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Re: test failure for mappers.cc
From: |
Jarno Rajahalme |
Subject: |
Re: test failure for mappers.cc |
Date: |
Wed, 10 Nov 2010 23:41:17 +0200 |
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