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From: Tom.Lane@G.GP.CS.CMU.EDU
Newsgroups: comp.compilers
Subject: Re: Portable Fast Direct Threaded Code
Keywords: interpreter, design
Message-ID: <18809.670519274@G.GP.CS.CMU.EDU>
Date: 1 Apr 91 15:21:14 GMT
References: <3035@redstar.cs.qmw.ac.uk> <1991Mar31.180635.5944@cs.rochester.edu>
Sender: compilers-sender@iecc.cambridge.ma.us
Reply-To: Tom.Lane@G.GP.CS.CMU.EDU
Organization: Compilers Central
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Approved: compilers@iecc.cambridge.ma.us

Lawrence Crowl points out one important problem with Eliot Miranda's
scheme for global register use in C.  There's an even more fundamental
problem, though: there is *no guarantee whatever* that the compiler will
assign the same registers to the global variables in every routine.

Compilers that do intelligent allocation of variables to registers may
refuse to honor the "register" declarations at all if the global variables
are not heavily used in a given routine, and in any case the globals need
not be assigned to the same registers every time.  Miranda's scheme thus
relies heavily on the assumption of a dumb register allocator (or else a
compiler that supports global register variable declarations).

This scheme may be "fast" direct threaded code, but it's hardly "portable".
-- 
			tom lane
Internet: tgl@cs.cmu.edu	BITNET: tgl%cs.cmu.edu@cmuccvma
[GCC lets you specify what register to use for global register variables, but
that is of course both machine and compiler specific. -John]
-- 
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