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;;;; tags which are set during the build process and which end up in
;;;; CL:*FEATURES* in the target SBCL, plus some comments about other
;;;; CL:*FEATURES* tags which have special meaning to SBCL or which
;;;; have a special conventional meaning

;;;; This software is part of the SBCL system. See the README file for
;;;; more information.
;;;; This software is derived from the CMU CL system, which was
;;;; written at Carnegie Mellon University and released into the
;;;; public domain. The software is in the public domain and is
;;;; provided with absolutely no warranty. See the COPYING and CREDITS
;;;; files for more information.

 ;; features present in all builds

 ;; our standard
 :ansi-cl :common-lisp
 ;; FIXME: Isn't there a :x3jsomething feature which we should set too?

 ;; our dialect

 ;; Douglas Thomas Crosher's conservative generational GC (the only one
 ;; we currently support)

 ;; We're running under a UNIX. This is sort of redundant, and it was also
 ;; sort of redundant under CMU CL, which we inherited it from: neither SBCL
 ;; nor CMU CL supports anything but UNIX (and "technically not UNIX"es
 ;; such as *BSD and Linux). But someday, maybe we might, and in that case
 ;; we'd presumably remove this, so its presence conveys the information
 ;; that the system isn't one which follows such a change.

 ;; features present in this particular build

 ;; Setting this enables the compilation of documentation strings
 ;; from the system sources into the target Lisp executable.
 ;; Traditional Common Lisp folk will want this option set.
 ;; I (WHN) made it optional because I came to Common Lisp from
 ;; C++ through Scheme, so I'm accustomed to asking
 ;; Emacs about things that I'm curious about instead of asking
 ;; the executable I'm running.

 ;; When this is set, EVAL is implemented as an "IR1 interpreter": 
 ;; code is compiled into the compiler's first internal representation,
 ;; then the IR1 is interpreted. When this is not set, EVAL is implemented
 ;; as a little bit of hackery wrapped around a call to COMPILE, i.e.
 ;; the system becomes a "compiler-only implementation" of Common Lisp.
 ;; As of sbcl-0.6.7, the compiler-only implementation is prototype code,
 ;; and much less mature than the old IR1 interpreter. Thus, the safe
 ;; thing is to leave :SB-INTERPRETER set. However, the compiler-only
 ;; system is noticeably smaller, so you might want to omit
 ;; :SB-INTERPRETER if you have a small machine.
 ;; Probably, the compiler-only implementation will become more
 ;; stable someday, and support for the IR1 interpreter will then be
 ;; dropped. This will make the system smaller and easier to maintain
 ;; not only because we no longer need to support the interpreter,
 ;; but because code elsewhere in the system (the dumper, the debugger,
 ;; etc.) no longer needs special cases for interpreted code.

 ;; Do regression and other tests when building the system. You
 ;; might or might not want this if you're not a developer,
 ;; depending on how paranoid you are. You probably do want it if
 ;; you are a developer.

 ;; Setting this makes more debugging information available.
 ;; If you aren't hacking or troubleshooting SBCL itself, you
 ;; probably don't want this set.
 ;; At least two varieties of debugging information are enabled by this
 ;; option:
 ;;   * SBCL is compiled with a higher level of OPTIMIZE DEBUG, so that
 ;;     the debugger can tell more about the state of the system.
 ;;   * Various code to print debugging messages, and similar debugging code,
 ;;     is compiled only when this feature is present.
 ;; Note that the extra information recorded by the compiler at
 ;; this higher level of OPTIMIZE DEBUG includes the source location
 ;; forms. In order for the debugger to use this information, it has to
 ;; re-READ the source file. In an ordinary installation of SBCL, this
 ;; re-READing may not work very well, for either of two reasons:
 ;;   * The sources aren't present on the system in the same location that
 ;;     they were on the system where SBCL was compiled.
 ;;   * SBCL is using the standard readtable, without the added hackage
 ;;     which allows it to handle things like target features.
 ;; If you want to be able to use the extra debugging information,
 ;; therefore, be sure to keep the sources around, and run with the
 ;; readtable configured so that the system sources can be read.
 ; :sb-show

 ;; Enable extra debugging output in the assem.lisp assembler/scheduler
 ;; code. (This is the feature which was called :DEBUG in the
 ;; original CMU CL code.)
 ; :sb-show-assem

