[Bug libc/16625] New: sin gives wrong result with 2.19 on i386

[Bug libc/16625] New: sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

            Bug ID: 16625
           Summary: sin gives wrong result with 2.19 on i386
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: jtaylor.debian at googlemail dot com
                CC: drepper.fsp at gmail dot com

in ubuntu 14.04 i386 with glibc2.19 we are seeing wrong results for calls to
sin that don't appear with 2.18:
https://launchpadlibrarian.net/167193519/buildlog_ubuntu-trusty-i386.python-cffi_0.8.1-1ubuntu3_FAILEDTOBUILD.txt.gz


>       assert library.sin(12.3) == math.sin(12.3)
E       assert 0.8444952930965295 == -0.26323179136580094

I failed to create a testcase for as it seems to depend on global state.

to reproduce via the packages in 14.04 o

apt-get source python-cffi
cd python-cffi*
apt-get build-dep python-cffi
gdb --args python2.7 -m pytest testing/test_function.py
testing/test_zdistutils.py

from what I can tell with my limited assembly skills is that it computes the
right result but clobbers it in libc_feresetround_387_ctx

0xf7da2028 in libc_feresetround_387_ctx (ctx=0xffffa0a0) at
../sysdeps/i386/fpu/fenv_private.h:452
452      if (__glibc_unlikely (ctx->updated_status))
3: $st2 = 0.26251239976915330398554715429781936
2: $st1 = 7.2970669792260636173761756580191662e-08
1: $st0 = 0.84449529309652948683151407749392092


$st2 contains the right result, but its not returned to $st0 after the function
returns so cffi gets the wrong result.

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #1 from Julian Taylor <jtaylor.debian at googlemail dot com> ---
hm no $st2 does not contain the right result but its related to the rounding
context
retval in ../sysdeps/ieee754/dbl-64/s_sin.c:391 does contain the result, after
that it jumps to the rounding context after which the result is wrong.

some better debugging instructions for cffi:
gdb --args python2.7 -m pytest testing/test_function.py
testing/test_zdistutils.py
break sin
# calls sin 8 times with before it x=1.23 reaches the problematic case of
x=12.3
ignore 1 8

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[adconrad at 0c3 dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

Adam Conrad <adconrad at 0c3 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |adconrad at 0c3 dot net,
                   |                            |jsm28 at gcc dot gnu.org

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[schwab@linux-m68k.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
I'm unable to reproduce that.  Compiler bug?

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #3 from Julian Taylor <jtaylor.debian at googlemail dot com> ---
possible, I just rebuilt the packaged libc with -Og instead of -O2 and the
issue disappears...
the compiler I use is is 4.8.2-16ubuntu2 20140221 (r208010) from the
gcc-4_8-branch.

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #4 from Julian Taylor <jtaylor.debian at googlemail dot com> ---
I have bisected the issue to this commit:

commit 392dd2de03c054b1b32358561570be14f55e9ae9
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date:   Fri Dec 20 16:01:03 2013 +0530

    Consolidate code to compute sin and cos from lookup tables

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #5 from Julian Taylor <jtaylor.debian at googlemail dot com> ---
but building the commit with gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-11ubuntu1)
fixes it again, really does smell like a compiler bug

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #6 from Julian Taylor <jtaylor.debian at googlemail dot com> ---
gcc-4.9 (Ubuntu 4.9-20140303-0ubuntu3) 4.9.0 20140304 (experimental) [trunk
revision 208299] works too, so its just 4.8.2 thats broken

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[adconrad at 0c3 dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

Adam Conrad <adconrad at 0c3 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |doko at debian dot org

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jtaylor.debian at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

--- Comment #7 from Julian Taylor <jtaylor.debian at googlemail dot com> ---
r204212 of gcc fixes it
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg02422.html

though it seems like an accidental fix be removing the -fregmove pass
building with gcc 4.8.2 and -fno-regmove fixes the issue too.

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

Florian Weimer <fweimer at redhat dot com> changed:

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

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[Bug libc/16625] sin gives wrong result with 2.19 on i386

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=16625

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

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

--- Comment #8 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Resolved as a compiler bug, not a glibc bug.

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