Re: SPARC tagged data

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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