[Bug math/10709] New: sin() returns incorrect rounding

[Bug math/10709] New: sin() returns incorrect rounding

[zimmerma+gcc at loria dot fr]

for the double precision number closest to 2.522464e-1, the glibc sin() function
returns 2.4957989804940914e-01 (when printed to 17 digits, which identifies
uniquely double-precision numbers), whereas the correct rounding to nearest is
2.4957989804940911e-01.

According to glibc/sysdeps/ieee754/dbl-64/s_sin.c, the result should be
always correctly rounded.

-- 
           Summary: sin() returns incorrect rounding
           Product: glibc
           Version: 2.10
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: zimmerma+gcc at loria dot fr
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: x86_64-redhat-linux
  GCC host triplet: x86_64-redhat-linux
GCC target triplet: x86_64-redhat-linux


http://sourceware.org/bugzilla/show_bug.cgi?id=10709

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[Bug math/10709] sin() returns incorrect rounding

[zimmerma+gcc at loria dot fr]

------- Additional Comments From zimmerma+gcc at loria dot fr  2009-09-30 12:57 -------
Created an attachment (id=4236)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=4236&action=view)
test case

compile with -fno-builtin to ensure the sin() call is not folded at
compile-time

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=10709

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[Bug math/10709] sin() returns incorrect rounding

[zimmerma+gcc at loria dot fr]

------- Additional Comments From zimmerma+gcc at loria dot fr  2009-09-30 13:07 -------
This bug is analyzed and a fix is proposed at
<http://www.loria.fr/~zimmerma/papers/bug10709.pdf>.

In short, it suffices to replace the line:
return (res==res+1.025*cor)? res : slow1(x);
in file glibc/sysdeps/ieee754/dbl-64/s_sin.c by:
return (res==res+1.096*cor)? res : slow1(x);

Paul Zimmermann

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=10709

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[Bug math/10709] sin() returns incorrect rounding

[vincent+libc at vinc17 dot org]

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


http://sourceware.org/bugzilla/show_bug.cgi?id=10709

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