perm filename BAIL.APP[DOC,AIL] blob sn#158786
filedate 1975-05-12 generic text, type T, neo UTF8
Black Artist's Appendix
Producing and maintaining languages and compilers for
production use is still a form of witchcraft. The following are
offered for the edification of future black artists.
Execution-time objects whose representation requires more
than one word of memory must have a one-word "handle". The handle
contains a pointer to the object and may contain flag bits or counts,
etc. Indirection through the handle should be valid and should
produce meaningful results.
In SAIL, strings are represented by two words:
a word with character count and garbage collector mark
bits, followed by a PDP-10 byte pointer. However, there is no
handle. This has caused numerous problems and makes the implementation
inelegant. There is a separate register dedicated at runtime to the
maintenance of a string stack. String operations are expensive
in space, since two words must be moved around for each string.
Many compiler bugs had (have) their origin in the fact that a
string itemvar, a string, an integer itemvar, and an integer are
in four distinct classes, rather than two (itemvar or non-itemvar).
A better representation for strings is
handle: BYTE (13)length (23)@bytepointer.
Then the length, byte pinter, and first character of the string are
all accessible in one instruction once the handle is known.
[The accesses are LDB AC,[POINT 13,HANDLE,12]; MOVE AC,@HANDLE;
and ILDB AC,@HANDLE.] The garbage collector can use the left
half of the byte pointer for marking, since there is redundant
information which can be temporarily destroyed and later reconstructed.
SAIL does use handles for arrays. Each array has an allocation
cell which points to the array descriptor. However array descriptors
are contiguous to the array data. Thus it is impossilbe to pass
subarrays or array slices as arguments. Moral: array descriptors
should be separate from array data. A good representation is
dimension number, bounds, multipliers, offset
data: BLOCK total size
Then MOVE AC,HANDLE gets the address of the descriptor
MOVEI AC,@HANDLE gets the address of the first data word
SAIL code for expression evaluation is not very good. This is due
to strict one-pass compilation. A very messy (and buggy) part of the
compiler has to do with generating code "on the fly" for boolean expressions.
To retain the speed advantage of one pass but generate much better code,
the solution is to adopt one-and-a-half-pass compilation. The parser
should construct parse trees until an assignment expression is detected,
and then code for the assignment should be generated. This would easily
allow compilations such as
I←I+1; AOS AC,I
I←I+J; MOVE AC,J
I←IF B THEN K ELSE J MOVE AC,K
rather than the longer, straight-forward way they are now compiled.
Similarly, doubled-up skips and redundant subscripting could be
improved. The effect would be substantially getter code, for
a very modest increase in compilation time.