Re: test failure for mappers.cc

[Jarno Rajahalme]

On Nov 10, 2010, at 14:04 , ext Ben Abbott wrote:

> On Nov 10, 2010, at 7:48 PM, Jarno Rajahalme wrote:
> 
>> On Nov 10, 2010, at 9:17 , ext Ben Abbott wrote:
>> 
>>> 
>>> I get the following
>>> 
>>> M_PI = 3.141592741012573
>>> (float) M_PI = 3.1415927 (0x40490fdb)
>>> atan2f (0.0f, -1.0f) = 3.14159250 (0x40490fda)
>>> atan2f (0.0f, -1.0f) - (float) M_PI = -0.00000024 (0xb4800000)
>>> 
>>> Looking through Apple's sources, I don't see a atan2f.c. But I do see 
>>> atan2f.s ...
>>> 
>>>     
>>> http://www.opensource.apple.com/source/Libm/Libm-315/Source/Intel/atan2f.s
>> 
>> Looking at that code, it seems it is working as specified. Note:
>> 
>> "Return a value in [-pi, +pi] (C 7.12.4.4 3).  Note that this
>> prohibits returning correctly rounded values for -pi and +pi, since
>> pi rounded to a float lies outside that interval."
>> 
>> On octave, I get:
>> 
>> octave:5> pi - double(single(pi))
>> ans = -8.7423e-08
>> 
>> which shows that pi rounded to a single actually is larger than pi as a 
>> double, and therefore outside of the range [-pi, pi].
>> 
>> Regards,
>> 
>> Jarno
> 
> ok. Perhaps this qualifies as an unintended feature?
> 
> Is there something that can be done to correct this behavior?
> 
> Ben
>               
> 


I should have said, that PI rounded to a single precision float is larger than 
PI, and therefore outside of range [-PI,PI], and an illegal return value for 
atan2 (). To have the result in the correct range, PI should be truncated to 
float, instead of rounding.

  Jarno