[FFmpeg-devel] -fsingle-precision-constant

[Jason Garrett-Glaser]

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.

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