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You can work productively with SBCL without knowing anything understanding anything about where it came from, how it is implemented, or how it extends the ANSI Common Lisp standard. However, a little knowledge can be helpful in order to understand error messages, to troubleshoot problems, to understand why some parts of the system are better debugged than others, and to anticipate which known bugs, known performance problems, and missing extensions are likely to be fixed, tuned, or added.
SBCL is descended from CMUCL, which is itself descended from Spice Lisp, including early implementations for the Mach operating system on the IBM RT, back in the 1980s. Some design decisions from that time are still reflected in the current implementation:
SBCL also inherited some newer architectural features from CMUCL. The most important is that on some architectures it has a generational garbage collector (“GC”), which has various implications (mostly good) for performance. These are discussed in another chapter, Efficiency.
SBCL has diverged from CMUCL in that SBCL is now essentially a
“compiler-only implementation” of Common Lisp. This is a change in
implementation strategy, taking advantage of the freedom “any of these
facilities might share the same execution strategy” guaranteed in the
ANSI specification section 3.1 (“Evaluation”). It does not mean SBCL
can't be used interactively, and in fact the change is largely invisible
to the casual user, since SBCL still can and does execute code
interactively by compiling it on the fly. (It is visible if you know how
to look, like using compiled-function-p
; and it is visible in the
way that that SBCL doesn't have many bugs which behave differently in
interpreted code than in compiled code.) What it means is that in SBCL,
the eval
function only truly “interprets” a few easy kinds of
forms, such as symbols which are boundp
. More complicated forms
are evaluated by calling compile
and then calling funcall
on the returned result.
The direct ancestor of SBCL is the x86 port of CMUCL. This port was in some ways the most cobbled-together of all the CMUCL ports, since a number of strange changes had to be made to support the register-poor x86 architecture. Some things (like tracing and debugging) do not work particularly well there. SBCL should be able to improve in these areas (and has already improved in some other areas), but it takes a while.
On the x86 SBCL – like the x86 port of CMUCL – uses a conservative GC. This means that it doesn't maintain a strict separation between tagged and untagged data, instead treating some untagged data (e.g. raw floating point numbers) as possibly-tagged data and so not collecting any Lisp objects that they point to. This has some negative consequences for average time efficiency (though possibly no worse than the negative consequences of trying to implement an exact GC on a processor architecture as register-poor as the X86) and also has potentially unlimited consequences for worst-case memory efficiency. In practice, conservative garbage collectors work reasonably well, not getting anywhere near the worst case. But they can occasionally cause odd patterns of memory usage.
The fork from CMUCL was based on a major rewrite of the system bootstrap process. CMUCL has for many years tolerated a very unusual “build” procedure which doesn't actually build the complete system from scratch, but instead progressively overwrites parts of a running system with new versions. This quasi-build procedure can cause various bizarre bootstrapping hangups, especially when a major change is made to the system. It also makes the connection between the current source code and the current executable more tenuous than in other software systems – it's easy to accidentally “build” a CMUCL system containing characteristics not reflected in the current version of the source code.
Other major changes since the fork from CMUCL include