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From: meissner@dg_rtp.UUCP
Newsgroups: comp.arch
Subject: Re: Available no. of registers
Message-ID: <884@dg_rtp.UUCP>
Date: Fri, 23-Jan-87 11:29:06 EST
Article-I.D.: dg_rtp.884
Posted: Fri Jan 23 11:29:06 1987
Date-Received: Wed, 28-Jan-87 06:20:34 EST
References: <3810002@nucsrl.UUCP> <926@mips.UUCP> <759@vaxb.calgary.UUCP> <1029@cuuxb.UUCP>
Reply-To: meissner@dg_rtp.UUCP (Michael Meissner)
Organization: Data General (Languages @ Research Triangle Park, NC.)
Lines: 31

In article <1029@cuuxb.UUCP> mwm@cuuxb.UUCP (Marc W. Mengel) writes:
> 
> Unfortunately, you don't have to take the address of a given variable
> to use it, you merely have to take the address of a variable near it
> and add an offset to it.  The way C is defined, this is quite legal.
> 
> For example, suppose I have a function f, declared as follows:
> 	f(p)
> 		struct { int a, b, c; } *p;
> 	{
> 	...
> 	}
> And I call it as follows:
> 	b()
> 	{
> 		int a, b, c;
> 
> 		f( &a );
> 	}
> According to our venerable friends kerningham&ritchie, this is legal;
> it is also used to some great extent in the older (v6 & v7) unix kernels.

    And this poor coding method will produce obscure results when used on
compilers that try to optimize the variables (group small items near the
beginning of the stack frame so instructions with small displacements can
be used, or to try and group frequently used items together).  Even if
the compiler doesn't reorganize things, it plays havoc if the stack goes
in the oppisite direction.
-- 
	Michael Meissner, Data General
	...mcnc!rti-sel!dg_rtp!meissner