[Bug math/17907] New: fma modifies and does not reset SSE rounding mode on 32bit machines

[Bug math/17907] New: fma modifies and does not reset SSE rounding mode on 32bit machines

[simonbyrne at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17907

            Bug ID: 17907
           Summary: fma modifies and does not reset SSE rounding mode on
                    32bit machines
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: simonbyrne at gmail dot com

Created attachment 8091
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8091&action=edit
fma rounding mode test function

The software fma function modifies the rounding mode for SSE floating point
instructions (to round-toward-zero), but does not reset it correctly, on 32bit
(686) processors. The x87 rounding mode is unaffected.

Attached is a sample program: compile on a 32bit machine with gcc:

$ gcc fmacheck.c -o fmacheck -lm -msse2 -mfpmath=sse

or clang:

$ clang fmacheck.c -o fmacheck -lm

When run as

$ ./fmacheck 0x1.0000000000001p+0

it should give
0x1.ffffffffffffep-1
if rounding mode is the default (round-to-nearest). If the rounding mode has
been incorrectly modified to be round-toward-zero, then you will get
0x1.ffffffffffffdp-1

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[Bug math/17907] fma modifies and does not reset SSE rounding mode on 32bit machines

[simonbyrne at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17907

Simon Byrne <simonbyrne at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |simonbyrne at gmail dot com

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[Bug math/17907] fma modifies and does not reset SSE rounding mode on 32bit machines

[jsm28 at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=17907

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

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

--- Comment #1 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
This looks like it would have been a consequence of bug 16064, fixed for 2.20.

*** This bug has been marked as a duplicate of bug 16064 ***

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[Bug math/17907] fma modifies and does not reset SSE rounding mode on 32bit machines

[joseph at codesourcery dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17907

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
(More specifically: i386 uses sysdeps/ieee754/ldbl-96/s_fma.c, which saves 
state with feholdexcept, sets the rounding mode (twice) with fesetround, 
and then restores state, with exceptions merged, with feupdateenv.  This 
was the sort of pattern that was mishandled before bug 16064 was fixed, 
because fesetround sets the SSE rounding mode but the fenv_t functions 
used not to handle it.)

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[Bug math/17907] fma modifies and does not reset SSE rounding mode on 32bit machines

[simonbyrne at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17907

--- Comment #3 from Simon Byrne <simonbyrne at gmail dot com> ---
Great, thanks for letting me know.

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