 ;; Setting this makes SBCL more "fluid", i.e. more amenable to
 ;; modification at runtime, by suppressing various INLINE declarations,
 ;; compiler macro definitions, FREEZE-TYPE declarations; and by
 ;; suppressing various burning-our-ships-behind-us actions after
 ;; initialization is complete; and so forth. This tends to clobber the
 ;; performance of the system, so unless you have some special need for
 ;; this when hacking SBCL itself, you don't want this set.
 ; :sb-fluid

 ;; Enable code for collecting statistics on usage of various operations,
 ;; useful for performance tuning of the SBCL system itself. This code
 ;; is probably pretty stale (having not been tested since the fork from
 ;; base CMU CL) but might nonetheless be a useful starting point for
 ;; anyone who wants to collect such statistics in the future.
 ; :sb-dyncount

 ;; Peter Van Eynde's increase-bulletproofness code
 ;; This is not maintained or tested in current SBCL, but I haven't
 ;; gone out of my way to remove or break it, either.
 ; :high-security
 ; :high-security-support

 ;; multiprocessing support
 ;; This is not maintained or tested in current SBCL. I haven't gone out
 ;; of my way to break it, but since it's derived from an old version of 
 ;; CMU CL where multiprocessing was pretty shaky, it's likely to be very
 ;; flaky now.
 ;;   :MP enables multiprocessing
 ;;   :MP-I486 is used, only within the multiprocessing code, to control
 ;;            what seems to control processor-version-specific code. It's
 ;;            probably for 486 or later, i.e. could be set as long as
 ;;            you know you're not running on a 386, but it doesn't seem
 ;;            to be documented anywhere, so that's just a guess.
 ; :mp
 ; :mp-i486

 ;; KLUDGE: used to suppress stale code related to floating point infinities.
 ;; I intend to delete this code completely some day, since it was a pain
 ;; for me to try to work with and since all benefits it provides are
 ;; non-portable. Until I actually pull the trigger, though, I've left
 ;; various stale code in place protected with #!-SB-INFINITIES.
 ; :sb-infinities

 ;; This affects the definition of a lot of things in bignum.lisp. It
 ;; doesn't seem to be documented anywhere what systems it might apply to.
 ;; It doesn't seem to be needed for X86 systems anyway.
 ; :32x16-divide

 ;; This is probably true for some processor types, but not X86. It affects
 ;; a lot of floating point code.
 ; :negative-zero-is-not-zero

 ;; This is mentioned in cmu-user.tex, which says that it enables
 ;; the compiler to reason about integer arithmetic. It also seems to
 ;; control other fancy numeric reasoning, e.g. knowing the result type of
 ;; a remainder calculation given the type of its inputs.
 ;; KLUDGE: Even when this is implemented for the target feature list,
 ;; the code to implement this feature will not generated in the
 ;; cross-compiler (i.e. will only be generated in the target compiler).
 ;; The reason for this is that the interval arithmetic routines used
 ;; to implement this feature are written under the assumption that
 ;; Lisp arithmetic supports plus and minus infinity, which isn't guaranteed by
 ;; ANSI Common Lisp. I've tried to mark the conditionals which implement
 ;; this kludge with the string CROSS-FLOAT-INFINITY-KLUDGE so that
 ;; sometime it might be possible to undo them (perhaps by using
 ;; nice portable :PLUS-INFINITY and :MINUS-INFINITY values instead of
 ;; implementation dependent floating infinity values, which would
 ;; admittedly involve extra consing; or perhaps by finding some cleaner
 ;; way of suppressing the construction of this code in the cross-compiler).
 ;; KLUDGE: Even after doing the KLUDGE above, the cross-compiler doesn't work,
 ;; because some interval operations are conditional on PROPAGATE-FUN-TYPE
 ;; instead of PROPAGATE-FLOAT-TYPE. So for now, I've completely turned off
 ;; a compiler which works, then I can think about getting the optimization
 ;; to work.) -- WHN 19990702
 ; :propagate-float-type

 ;; According to cmu-user.tex, this enables the compiler to infer result
 ;; types for mathematical functions a la SQRT, EXPT, and LOG, allowing
 ;; it to e.g. eliminate the possibility that a complex result will be
 ;; generated.
 ;; KLUDGE: turned off as per the comments for PROPAGATE-FLOAT-TYPE above
 ; :propagate-fun-type

 ;; It's unclear to me what this does (but it was enabled in the code that I
 ;; picked up from Peter Van Eynde). -- WHN 19990224

 ;; This is set in classic CMU CL, and presumably there it means
 ;; that the floating point arithmetic implementation
 ;; conforms to IEEE's standard. Here it definitely means that the
 ;; floating point arithmetic implementation conforms to IEEE's standard.
 ;; I (WHN 19990702) haven't tried to verify
 ;; that it does conform, but it should at least mostly conform (because
 ;; the underlying x86 hardware tries).

