Re: Use of lrint() in 'gdk-pixbuf/pixops/pixops.c'
- Date: Sat, 4 Feb 2017 19:59:49 -0800
- From: Yale Zhang <yzhang1985@xxxxxxxxx>
- Subject: 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
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
"cvtss2si %1, %0\n" // most compilers are smart enough
to convert this to vcvtss2si to avoid expensive AVX-SSE transition
On Sat, Feb 4, 2017 at 12:17 PM, John Emmas via gtk-devel-list
> On 4 Feb 2017, at 19:44, Emmanuele Bassi wrote:
>> Please, file a bug against 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+
> Thanks Emmanuele - just filed bug #778181
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