[Bug math/20255] [i386] fdim, fdimf return with excess range and precision / double rounding

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https://sourceware.org/bugzilla/show_bug.cgi?id=20255

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=228a78c21b9ed27004a1ccb5bd48a3263d7cd67d

commit 228a78c21b9ed27004a1ccb5bd48a3263d7cd67d
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Tue Jun 14 16:41:50 2016 +0000

    Fix i386 fdim double rounding (bug 20255).

    fdim suffers from double rounding on i386 because subtracting two
    double values can produce an inexact long double value exactly half
    way between two double values.  This patch fixes this by creating an
    i386-specific version of fdim - C, based on the generic version,
    unlike the previous .S version - which sets the x87 precision control
    to double precision for the subtraction and then restores it
    afterwards.  As noted in the comment added, there are no issues of
    double rounding for subnormals (a case that setting precision control
    does not address) because subtraction cannot produce an inexact result
    in the subnormal range.

    Tested for x86_64 and x86.

        [BZ #20255]
        * sysdeps/i386/fpu/s_fdim.c: New file.  Based on math/s_fdim.c.
        * math/libm-test.inc (fdim_test_data): Add another test.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                           |    4 ++++
 math/libm-test.inc                  |    3 +++
 {math => sysdeps/i386/fpu}/s_fdim.c |   14 ++++++++++++--
 3 files changed, 19 insertions(+), 2 deletions(-)
 copy {math => sysdeps/i386/fpu}/s_fdim.c (67%)

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