[FFmpeg-devel] -fsingle-precision-constant

Michael Niedermayer michaelni
Thu May 13 19:44:04 CEST 2010

On Thu, May 13, 2010 at 01:31:07PM -0400, Ronald S. Bultje wrote:
> Hi,
> 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

float func(float f){
    return f*1.2;

results in:

        unpcklps        %xmm0, %xmm0
        cvtps2pd        %xmm0, %xmm0
        mulsd   .LC0(%rip), %xmm0
        unpcklpd        %xmm0, %xmm0
        cvtpd2ps        %xmm0, %xmm0

vs with -fsingle-precision-constant
        mulss   .LC0(%rip), %xmm0

this is nuts

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is dangerous to be right in matters on which the established authorities
are wrong. -- Voltaire
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