cdecl and pascal keywords

[Risto Lankinen]

kvitekp at jvd.msk.su (Pete Kvitek) writes:

>  Another reason to use pascal calling convections is to
>  conserve stack space with heavily nested procedures
>  (for example recourcive ones).

Hi!

This is a false assumption, at least in case of Microsoft C (using _cdecl
versus _pascal).  In either way, the stack contains the arguments of the
(possibly recursively) called function, the backed-up local frame pointer
and the return address.  There's no penalty in stack usage for using the
_cdecl even in recursive functions.

The size difference is, indeed, significant in programs using a lot of
function calls:  Using _pascal, each function *code* carries an overhead
of 1 byte (by RET nn instead of plain RET), while with _cdecl each *call*
to a function spends 3 extra bytes (by an inserted ADD SP,nnnn) for stack
clean-up.

The _cdecl is good for functions, which have variable number of arguments.
I have also seen it claimed faster than _pascal, although with i286 I dare
to doubt the significance of the difference.  Because with _cdecl the next
instruction is 'guaranteed' to be 3 bytes long (and spend a few additional
cycles for pre-fetch), while with _pascal there is a good chance the next
instruction is smaller than that (say, PUSH AX = 1 byte within nested calls,
for example).

By the way, an 'extremely optimizing' compiler could fight back a bit with
_cdecl, by leaving the stack arguments intact between calls, should there be
a (rare) piece of code, where exactly (or almost exactly) the same arguments
are used in subsequent calls, and the arguments were declared as const.

Terveisin: Risto Lankinen
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