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See http://www.c2.com/cgi/wiki?OnceAndOnlyOnce for more information. As pointed out there, Common Lisp is expressive enough that it leaves few good excuses for putting information in more than one place.

SBCL tries to do better at this than the original CMU CL code did. E.g. we keep the Memory Map constants in one place, not two, various repeated idioms and out-and-out cut-and-pasted code has been factored out into shared functions, and new definitions like *OPTIMIZE-BYTE-COMPILATION* (at least until the byte compiler went away around sbcl-0.7.0) are used liberally to collect common new idioms.

Other possible targets for future duplicate code reduction are

• the architecture-specific compiler backends
• bits of the architecture-specific runtime. I haven't given a lot of thought to exactly how this could be refactored, but I'm looking at my second backend now and it's all scarily similar to the first.
• valid_dynamic_space_pointer() in purify.c and possibly_valid_dynamic_space_pointer() in gencgc.c seem to be the same thing, just with more post-fork maintenanance done on the gencgc.c version.
• scavtab[] seems to be the same thing in gc.c and gencgc.c. Probably it should be factored out into its own scavtab.c file.
• WHN introduced his own cut-and-paste monstrosity in DEF-BOOLEAN-ATTRIBUTE. Now that EVAL-WHEN works, maybe he or someone else will finally be able to figure out how to fix it.
• ldb's version of object layout and other low-level information is hand-maintained, not propagated automatically from the master copies stored in Lisp (e.g. in objdef.lisp). It'd be better to make ldb get its information from sbcl.h (and, probably, to make GENESIS put more information into sbcl.h to support this).
• The Alpha 32/64-bit differences are coded as a bunch of cut-and-pasted expressions like #!-ALPHA SB!VM:WORD-BITS #!+ALPHA 64. These could be defined once (and so associated with mnemonic names and/or explanatory comments).

-- Daniel Barlow and Bill Newman

This page is linked from: Bill Newman   Registers   Wishlist  

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