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From: eb%watergate@lucid.com (Eric Benson)
Newsgroups: comp.compilers
Subject: Re: SPARC tagged data
Keywords: architecture, sparc
Message-ID: <9104300702.AA01038@watergate>
Date: 30 Apr 91 07:02:38 GMT
References: <9104291542.AA11213@flora.techfak.uni-bielefeld.de>
Sender: compilers-sender@iecc.cambridge.ma.us
Reply-To: Eric Benson <eb%watergate@lucid.com>
Organization: Compilers Central
Lines: 34
Approved: compilers@iecc.cambridge.ma.us

In article <9104291542.AA11213@flora.techfak.uni-bielefeld.de> horst@techfak.uni-bielefeld.de wrote:
> Does anyone know what TAGGED DATA instructions are useful for and how to
> use them? Tagged data is assumed to be 30 bits wide followed by two bits
> set to zero. The SPARC allows add and subtract instructions on tagged data.
> 
> [Most likely it's for immediate integers in a Lisp-like system that uses
> tagged pointers, but I hope someone who actually knows will tell us. -John]

Yes, the tagged arithmetic instructions were put in the SPARC architecture
for Lucid Common Lisp.  If the low-order two bits of a Lisp object
reference are zero, it is a 30-bit immediate fixnum.  If some of those
bits are non-zero, it may be a pointer to a floating point number or a
bignum (arbitrary-precision integer).  Generic arithmetic is generally
optimized for the fixnum case, since the overwhelming majority of
arithmetic is performed on small integers.  On many machines + is compiled
inline as

   Test low order two bits of first operand.
     If nonzero, use general case. (Operand could be a float or bignum.)
   Test low order two bits of second operand.
     If nonzero, use general case. (Operand could be a float or bignum.)
   Add two operands.
   If overflow, use general case.  (Result is a bignum).

On the SPARC this is done as one instruction (TADDCC) followed by a
conditional branch rarely taken.

eb@lucid.com 	           	 	Eric Benson
415/329-8400 x5523                      Lucid, Inc.
Telex 3791739 LUCID                     707 Laurel Street
Fax 415/329-8480                        Menlo Park, CA 94025
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