RE: Problems VPATH, pow

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

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