[Bug target/56831] New: [x86] sNaN value as a function call argument

[tschwinge at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56831

             Bug #: 56831
           Summary: [x86] sNaN value as a function call argument
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tschwinge@gcc.gnu.org
            Target: i686


On 32-bit x86, using a sNaN value as a function call argument may turn it
into a qNaN (whilst raising an INVALID exception) already at the call
site, which is unexpected.  This is due to using a flds, fstps sequence
(which according to my old Intel manuals correctly processes a float and
double sNaN value in this way), instead of just using a movl for pushing
the sNaN value onto the stack.

This only happens for some optimization flags together with volatile
declaration of the variable keeping the sNaN value; see the discussion in
<http://news.gmane.org/find-root.php?message_id=%3c87txonzdtd.fsf%40kepler.schwinge.homeip.net%3e>
and thereabouts, and the SNAN_TESTS_float and SNAN_TESTS_double usage in
glibc commit 5aa4a1a1fd742479818a668d42d91ca9ec4a6318,
<http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=5aa4a1a1fd742479818a668d42d91ca9ec4a6318>.