded up in one case and down in the
> > > other.
> > > 
> > > This is with -O3. Same with -O2, -O1, but OK without optimisation, and
> > > also OK if I use -O3 -march=pentium4.
> > > 
> > > Is this to be expected, and should I use floorf() to avoid it, or is it
> > > something that should not happen ?
> > 
> > It might not be a bug. You are not guaranteed of the same order of
> > evaluation of arithmetic expressions in C. If you want that you have
> > to use FORTRAN or assembler (maybe some other language has it also).
> 
> 1. There is absolutely no ambiguity in order of execution of the statement
>    above.
> 
> 	- subtract a from x
> 	- multiply by b
> 	- add 0.5
>         - cast to int

No, but as others has discussed, you have used an optimiser.
Also, what are the values of x, a, b and f, or, what is the numerical
error of the sum? If it is greater than 0.004, you are basically
looking at noise. That is the reason it suggested using doubles.

> 2. In both cases the floating point result was the same. Only the case to int
>    was different.

Which cases?

Regards,
/Karl

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