id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4391	forkIO threads do not properly save/restore the floating point environment	draconx		"When using forkIO threads, the floating point environment is not correctly
saved and/or restored on context switches.  This causes floating point state
to leak between the threads.

For example, if two threads (call them A and B) are created, and thread A
changes the rounding direction, computations in thread B will be carried out
under the rounding direction set by A, and vice versa.  Likewise, any
exceptions raised by computations in thread A are visible in thread B.

If forkIO threads are not intended to be used with floating point, this should
be spelled out explicitly in the documentation rather than some vague notion of
avoiding ""thread local state"".

I've attached a short test program which demonstrates this issue.  For
portability, it uses standard C library functions to manipulate the floating
point environment.  It prints the following when compiled with GHC
(parenthetical statements added):

{{{
A: ToNearest   (default rounding mode)
A: True
B: ToNearest
B: TowardZero
B: Infinity    (result of division by 0)
A: [DivByZero] (A sees B's exception)
A: TowardZero  (A sees B's rounding direction)
A: False
}}}

If we use OS threads (which properly save/restore the floating point
environment):

{{{
A: ToNearest
A: True
B: ToNearest
B: TowardZero
B: Infinity   (result of division by zero)
A: []         (no new exceptions are visible)
A: ToNearest  (rounding direction is unchanged)
A: True
}}}"	bug	closed	normal	7.4.1	Runtime System	6.12.3	fixed			Unknown/Multiple	x86_64 (amd64)	None/Unknown