Re: BUG in libm's powf

From: Mark Murray <markm_at_FreeBSD.org>
Date: Tue, 7 Sep 2021 09:00:24 +0100
> On 6 Sep 2021, at 21:22, Gordon Bergling <gbergling_at_gmail.com> wrote:
> 
> could you turn to test program into an AFT test to prevent further regressions?

Certainly!

M

> 
> —Gordon
> 
>> Am 06.09.2021 um 20:04 schrieb Steve Kargl <sgk_at_troutmask.apl.washington.edu>:
>> 
>> No, thank you for the quick response.
>> 
>> Of course, a one character diff might be easier to review. :-)
>> 
>> --
>> steve
>> 
>> On Mon, Sep 06, 2021 at 06:55:07PM +0100, Mark Murray wrote:
>>> Thanks!
>>> 
>>> And it's committed!
>>> 
>>> M
>>> 
>>>> On 6 Sep 2021, at 18:53, Steve Kargl <sgk_at_troutmask.apl.washington.edu> wrote:
>>>> 
>>>> Fine with me.  I don't have a phabricator account and
>>>> bugzilla reports seems to get lost in the ether.
>>>> 
>>>> --
>>>> steve
>>>> 
>>>> On Mon, Sep 06, 2021 at 06:45:11PM +0100, Mark Murray wrote:
>>>>> Hi
>>>>> 
>>>>> I've opened a Phab ticket for this. I hope that's OK?
>>>>> 
>>>>> https://reviews.freebsd.org/D31865
>>>>> 
>>>>> M
>>>>> 
>>>>>> On 6 Sep 2021, at 16:28, Steve Kargl <sgk_at_troutmask.apl.washington.edu> wrote:
>>>>>> 
>>>>>> Paul Zimmermann has identified a bug in Openlibm's powf(),
>>>>>> which is identical to FreeBSD's libm.  Both derived from
>>>>>> fdlibm. https://github.com/JuliaMath/openlibm/issues/212.
>>>>>> 
>>>>>> Consider
>>>>>> 
>>>>>> % cat h.c
>>>>>> #include <math.h>
>>>>>> #include <stdio.h>
>>>>>> int
>>>>>> main(void)
>>>>>> {
>>>>>> float x, y, z;
>>>>>> x =  0x1.ffffecp-1F;
>>>>>> y = -0x1.000002p+27F;
>>>>>> z =  0x1.557a86p115F;
>>>>>> printf("%e %e %e <-- should be %e\n", x, y, powf(x,y), z);
>>>>>> return 0;
>>>>>> }
>>>>>> 
>>>>>> % cc -o h -fno-builtin h.c -lm && ./h
>>>>>> 9.999994e-01 -1.342177e+08 inf <-- should be 5.540807e+34
>>>>>> 
>>>>>> Note, clang seems to have a builtin for powf(), but one cannot
>>>>>> count of clang being the only consumer of libm.  With the patch
>>>>>> at the end of this email, I get
>>>>>> 
>>>>>> % cc -o h -fno-builtin h.c -L/home/kargl/trunk/math/libm/msun -lmath && ./h
>>>>>> 9.999994e-01 -1.342177e+08 5.540807e+34 <-- should be 5.540807e+34
>>>>>> 
>>>>>> Watch for copy and paste whitespace corruption.
>>>>>> 
>>>>>> --- /usr/src/lib/msun/src/e_powf.c	2021-02-21 03:29:00.956878000 -0800
>>>>>> +++ src/e_powf.c	2021-09-06 08:17:09.800008000 -0700
>>>>>> _at__at_ -136,7 +136,7 _at__at_
>>>>>>  /* |y| is huge */
>>>>>> 	if(iy>0x4d000000) { /* if |y| > 2**27 */
>>>>>> 	/* over/underflow if x is not close to one */
>>>>>> -	    if(ix<0x3f7ffff7) return (hy<0)? sn*huge*huge:sn*tiny*tiny;
>>>>>> +	    if(ix<0x3f7ffff6) return (hy<0)? sn*huge*huge:sn*tiny*tiny;
>>>>>> 	    if(ix>0x3f800007) return (hy>0)? sn*huge*huge:sn*tiny*tiny;
>>>>>> 	/* now |1-x| is tiny <= 2**-20, suffice to compute
>>>>>> 	   log(x) by x-x^2/2+x^3/3-x^4/4 */
>>>>>> 
>>>>>> 
>>>>>> --
>>>>>> Steve
>>>>>> 
>>>>> 
>>>>> --
>>>>> Mark R V Murray
>>>>> 
>>>> 
>>>> 
>>>> 
>>>> --
>>>> Steve
>>>> 
>>> 
>>> --
>>> Mark R V Murray
>>> 
>> 
>> 
>> 
>> --
>> Steve
>> 
> 

--
Mark R V Murray


Received on Tue Sep 07 2021 - 08:00:24 UTC

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