[cairo] Re: Some other bits...

[Bill Spitzak]

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.