 ;; This seems to be the pre-GENCGC garbage collector for CMU CL, which was
 ;; AFAIK never supported for the X86.
 ; :gengc

 ;; CMU CL had, and we inherited, code to support 80-bit LONG-FLOAT on the x86
 ;; architecture. Nothing has been done to actively destroy the long float
 ;; support, but it hasn't been thoroughly maintained, and needs at least
 ;; some maintenance before it will work. (E.g. the LONG-FLOAT-only parts of
 ;; genesis are still implemented in terms of unportable CMU CL functions
 ;; which are not longer available at genesis time in SBCL.) A deeper
 ;; problem is SBCL's bootstrap process implicitly assumes that the
 ;; cross-compilation host will be able to make the same distinctions
 ;; between floating point types that it does. This assumption is
 ;; fundamentally sleazy, even though in practice it's unlikely to break down
 ;; w.r.t. distinguishing SINGLE-FLOAT from DOUBLE-FLOAT; it's much more
 ;; likely to break down w.r.t. distinguishing DOUBLE-FLOAT from LONG-FLOAT.
 ;; Still it's likely to be quite doable to get LONG-FLOAT support working
 ;; again, if anyone's sufficiently motivated.
 ; :long-float

 ;; miscellaneous notes on other things which could have special significance
 ;; in the *FEATURES* list

 ;; notes on the :NIL and :IGNORE features:
 ;; #+NIL is used to comment out forms. Occasionally #+IGNORE is used
 ;; for this too. So don't use :NIL or :IGNORE as the names of features..

 ;; notes on :SB-XC and :SB-XC-HOST features (which aren't controlled by this
 ;; file, but are instead temporarily pushed onto *FEATURES* or
 ;; *TARGET-FEATURES* during some phases of cross-compilation):
 ;; :SB-XC-HOST stands for "cross-compilation host" and is in *FEATURES*
 ;; during the first phase of cross-compilation bootstrapping, when the
 ;; host Lisp is being used to compile the cross-compiler.
 ;; :SB-XC stands for "cross compiler", and is in *FEATURES* during the second
 ;; phase of cross-compilation bootstrapping, when the cross-compiler is
 ;; being used to create the first target Lisp.

 ;; notes on the :SB-ASSEMBLING feature (which isn't controlled by
 ;; this file):
 ;; This is a flag for whether we're in the assembler. It's
 ;; temporarily pushed onto the *FEATURES* list in the setup for
 ;; the ASSEMBLE-FILE function. It would be a bad idea
 ;; to use it as a name for a permanent feature.

 ;; notes on local features (which are set automatically by the
 ;; configuration script, and should not be set here unless you
 ;; really, really know what you're doing):
 ;; machine architecture features:
 ;;   :x86 ; any Intel 386 or better, or compatibles like the AMD K6 or K7
 ;;   (No others are supported by SBCL as of 0.6.7, but :alpha or 
 ;;   :sparc support could be ported from CMU CL if anyone is
 ;;   sufficiently motivated to do so.)
 ;;   (CMU CL also had a :pentium feature, which affected the definition 
 ;;   of some floating point vops. It was present but not enabled in the
 ;;   CMU CL code that SBCL is derived from, and is present but stale
 ;;   in SBCL as of 0.6.7.)
 ;; operating system features:
 ;;   :linux   = We're intended to run under some version of Linux.
 ;;   :bsd     = We're intended to run under some version of BSD Unix. (This
 ;;              is not exclusive with the features which indicate which
 ;;              particular version of BSD we're intended to run under.)
 ;;   :freebsd = We're intended to run under FreeBSD.
 ;;   :openbsd = We're intended to run under FreeBSD.
 ;; (No others are supported by SBCL as of 0.6.7, but :hpux or
 ;; :solaris support could be ported from CMU CL if anyone is
 ;; sufficiently motivated to do so.)

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