Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!sdd.hp.com!hp-pcd!hpfcso!mjs
From: mjs@hpfcso.HP.COM (Marc Sabatella)
Newsgroups: comp.sys.hp
Subject: Re: alloca() on the HP9000/300
Message-ID: <7370189@hpfcso.HP.COM>
Date: 9 Aug 90 16:26:07 GMT
References: <1455@tub.UUCP>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 26

>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.

The Series 300 C compiler is smart enough to remove link/unlk pairs and convert
references off a6 into references off a7.  For this, it needs complete
information about a7, and alloca() breaks the basic assumption that the stack
pointer is preserved across the call.  If you do not optimize (not even with
"+O1"), you may be able to get away with using the standard alloca().

But I strongly advise against it.  By its very nature, alloca() is non-portable
(ie, it won't even work on machines that don't use the fp/sp model), and if
one day the "remove link/unlk" feature is moved out of the optimizer and into
the compiler proper, it will cease to function on the 300.

--------------
Marc Sabatella (marc@hpmonk.fc.hp.com)
Disclaimers:
	2 + 2 = 3, for suitably small values of 2
	Bill and Dave may not always agree with me