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    SubjectRe: [BK-2.5] Move "used FPU status" into new non-atomic thread_info->status field.
    On Mon, 10 Mar 2003 11:25:55 -0800 (PST), Linus Torvalds wrote:
    >We could _probably_ do it on x86 too. The standard C calling convention on
    >x86 says FPU register state is clobbered, if I remember correctly.
    >However, some of the state is "long-term", like rounding modes, exception
    >masking etc, and even if we didn't save the register state we would have
    >to save that part.
    ...
    >As it was, the x86 state was pretty much random after fork(), and that can
    >definitely lead to problems for real programs if they depend on things
    >like silent underflow etc.

    Do you mean x87 control or the x87 stack here?

    >(Now, in _practice_ all processes on the machine tends to use the same
    >rounding and exception control, so the "random" state wasn't actually very
    >random, and would not lead to problems. It's a security issue, though).

    Sorry for being dense, but can you clarify: will current 2.{2,4,5}
    kernels preserve or destroy the parent process' FPU control at fork()?

    We're using unmasked FPU exceptions on x86 (and Solaris/SPARC) in the
    runtime system for the Erlang telecom systems programming language.
    This gives a noticeable performance improvement, but it relies on
    the FPU control not changing beneath it: the FPU control is only
    initialised at startup and when SIGFPU has occurred.

    /Mikael
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