Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!iuvax!purdue!decwrl!amdcad!crackle!tim
From: tim@crackle.amd.com (Tim Olson)
Newsgroups: comp.lang.c
Subject: Re: Short code to determine compiler's
Message-ID: <26470@amdcad.AMD.COM>
Date: 23 Jul 89 20:55:20 GMT
References: <396@uop.uop.EDU> <225800197@uxe.cso.uiuc.edu> <579@targon.UUCP> <171@bms-at.UUCP> <14497@dartvax.Dartmouth.EDU> <1038@cernvax.UUCP>
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Reply-To: tim@amd.com (Tim Olson)
Organization: Advanced Micro Devices, Inc. Sunnyvale CA
Lines: 22

In article <1038@cernvax.UUCP> hjm@cernvax.UUCP (Hubert Matthews) writes:
| I have a question for the authors of smart C compilers: is the
| register declaration useful at all for such compilers?  It indicates
| that a variable is non-volatile, noalias and that taking its address
| is illegal, all of which should make things easier for the optimiser.

Yes, the register keyword does indicate such things, but they can all be
determined by flow analysis anyway.  If a local variable does not have
its address taken, and it is non-volatile, then it is equivalent to
"register".

| So, does "register" help or hinder?  Has it become a source-code
| "no-op"?  Can I merrily leave it out of my programs and let the
| compiler do the guessing for me :-).

It's a no-op, except for error messages that must be reported if a
register variable's address is taken.  In other words, it's more for
diagnostic purposes than performance in "smart" C compilers. 

	-- Tim Olson
	Advanced Micro Devices
	(tim@amd.com)