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    SubjectRe: RFC: changing precision control setting in initial FPU context
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    Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
    >
    > You want peoples existing applications to suddenely and magically change
    > their results. Umm problem.

    So, how would you feel about a mechanism whereby the kernel could be
    passed a default FPU control word by the binary (with old binaries, by
    default, using the old default control word)? There's already an ELF
    AT_FPUCW auxv entry type. What if this was used by the kernel, rather
    than the C library (as it is now), to set a default to be used in
    "init_fpu()" when and if the program executed a floating point
    instruction?

    Then, a compiler startup-code writer would be able to specify a
    default control word for binaries that was appropriate for (new)
    programs generated by that compiler *WITHOUT* worrying about whether
    he was accidentally turning a non-FP program into an FP program by
    introducing "fnstcw" as its only FPU instruction.

    The C library is already trying to do this (setting the CW based on
    the AT_FPUCW vector). It just can't do it *right* because it doesn't
    know if the program is really FP. It just guesses that if the
    AT_FPUCW vector contains something other than the hard-coded
    _FPU_DEFAULT (which is supposed to be equal to the kernel default: it
    isn't, but it's close enough), it must be set; otherwise, it's left
    alone.

    Kevin <buhr@stat.wisc.edu>
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