🚀 go-pugleaf

In article <4hv05f$j9i@solutions.solon.com>,
Konrad Schwarz <schwarz@a2.complang.tuwien.ac.at> wrote:
:In article <4hqed0$1lc@solutions.solon.com>, kanze@gabi-soft.fr (J. Kanze) writes:
:|>     [Concerning alloca...]
:I: As far as I know, implementing alloca via the stack is trivial on
:I: any general-purpose CPU, but I am interested in hearing of counter examples.
:|> 
:|> So what do you do on machines that don't have a stack?
:
:General-purpose CPU's have stacks, at least in my book.  How do you
:implement C on machines that don't have a stack?

How about calling malloc() at function entry, and free() at function
exit?  That should work.  (I mean that the generated code does
this, not that the programmer does this; it's invisible to him/her.)
setjmp/longjmp becomes tricky to implement to avoid memory leaks, but
is possible.

(I don't know if my previous post re alloca() made it out or not;
it didn't show here.  Our newsfeed(s) is/are crummy; some stuff
gets here right away, some takes nearly a week, and some never
seems to show up at all.  Your article and its great-grandparent
are here, but not the two in between.  This is not a function of
which group is involved.  Sorry.)

There are several problems with alloca().  I couldn't think of
any when I previously wrote.  Here are three:
	1.  Think of how it would be implemented such that it would
	    work correctly when assigned to a function pointer.  For
	    example, call some other function with malloc sometimes
	    and alloca sometimes as an argument.

	2.  Try this:
	        char *base, *interesting_stuff;
	        int f1(), f2();
	        interesting_stuff = f1() + (base=alloca(SIZE)) + f2();
	    How would you implement alloca() to handle this?

	3. Since the standard chose (probably for good reasons :)
	    to omit alloca, porting code that uses it can be a real
	    b**ch.  I once had to deal with some code that had called
	    alloca() IN A LOOP (otherwise, I could have just replaced:
	        alloca         with    malloc, saving ptr
	        return         with    goto EXIT
	    and added:
	        EXIT: free(ptr); return;
	    which is easy (in the absence of setjmp/longjmp, anyway).)

Cheers,
Stan.
--
Stan Ryckman  (stanr@tiac.net) (new e-mail address)