[Bug math/3866] New: pow(double, double) does not fulfil several of C99's requirements

[Bug math/3866] New: pow(double, double) does not fulfil several of C99's requirements

[kreckel at ginac dot de]

Here is a list of bugs in GlibCs pow implementation.


1) pow(-1, 1e100):

Returns NaN and raises the "invalid" floating-point exception.

It should return 1 and raise no exception. (This is because all large
floating-point numbers are even integers if the radix is even. This
motivates pow(-1, inf) = +1, which is treated correctly by GlibC).


2) pow(-1, -1e100):

Same as above.


3) pow(-3.141592, 1e100):

Returns NaN and raises the "invalid" floating-point exception.

It should return +inf and raise the "overflow" floating-point exception flag.


4) pow(-3.141592, -1e100):

Returns NaN and raises the "invalid" floating-point exception.

It should return +0 and raise the "underflow" floating-point exception flag.



4) pow(-0, -1e100):

Raises the "invalid" floating point exception.

It shouldn't do that: this is just a case of exact division by
zero. Division by zero is correctly flagged and +inf is returned, but
the "invalid" flag is wrong.



5) pow(-inf, -1e100):

Raises the "invalid" floating point exception.

It shouldn't do that. After all, it correctly returns 0.


6) pow(-inf, 1e100):

Raises the "invalid" floating point exception.

It shouldn't do that. After all, it correctly returns +inf.


7) pow(-0, -9.0071991544401990e+15)

This returns +inf.

It should return -inf, since the double precision exponent is an odd number
(fabs(fmod(-9.0071991544401990e+15,2.0))==1.0 exactly).

-- 
           Summary: pow(double, double) does not fulfil several of C99's
                    requirements
           Product: glibc
           Version: 2.4
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: kreckel at ginac dot de
                CC: glibc-bugs at sources dot redhat dot com
  GCC host triplet: i386-linux-gnu


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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[mtk dot manpages at gmail dot com]

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http://sourceware.org/bugzilla/show_bug.cgi?id=3866

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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[mtk dot manpages at gmail dot com]

------- Additional Comments From mtk dot manpages at gmail dot com  2008-08-05 14:05 -------
Created an attachment (id=2892)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=2892&action=view)
test program

Just to add a little detail here regarding these four points:

1) pow(-1, 1e100):
2) pow(-1, -1e100):
3) pow(-3.141592, 1e100):
4) pow(-3.141592, -1e100):

If x is less than -0, then (on x86, glibc 2.8) pow() fails with EDOM/FE_INVALID
for any y whose absolute value is greater than about 9.223373e18. 
(EDOM/FE_INVALID == pole error, which is the error that occurs if x == 0, and y
is negative)

-- 


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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[mtk dot manpages at gmail dot com]

------- Additional Comments From mtk dot manpages at gmail dot com  2008-08-05 14:19 -------
Regarding:
4) pow(-0, -1e100)

this problem seems to be absent on glibc 2.8.  There, pow(-0, -1e100) does the
right thing, returning -Inf and giving EDOM/FE_DIVBYZERO.

-- 


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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[mtk dot manpages at gmail dot com]

------- Additional Comments From mtk dot manpages at gmail dot com  2008-08-05 14:21 -------
Regarding:
5) pow(-inf, -1e100):
6) pow(-inf, 1e100):

The problem persists in glibc 2.8, and again the cut-off y value that
demonstrates this behavior is |y| greater than about  9.223373e18.


-- 


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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[kreckel at ginac dot de]

------- Additional Comments From kreckel at ginac dot de  2008-08-05 22:04 -------
(In reply to comment #2)
> 4) pow(-0, -1e100)
> 
> this problem seems to be absent on glibc 2.8.  There, pow(-0, -1e100) does the
> right thing, returning -Inf and giving EDOM/FE_DIVBYZERO.

But the right thing to do would return +Inf and giving EDOM/FE_DIVBYZERO, as
-1e100 is < 0 and not an odd integer (specified in. F.9.4.4, second dash.)

-- 


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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[mtk dot manpages at gmail dot com]

------- Additional Comments From mtk dot manpages at gmail dot com  2008-08-08 05:48 -------
(In reply to comment #4)
> (In reply to comment #2)
> > 4) pow(-0, -1e100)
> > 
> > this problem seems to be absent on glibc 2.8.  There, pow(-0, -1e100) does 
the
> > right thing, returning -Inf and giving EDOM/FE_DIVBYZERO.
> But the right thing to do would return +Inf and giving EDOM/FE_DIVBYZERO, as
> -1e100 is < 0 and not an odd integer (specified in. F.9.4.4, second dash.)

Oops.  Yes, of course you are right.  And glibc 2.8 is doing the right thing, 
I just wrote the wrong thing. pow(-0, -1e100) gives +Inf with 
EDOM/FE_DIVBYZERO.

-- 


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[Bug math/3866] pow(double, double) does not fulfil several of C99's requirements

[vincent+libc at vinc17 dot org]

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