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From: mwm@cuuxb.UUCP (Marc W. Mengel)
Newsgroups: comp.arch
Subject: Re: Available no. of registers
Message-ID: <1029@cuuxb.UUCP>
Date: Thu, 22-Jan-87 00:04:27 EST
Article-I.D.: cuuxb.1029
Posted: Thu Jan 22 00:04:27 1987
Date-Received: Thu, 22-Jan-87 19:56:52 EST
References: <3810002@nucsrl.UUCP> <926@mips.UUCP> <759@vaxb.calgary.UUCP>
Reply-To: mwm@cuuxb.UUCP (Marc W. Mengel)
Organization: AT&T-IS, Software Support, Lisle IL
Lines: 33

In article <759@vaxb.calgary.UUCP> radford@calgary.UUCP writes:
>Shouldn't a compiler smart enough to allocate variables to registers be
>smart enough to see that a local variable is never the operand of the
>& (address-of) operator, and thus cannot be referenced by a pointer?
>Both of these tasks seem to require that the procedure be completely
>scanned before code generation.

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.
>    Radford Neal
>    The University of Calgary


-- 
 Marc Mengel
 ...!ihnp4!cuuxb!mwm