1. Save and restore the control part of the MXCSR in addition to the
i387 control word to ensure that the two are consistent.
Note that standards don't require longjmp to restore either control
word, and none of Linux, MacOS X 10.3 and earlier, NetBSD, OpenBSD,
or Solaris do it. However, it is historical FreeBSD behavior, and
bde points out that it is needed to make longjmping out of a signal
handler work properly, given the way FreeBSD clobbers the FPU state
on signal handler entry.
2. Don't clobber the FPU exception flags in longjmp. C99 requires them
to remain unchanged.
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