RE: Problems VPATH, pow

["Davies, Daniel" <Daniel dot Davies at xerox dot com>]

Jeff,

Thanks!  I am building with --enable-newlib-hw-fp.

Dan 

-----Original Message-----
From: Jeff Johnston [mailto:jjohnstn@redhat.com] 
Sent: Thursday, September 01, 2005 11:00 AM
To: Davies, Daniel
Cc: newlib@sources.redhat.com
Subject: Re: Problems VPATH, pow

Ok Daniel, if I understand correctly, you are building newlib with
--enable-newlib-hw-fp.  Correct?

Anyway, I did find a problem with the mathfp pow routines as you had
mentioned.  They were attempting to do log (fabs (x)) in the x == 0.0
case which is incorrect.  I just checked in the attached patch so feel
free to try it out.

Using the regular libm/math pow routines, I am seeing the correct
result.

-- Jeff J.

Davies, Daniel wrote:
> Jeff,
> 
> Thanks for your very quick response!
> 
> Thanks also for pushing me into making a simple test case.  I 
> discovered along the way that there was a tangle of dependencies 
> between my RTOS and newlib that resulted in using the RTOS's buggy
version of modf.
> 
> When that was straightened out, I discovered that pow(0,2) works fine,

> but pow(0,0.5) fails.  That's what had actually triggered my first 
> bug, but I generalized the problem incorrectly based on my mis-reading

> of the pow source code.  Sorry!
> 
> #include <stdio.h>
> #include <math.h>
> 
> int
> main(int argc, char **argv)
> {
>     printf("pow (1,2) is %f\n", pow(1,2));
> 
>     printf("pow (0,2) is %f\n", pow(0,2));
> 
>     printf("pow (0,1) is %f\n", pow(0,1));
> 
>     printf("pow (0,0.5) is %f\n", pow(0,0.5));
> 
>     printf("pow (0,0.1) is %f\n", pow(0,0.1));
> 
>     printf("pow (4,0.5) is %f\n", pow(4,0.5));
> 
>     printf("pow (2,2) is %f\n", pow(2,2));
> 
>     return 0;
> }
> 
> Produces the following output on my system 
> (powerpc-unknown-eabialtivec, an embedded board.  The boards has a 
> 7447A.  The code is compiled with
> gcc-4.0.1 using hard floating point).
> 
> pow (1,2) is 1.000000
> pow (0,2) is 0.000000
> pow (0,1) is 0.000000
> pow (0,0.5) is 1.000000		<wrong>
> pow (0,0.1) is 1.000000		<wrong>
> pow (4,0.5) is 2.000000
> pow (2,2) is 4.000000
> 
> Dan
> 
>  
> 
> -----Original Message-----
> From: Jeff Johnston [mailto:jjohnstn@redhat.com]
> Sent: Tuesday, August 30, 2005 10:11 AM
> To: Davies, Daniel
> Cc: newlib@sources.redhat.com
> Subject: Re: Problems VPATH, pow
> 
> Davies, Daniel wrote:
> 
>>Sorry for the subject-less message - I hit the send button too soon. 
>>
>>-----Original Message-----
>>From: Davies, Daniel
>>Sent: Monday, August 29, 2005 1:57 PM
>>To: 'newlib@sources.redhat.com'
>>Cc: Davies, Daniel
>>Subject: 
>>
>>First, let me thank everyone involved with newlib, it has certainly 
>>made my life easier!
>> 
>>I'm cross compiling newlib 1.13.0 for powerpc-unknown-eabialtivec on 
>>i386-pc-solaris2.10.  I'm using version 3.80 of gnu make and gcc
> 
> 4.0.1.
> 
>> 
>>Two nits:
>> 
>>1. This is probably a solairs-ism.  The VPATH declaration in 
>>newlib-1.13.0/libgloss/rs6000/Makefile.in is
>> 
>>VPATH = @srcdir@ @srcdir@/..
>> 
>>This does not work in Solairs.
>> 
>>VPATH = @srcdir@:@srcdir@/..
>> 
>>(replaced space with colon) works.
>> 
> 
> 
> I have changed this over to use colon as gnu make accepts either space

> or colon.
> 
> 
>>2. pow(0, y) = 1 for all y > 0, e.g. 0^2 = 1.  It looks like pow 
>>doesn't check for this case, takes the log of 0 (which is undefined), 
>>gets a number very close to 0 instead of an approximation to minus 
>>infinity, later takes exp of this number and gets 1.
>>
> 
> 
> Can you provide a test that demonstrates your problem?  I have a 
> simple test which when run on mn10300 and linux, works as expected 
> (result of pow (0,2) is 0).
> 
> #include <stdio.h>
> #include <math.h>
> 
> int main (void)
> {
>    printf ("pow (1,2) is %g\n", pow (1, 2));
>    printf ("pow (0,2) is %g\n", pow (0, 2));
>    printf ("pow (2,2) is %g\n", pow (2, 2));
>    return 0;
> }
> ~
> bash-3.00$ ./a.out
> pow (1,2) is 1
> pow (0,2) is 0
> pow (2,2) is 4
> 
> 
> 
>>Dan
> 
>