[llvm-commits] [llvm] r49458 - /llvm/trunk/lib/Transforms/IPO/SimplifyLibCalls.cpp

[Chris Lattner]

On Apr 10, 2008, at 9:51 AM, Chris Lattner wrote:
> On Apr 10, 2008, at 9:47 AM, Duncan Sands wrote:
>> On Thursday 10 April 2008 18:21:24 Chris Lattner wrote:
>>> On Apr 10, 2008, at 4:25 AM, Duncan Sands wrote:
>>>>>>> +      // x    pow(x, 0.5)  sqrt(x)
>>>>>>> +      // ---------------------------------------------
>>>>>>> +      // -0.0    +0.0       -0.0
>>>>>>> +      // -inf    +inf       NaN
>>>>>>
>>>>>> looks like pow is broken.  Is pow's behaviour documented
>>>>>> anywhere?  On my machine the man page says:
>>>>>>
>>>>>>     The pow() function can return the following error:
>>>>>>
>>>>>>     EDOM   The  argument x is negative and y is not an integral
>>>>>> value.  This would result in a complex num‐
>>>>>>            ber.
>>>>>>
>>>
>>> -0.0 is negative, but it *is* an integral value, so EDOM doesn't  
>>> apply.
>>
>> You are confusing x and y.
>
> Whoa, you're right. I'll ping my numeric guru and ask why. :)


His response:

I stand behind the claim that the behavior of the pow(x, 0.5) -> sqrt  
substitution is incompatible with C99.  Here are the exact citations:

 From C99, Annex F (normative):

	F.9.4.4 The pow functions

	– pow(+-0, y) returns +0 for y > 0 and not an odd integer.
	– pow(-inf, y) returns +inf for y > 0 and not an odd integer.

	F.9.4.5 The sqrt functions

	sqrt is fully specified as a basic arithmetic operation in IEC 60559

 From IEC 60559 (IEEE 754-1985), section 5.2:

	... the square root of -0 shall be -0.

Does that pin things down enough for you?

– Steve