absolute size, packing control

[Henry Spencer]

In article <2970 at tekcrl.CRL.TEK.COM> danw at tekchips.CRL.TEK.COM (Daniel E. Wilson;1432;58-790;;) writes:
>    It would seem that a well written compiler would already choose the
>best alignments of members in a structure.

Unfortunately, the definition of "best" is somewhat context-dependent.
All a compiler can do is try to pick a reasonable compromise (i.e. the
one that best fits the prejudices of its implementors).

> Also remember that some
>types of hardware can't address a word on a byte boundry (PDP-11) and
>so this option would end up not being implemented.

This interferes with *pointers* to such variables, but the variables
themselves are not a problem -- they just take longer to access.  This
can be a worthwhile tradeoff in some circumstances.  In particular, it
would be very nice to be able to define a precise storage layout for
things like network packets -- it wouldn't be entirely portable, but
having the compiler do the details of digging the data out would be
better than making the programmer do it.

>A good 80x86 compiler
>would probably not pad a structure because the hardware doesn't care how
>an int is aligned.

Ah, but it does!  Unaligned accesses *work*, but they are much slower.
A good 80x86 compiler should probably pad out to 16-bit alignment at least,
and perhaps 32.
-- 
Intel CPUs are not defective,  |     Henry Spencer at U of Toronto Zoology
they just act that way.        | uunet!attcan!utzoo!henry henry at zoo.toronto.edu