Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!hp-pcd!hpfcso!mev
From: mev@hpfcso.HP.COM (Mike Vermeulen)
Newsgroups: comp.sys.hp
Subject: Re: alloca() on the HP9000/300
Message-ID: <7370198@hpfcso.HP.COM>
Date: 27 Aug 90 15:07:36 GMT
References: <1455@tub.UUCP>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 25

> Recent versions of GNU Emacs include both Gwyn's package and
> assembly-level hacks for some specific machines, one of which is the 300
> series.

The 300 series assembly alloca in the GNU emacs distribution works by:
1) bumping SP and 2) copying the old stack top so that all SP based addresses
now refer to the copies of objects on top of the stack.

> I believe it stops working with the HPUX 7.0 cc if you turn optimization up
> past +O1

I believe it stopped working at HPUX 6.5/+O2 because the potential addition of
the save/restore of floating point registers meant that a larger stack top had
to be copied.  Increasing the stack top size to 88 bytes (6 data registers,
4 address registers, 6 floating point registers), makes emacs work witho
ut
complaint.

This alloca method is still fragile because if the peephole optimizer were to
change frame pointer based addressing of locals/parameters to stack pointer
based addressing; then the size of the stack top still may not be adequate when
given too many locals/parameters.

--mev

standard disclaimer: this is not an official HP response.