Unmasked exceptions (which can be fixed up using fpset*() before they
trap) are very rare, especially on amd64 since SSE exceptions trap
synchronously, but I want to merge the faster amd64 implementations of
fpset*() back to i386 without introducing the bug on i386.
The i386 implementation has always avoided the trap automatically by
changing things using load/store of the FP environment, but this is
very slow. Most changes only affect the control word, so they can
usually be done much more efficiently, and amd64 has always done this,
but loading the control word can trap.
This version use the fast method only in the usual case where it will
not trap. This only costs a couple of integer instructions (including
one branch which I haven't optimized carefully yet) in the usual case,
but bloats the inlines a lot. The inlines were already a bit too large
to handle both the FPU and SSE.
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