[Bug math/14471] New: Inaccurate y0f function on x86_32

[Bug math/14471] New: Inaccurate y0f function on x86_32

[liubov.dmitrieva at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14471

             Bug #: 14471
           Summary: Inaccurate y0f function on x86_32
           Product: glibc
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: liubov.dmitrieva@gmail.com
    Classification: Unclassified


Test case:

// gcc main.c -std=c99 -lm -m32

#include <stdio.h>
#include <fenv.h>
#include <stdlib.h>  // abs function for int argument

extern float y0f(float);
extern double y0(double);

#define i32(a)    (((int*)&(a))[0])

int main() {
    float a,r1,r2;

    i32(a)=0x4155c70d;    //argument

    fesetround(FE_TONEAREST);

    r1 = y0f(a);  //actual result
    r2 = y0(a);   //actual result 2

    printf("inputs: a = (0x%08x)\t%e\n", i32(a), a);
    printf("actual    = (0x%08x)\t%.13e\n", i32(r1), r1);
    printf("expected  = (0x%08x)\t%.13e\n", i32(r2), r2);
    printf("error     = %d ulp\n", abs(i32(r1) - i32(r2)));

    return 0;
}

Result:

(core 2 for example):


-bash-4.2$ gcc main_y0f.c -lm -odo -m32
-bash-4.2$ ./do
inputs: a = (0x4155c70d)        1.336110e+01
actual    = (0xb4801c51)        -2.3862460807322e-07
expected  = (0xb47ff7c8)        -2.3838867946324e-07
error     = 9353 ulp

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[Bug math/14471] Inaccurate y0f function on x86_32

[liubov.dmitrieva at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14471

--- Comment #1 from Liubov Dmitrieva <liubov.dmitrieva at gmail dot com> 2012-08-15 14:33:50 UTC ---
And x86_64 result (core2):

-bash-4.2$ gcc main_y0f.c -lm -odo
-bash-4.2$ ./do
inputs: a = (0x4155c70d)        1.336110e+01
actual    = (0xb4800855)        -2.3847920260778e-07
expected  = (0xb47ff7c8)        -2.3838867946324e-07
error     = 4237 ulp

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[Bug math/14471] Inaccurate y0f function on x86_32 and x86_64

[liubov.dmitrieva at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14471

Liubov Dmitrieva <liubov.dmitrieva at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Inaccurate y0f function on  |Inaccurate y0f function on
                   |x86_32                      |x86_32 and x86_64

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[Bug math/14471] Inaccurate y0f function on x86_32 and x86_64

[liubov.dmitrieva at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14471

--- Comment #2 from Liubov Dmitrieva <liubov.dmitrieva at gmail dot com> 2012-08-16 08:16:33 UTC ---
>> represent the test argument using a hex float value
Updated test:


#include <stdio.h>
#include <math.h>
#include <fenv.h>

extern float y0f(float);
extern double y0(double);

int main() {
    float a,r1;
    double r2;

    a=0x1.ab8e1ap3;    //argument

    fesetround(FE_TONEAREST); // FE_TONEAREST, FE_UPWARD, FE_DOWNWARD, and
FE_TOWARDZERO

    r1 = y0f(a);  //actual result
    r2 = y0(a);   //actual result 2

    printf("inputs: a = %.6a\n", a);
    printf("actual    = %.6a\n", r1);
    printf("expected  = %.6a\n", r2);
    printf("error     = %f ulp\n", fabs(r1 - r2) * exp2(23.0-logbf(r1)));
    return 0;
}


Result x86:
inputs: a = 0x1.ab8e1ap+3
actual    = -0x1.0038a2p-22
expected  = -0x1.ffef8fp-23
error     = 8301.219724 ulp

Result x86_64:
inputs: a = 0x1.ab8e1ap+3
actual    = -0x1.0010aap-22
expected  = -0x1.ffef8fp-23
error     = 3185.219724 ulp

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[Bug math/14471] Inaccurate y0f function on x86_32 and x86_64

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=14471

Paul Zimmermann <zimmerma+gcc at loria dot fr> changed:

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

--- Comment #3 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
another example:

Testing function y0 for exponent 0 [seed=4519].
 rounding mode MPFR_RNDZ:
      wrong underflow flag for x=e.4c1760@-1
      library gives 0
      mpfr    gives 3.274468@-7
      underflow: mpfr 0, library 0

The correct result given by mpfr (I hope) is far away from the subnormal
range (1.17472840344135e-8 vs 1.17549435082229e-38) thus the error is huge,
and moreover the glibc result 0 is inconsistent with the underflow flag.

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[Bug math/14471] Inaccurate y0f function

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=14471

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Inaccurate y0f function on  |Inaccurate y0f function
                   |x86_32 and x86_64           |

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[Bug math/14471] Inaccurate y0f function

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14471

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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[Bug math/14471] Inaccurate y0f function

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=14471

--- Comment #5 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
with glibc-2.32 the maximum error on x86_64 is now 4837658 ulps (for
x=8.935769796e-01)

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[Bug math/14471] Inaccurate y0f function

[zimmerma+gcc at loria dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=14471

Paul Zimmermann <zimmerma+gcc at loria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |2.34

--- Comment #6 from Paul Zimmermann <zimmerma+gcc at loria dot fr> ---
fixed by commit 9acda61

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