Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!snorkelwacker!apple!escher
From: escher@Apple.COM (Michael Crawford)
Newsgroups: gnu.gcc
Subject: Re: A question on local variable allocation
Message-ID: <6915@internal.Apple.COM>
Date: 27 Feb 90 00:44:33 GMT
References: <25E58087.11649@paris.ics.uci.edu>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 45

In article <25E58087.11649@paris.ics.uci.edu> rfg@paris.ics.uci.edu (Ronald Guilmette) writes:
>Can someone please explain to me why, in the following code, the space
>allocated for the two local array variables does not overlap?
>
...
>	void function ()
>	{
>	  {
>	     int inside1[1024];
>
>	     g (inside1);
>	  }
>
>	  {
>	     int inside2[1024];
>
>	     g (inside2);
>	  }
>	}
>

I believe the reason is that C allows one to goto the inside of a block from
the outside, as long as the goto is within the same function.  This would
allow one to jump back and forth between the two blocks given.

I admit, though, that the optimizer ought to note the absence of any goto's or
labels within the function block, and do the allocation as you suggest,
with it overlapping.

By the way, the allocation of the arrays on the stack is done at the time the
function is entered (in general -- I do not know much about gcc).  I could
image there could be some trouble with alloca() working properly if called
inside, or outside of the blocks.

-- 
Michael D. Crawford
Oddball Enterprises
694 Nobel Drive
Santa Cruz, CA 95060
oddball!mike@ucscc.ucsc.edu

Consulting for Apple Computer Inc.
escher@apple.com

The opinions expressed here are solely my own.