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    SubjectRe: [GIT] Networking
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    On Mon, Nov 2, 2015 at 5:58 PM, Andy Lutomirski <luto@amacapital.net> wrote:
    >
    > Based in part on an old patch by Sasha, what if we relied on CSE:
    >
    > if (mul_would_overflow(size, n))
    > return NULL;
    > do_something_with(size * n);

    I suspect we wouldn't even have to rely on CSE. Are these things in
    performance-critical areas? I suspect our current "use divides" is
    actually slower than just using two multiplies, even if one is only
    used for overflow checking.

    That said, I also think that for something like this, where we
    actually have a *reason* for using a special overflow helper function,
    we could just try to use the gcc syntax.

    I don't think it's wonderful syntax, but at least there's an excuse
    for odd/ugly code in those kinds of situations. The reason I hated the
    unsigned subtraction case so much was that the simple obvious code
    just avoids all those issues entirely, and there wasn't any real
    *reason* for the nasty syntax. For multiplication overflow, we'd at
    least have a reason.

    Sadly, the *nice* syntax, where we'd do something like "goto label"
    when the multiply overflows, does not mesh well with inline asm. Yes,
    gcc now has "asm goto", but you can't use it with an output value ;(

    But from a syntactic standpoint, the best syntax might actually be
    something like

    mul = multiply_with_overflow(size, n, error_label);
    do_something_with(mul);

    error_label:
    return NULL;

    and it would *almost* be possible to do this with inline asm if it
    wasn't for the annoying "no output values" case. There are many other
    cases where I'd have wanted to do this (ie the whole "fetch value from
    user space, but if an exception happens, point the exception handler
    at the label).

    Back in May, we talked to the gcc people about allowing output values
    that are unspecified for the "goto" case (so only a fallthrough would
    have them set), but I think that that doesn't match how gcc internally
    thinks about branching instructions..

    But you could still hide it inside a macro and make it expand to something like

    #define multiply_with_overflow(size, n, error_label) ({
    unsigned long result, error; \
    .. do multiply in asm, set result and error... \
    if (error) goto error_label;
    result; })

    which would allow the above kind of odd hand-coded try/catch model in
    C. Which I think would be pretty understandable and not very prone to
    getting it wrong. Hmm?

    Linus


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