[cairo] Re: Some other bits...
Bill Spitzak
spitzak at d2.com
Wed Aug 17 12:16:26 PDT 2005
Bertram Felgenhauer wrote:
> On 8/17/05, Bill Spitzak <spitzak at d2.com> wrote:
>
>>int
>> _cairo_fixed_integer_part (cairo_fixed_t f)
>> {
>> return f >> CAIRO_FIXED_FLOAT_BITS;
>> }
>>
>> int
>> _cairo_fixed_integer_floor (cairo_fixed_t f)
>> {
>> if (f >= 0)
>> return f >> CAIRO_FIXED_FLOAT_BITS;
>> else
>> return -((-f - 1) >> CAIRO_FIXED_FLOAT_BITS) - 1;
>> }
>>
>>These functions return identical values! I.e. integer_part() is
>>returning floor(), in a much more efficient way.
>
>
> A problem here is that the C standard does not define the
> result of >> when applied to negative numbers, making this
> optimization unportable. It's certainly a good optimization
> when it works though.
But then _integer_part would not work for negative numbers either, it
would have to be implemented like _integer_floor above. And all the
other code that uses >> would fail on negative numbers and have to be
rewritten as well.
In fact I'm fairly certain that this assumption happens tens of
thousands of times in Cairo and X and every program that will use Cairo,
thus trying to fix it is futile and serves only to slow programs down
and make the code hard to read.
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