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    SubjectRe: Patch to remove undefined C code
    On Mon, Oct 16, 2000 at 04:55:08PM -0400, Ben Pfaff wrote:
    > > > $ The order of evaluation of the function designator, the
    > > > $ arguments, and subexpressions within the arguments is
    > > > $ unspecified, ...
    > >
    > > I sit surprised and corrected. With every version of every C compiler on
    > > every OS I have ever used (over 100 combinations from AmigaDOS 1.1 using
    > > Manx C to E10000 using Kai C++) the behavior has been the same for parameter
    > > lists as for the comma operator in this respect.
    > Yes. In practice the usual question is whether the compiler will
    > evaluate the operands from left to right or from right to left,
    > but the compiler is within its rights to evaluate the operands in
    > any order it wants.

    That depends mainly on question: Does your stack grow up or down ?

    Machines where stack grows up are a bit rare, but they do exist.
    The CISC archetype, IBM S/360/370/390 has simple instruction to
    add a small positive offset value into pointer register, but not
    to substract it (nor have negative offsets). Thus most stackfull
    languages there have the stack growing up. Doing downwards-growing
    stack setup requires 1 extra register use, and one extra instruction
    per frame entry, plus considerably more for parameter push.
    (But it was a great beast to run Fortran-IV which didn't need stack,
    just subfunction invocation record -- at a static location unique
    for each subfunction --> no recursion supported.)

    > > Does this imply that even the evaluation of a function pointer
    > > is itself undefined in terms of ordering?
    > Yes.

    For things like lone pointer referral with pre/post inc/dec:
    it is well defined, but for things like:
    *p++ + *p
    it is not. (Will the second *p be evaluated before or after *p++
    post-increment ?)

    /Matti Aarnio
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