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Re: Use of lrint() in 'gdk-pixbuf/pixops/pixops.c'




I suggest adding a lrintf() fallback to fallback-c89.c too.

Currently, there are 2 rounding functions in the fall backs, round()
and rint(), with rint() having the better less biased IEEE
round-to-even behavior for the 0.5 case.

lrintf() is preferable because it can be done in a single instruction
(round and convert to int) on x86, while (int)round(x) would need 2
instructions.

I suggest the following implementation of lrintf():

inline int lrintf(float x)
{
    // warning: assumes processor's rounding mode is set to nearest int
#if defined(__SSE__) && defined(__GNUC__)
    // single instruction version that avoids conversion of float to SSE vector
    int rounded;
    __asm__ (
             "cvtss2si %1, %0\n"  // most compilers are smart enough
to convert this to vcvtss2si to avoid expensive AVX-SSE transition
penalties
             : "=r"(rounded)
             : "x"(x)
            );
    return rounded;
#elif defined(__SSE__)
    return _mm_cvtss_si32(_mm_set_ss(x));
#else
    return rintf(x);
#endif
}


On Sat, Feb 4, 2017 at 12:17 PM, John Emmas via gtk-devel-list
<gtk-devel-list@xxxxxxxxx> wrote:
> On 4 Feb 2017, at 19:44, Emmanuele Bassi wrote:
>
>>
>> Please, file a bug against gdk-pixbuf:
>>
>>  https://bugzilla.gnome.org/enter_bug.cgi?product=gdk-pixbuf
>>
>> I'd rather have a check at configure-time that looks if we have
>> lrint() available, and if not, provides a fallback. We used this
>> approach in GTK+ 3.x, and it makes it easier to remove the code once
>> we decide to bump the compiler requirements, like we did in GTK+
>> master.
>>
>
> Thanks Emmanuele - just filed bug #778181
>
> John
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> gtk-devel-list@xxxxxxxxx
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