[FFmpeg-devel] -fsingle-precision-constant

[Michael Niedermayer]

On Thu, May 13, 2010 at 12:51:53PM -0700, Jason Garrett-Glaser wrote:
> On Thu, May 13, 2010 at 12:02 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Thu, May 13, 2010 at 07:28:18PM +0100, M?ns Rullg?rd wrote:
> >> Michael Niedermayer <michaelni at gmx.at> writes:
> >>
> >> > On Thu, May 13, 2010 at 08:53:27PM +0300, Uoti Urpala wrote:
> >> >> On Thu, 2010-05-13 at 19:44 +0200, Michael Niedermayer wrote:
> >> >> > On Thu, May 13, 2010 at 01:31:07PM -0400, Ronald S. Bultje wrote:
> >> >> > > On Thu, May 13, 2010 at 1:13 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> >> >> > > > when i looked at the asm gcc generates for mp3float i noticed
> >> >> > > > it converts floats to doubles all over the place. ?Adding
> >> >> > > > -fsingle-precision-constant makes the code a whole 10% faster
> >> >> > > > overall on core2
> >> >> > >
> >> >> > > What if you add "f" as a postfix to relevant constants? Or can you
> >> >> > > give examples of functions where this is an issue so we can be more
> >> >> > > helpful?
> >> >> >
> >> >> > all functions i guess
> >> >> >
> >> >> > like
> >> >> > float func(float f){
> >> >> > ? ? return f*1.2;
> >> >> > }
> >> >>
> >> >> Here "1.2" is a constant of type "double", and since one operand is a
> >> >
> >> > sure
> >> > what pisses me off is that the compiler acts very unreasonable in practical
> >> > terms for an optimizing compiler.
> >> > floating point calculations are not exact theres no point to
> >> > convert every float that is multiplied by a constant to double before the
> >> > multiply and back to float afterwards. This just doesnt gain much, its
> >> > unreasonable and unexpected behavior.
> >> >
> >> >> double the multiplication is also a multiplication at double precision.
> >> >> As 1.2 is not exactly representable it is not possible to optimize it to
> >> >> a float without losing precision.
> >> >
> >> > floating point is inherently approximate, thus limiting optimizations to
> >> > being binary identical to some arbitrarily picked approximation is quite
> >> > silly besides gcc doesnt do this in other cases and i seriously doubt that
> >> > other compilers optimize floating point code so narrowly
> >>
> >> Silly as it is, that is what the relevant standards mandate, and GCC
> >> cannot do things differently. ?Complain to the ISO C and IEEE754
> >> committees instead.
> >
> > does C really mandate the implementation to use double precision?
> > i mean it can mandate the high level type be double but what is
> > preventing the compiler from using a very slightly lower precission
> > float implemenattion.
> > i ve heared intels c compiler will even use trigonometric identities
> > to optimize float code ...
> 
> That's what -ffast-math is for.

i meant that what ive heard is intels compiler does this by default
and as mans said -ffast-math does a few things too much

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When the tyrant has disposed of foreign enemies by conquest or treaty, and
there is nothing more to fear from them, then he is always stirring up
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