Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uwm.edu!rpi!brutus.cs.uiuc.edu!jarthur!uci-ics!rfg
From: rfg@ics.uci.edu (Ronald Guilmette)
Newsgroups: gnu.gcc
Subject: Re: A question on local variable allocation
Message-ID: <25EAFE2C.2545@paris.ics.uci.edu>
Date: 27 Feb 90 22:24:44 GMT
References: <25E58087.11649@paris.ics.uci.edu> <6915@internal.Apple.COM>
Reply-To: rfg@ics.uci.edu (Ronald Guilmette)
Organization: UC Irvine Department of ICS
Lines: 56

In article <6915@internal.Apple.COM> escher@Apple.COM (Michael Crawford) writes:
>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 cannot see what possible difference such jumps would make to anything.
Do you know of some secret implementation problem?  If so, please state it.

>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.

That should happen always.  Without regard to *anything* the optimizer knows
or does (or doesn't know or doesn't do).

>By the way, the allocation of the arrays on the stack is done at the time the
>function is entered

I knew that.  I'm not totally ignorant, just mostly so.

> (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.

Please state the problem so that us non-clarvoiants can think about it.

>Michael D. Crawford
>Oddball Enterprises

Do you manufacture oddballs or just repair them? :-)


// Ron Guilmette (rfg@ics.uci.edu)
// C++ Entomologist
// Motto:  If it sticks, force it.  If it breaks, it needed replacing anyway.