[Bug math/2678] New: pow returns incorrect results on underflow

[Bug math/2678] New: pow returns incorrect results on underflow

[jsm28 at gcc dot gnu dot org]

pow(-DBL_MIN, 3) should be -0; similarly, pow(-DBL_MAX, -3) should be -0.  glibc
gets the sign of the 0 wrong here and in some such cases with large integer
exponent wrongly returns NaN.  Tested with current CVS glibc on
i686-pc-linux-gnu, originally seen on PowerPC in testing an earlier glibc
version with the ucbtest testsuite.

I'll attach a testsuite patch that adds some tests for these cases to illustrate
the problem.  (There are already such tests where the first argument of pow is
actually -0 or -infinity rather than -DBL_MIN or -DBL_MAX.)  This shows failures
such as in test-float.out (test-double.out, test-ldouble.out similar):
Failure: Test: pow (-min_value, 3) == -0
Result:
 is:          0.00000000000000000000e+00   0x0.00000000000000000000p+0
 should be:  -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  0.0000
Failure: Test: pow (-min_value, 16777215) == -0
Result:
 is:          0.00000000000000000000e+00   0x0.00000000000000000000p+0
 should be:  -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  0.0000
Failure: pow (-min_value, 1e10) == 0.0: Exception "Invalid operation" set
Failure: Test: pow (-min_value, 1e10) == 0.0
Result:
 is:          nan   nan
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
Failure: pow (-min_value, 1e20) == 0.0: Exception "Invalid operation" set
Failure: Test: pow (-min_value, 1e20) == 0.0
Result:
 is:          nan   nan
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
Failure: Test: pow (-max_value, -3) == -0
Result:
 is:          0.00000000000000000000e+00   0x0.00000000000000000000p+0
 should be:  -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  0.0000
Failure: Test: pow (-max_value, -16777215) == -0
Result:
 is:          0.00000000000000000000e+00   0x0.00000000000000000000p+0
 should be:  -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  0.0000
Failure: pow (-max_value, -1e10) == 0.0: Exception "Invalid operation" set
Failure: Test: pow (-max_value, -1e10) == 0.0
Result:
 is:          nan   nan
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
Failure: pow (-max_value, -1e20) == 0.0: Exception "Invalid operation" set
Failure: Test: pow (-max_value, -1e20) == 0.0
Result:
 is:          nan   nan
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0

-- 
           Summary: pow returns incorrect results on underflow
           Product: glibc
           Version: 2.4
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: jsm28 at gcc dot gnu dot org
                CC: glibc-bugs at sources dot redhat dot com


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[Bug math/2678] pow returns incorrect results on underflow

[jsm28 at gcc dot gnu dot org]

------- Additional Comments From jsm28 at gcc dot gnu dot org  2006-05-21 00:31 -------
Created an attachment (id=1035)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=1035&action=view)
Testsuite patch


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[Bug math/2678] pow returns incorrect results on underflow

[vincent+libc at vinc17 dot org]

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           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vincent+libc at vinc17 dot
                   |                            |org


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[Bug math/2678] pow returns incorrect results on underflow

[zimmerma+gcc at loria dot fr]

------- Additional Comments From zimmerma+gcc at loria dot fr  2009-09-24 09:42 -------
Created an attachment (id=4227)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=4227&action=view)
test case


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[Bug math/2678] pow returns incorrect results on underflow

[zimmerma+gcc at loria dot fr]

------- Additional Comments From zimmerma+gcc at loria dot fr  2009-09-24 09:43 -------
This is still wrong with glibc 2.9, and with 2.10.1 (development version):

pow(-2.225074e-308,3.000000e+00) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-2.225074e-308,1.677722e+07) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-2.225074e-308,1.000000e+10) = 0.000000 (0 expected) [mpfr 0.000000]
pow(-2.225074e-308,1.000000e+20) = 0.000000 (0 expected) [mpfr 0.000000]
pow(-1.797693e+308,-3.000000e+00) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-1.797693e+308,-1.677722e+07) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-1.797693e+308,-1.000000e+10) = 0.000000 (0 expected) [mpfr 0.000000]
pow(-1.797693e+308,-1.000000e+20) = 0.000000 (0 expected) [mpfr 0.000000]

Moreover when compiling with -O1 with GCC 4.3.2 (i.e., when constant folding is
activated) all tests except the first one are wrong too (but the values computed
by MPFR are correct):

libc version: 2.10.1
pow(-2.225074e-308,3.000000e+00) = -0.000000 (-0 expected) [mpfr -0.000000]
pow(-2.225074e-308,1.677722e+07) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-2.225074e-308,1.000000e+10) = 0.000000 (0 expected) [mpfr 0.000000]
pow(-2.225074e-308,1.000000e+20) = 0.000000 (0 expected) [mpfr 0.000000]
pow(-1.797693e+308,-3.000000e+00) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-1.797693e+308,-1.677722e+07) = 0.000000 (-0 expected) [mpfr -0.000000]
pow(-1.797693e+308,-1.000000e+10) = 0.000000 (0 expected) [mpfr 0.000000]
pow(-1.797693e+308,-1.000000e+20) = 0.000000 (0 expected) [mpfr 0.000000]


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[Bug math/2678] pow returns incorrect results on underflow

[zimmerma+gcc at loria dot fr]

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           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zimmerma+gcc at loria dot fr


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[Bug math/2678] pow returns incorrect results on underflow

[vincent+libc at vinc17 dot org]

------- Additional Comments From vincent+libc at vinc17 dot org  2009-09-24 10:43 -------
(In reply to comment #3)
> Moreover when compiling with -O1 with GCC 4.3.2 (i.e., when constant folding
> is activated)

It seems that constant folding on pow() isn't done by GCC yet. I interpret the
change due to -O1 by an optimization like pow(x,3) -> x * x * x (and glibc isn't
concerned since this is a GCC optimization).

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[Bug math/2678] pow returns incorrect results on underflow

[zimmerma+gcc at loria dot fr]

------- Additional Comments From zimmerma+gcc at loria dot fr  2009-09-24 14:06 -------
(In reply to comment #4)
> It seems that constant folding on pow() isn't done by GCC yet.

according to http://gcc.gnu.org/gcc-4.3/changes.html#mpfropts, pow is also
handled by the constant folding mechanism.



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[Bug math/2678] pow returns incorrect results on underflow

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=2678

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #7 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-03-28 15:03:05 UTC ---
Fixed by:

commit d6270972f79fe89a96fa7a3909991dad2e317033
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Mar 28 14:57:58 2012 +0000

    Fix pow of negative numbers to integer exponents (bugs 369, 2678, 3866).

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[Bug math/2678] pow returns incorrect results on underflow

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=2678

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|aj at suse dot de           |unassigned at sourceware
                   |                            |dot org

--- Comment #6 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-02-28 16:40:27 UTC ---
Confirmed still present.

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