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From: shankar@hpclscu.HP.COM (Shankar Unni)
Newsgroups: comp.sys.hp
Subject: Re: alloca() on the HP9000/300
Message-ID: <1340135@hpclscu.HP.COM>
Date: 14 Aug 90 03:14:12 GMT
References: <1455@tub.UUCP>
Organization: Hewlett-Packard Calif. Language Lab
Lines: 40

> >Is it possible to write a "real" alloca() for the HP at all, or is
> >there some sort of conflict with the C compiler (i.e. does the way
> >the stack pointer is handled make usage of alloca() impossible)?
> 
> The conventional alloca() depends on the frame pointer / stack pointer model,
> in which a function references its local and parameters off of the frame
> pointer.  alloca() diddles the stack pointer and returns, hoping that no one
> will notice, since all references are off a6 not a7, and a7 will be restored
> on the unlk.

And this is exactly why it is hard to make a "real" alloca() (whatever that
is) work on an 800.

The 800 does not have a separate frame pointer - every local variable,
including parameters, is referenced off SP (the top-of-stack pointer).
Changing SP thus has terrible consequences.

I suppose it would be possible to build alloca() into the compiler itself
(i.e. affect the code generation for a function based on whether it calls
alloca() or not), but alloca() being so non-standard, it was decided not to
do so.

Now that the GNU C compiler has been ported to the 800 series, that would
be a good place to try out such techniques, and evaluate them.

All in all, it's probably best to stay as far away from alloca() as
possible. If you *have* to use alloca() (i.e. someone else's software, and
you can't / won't change it), you can use some good PD alloca.c that does
not fiddle with SP. The code generated will be no worse than if you
actually modified the code to call malloc()/free().

-----
Shankar Unni                                   E-Mail: 
Hewlett-Packard California Language Lab.     Internet: shankar@hpda.hp.com
Phone : (408) 447-5797                           UUCP: ...!hplabs!hpda!shankar

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