Performance and Evaluation of Lisp Systems - Dreamsongs

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bits in the tag field of every object indicate where the CDR of that object (if any
exists) is to be found. If the CDR-code has the value CDR-normal, then the CDR
of the object is found by following the pointer in the next higher location. If
the CDR-code has the value CDR-next, then the CDR object is the next higher
location. If the CDR-code is CDR-NIL, then the CDR of the object is NIL. CDR-
coding is completely transparent to the user and reduces the space needed to store
list structure by nearly 50%. This scheme is essentially the same as that used on
the 3600.
2.4.4 The Microcompiler
The optimizing microcompiler eliminates all the overhead of macroinstruc-
tion processing and speeds up control flow and most forms of data manipulation.
Compile-time declarations (using Common Lisp syntax) allow further optimiza-
tion of function calling and open-coding of fixnum arithmetic.
As has been often noted, function-calling speed is quite central to the per-
formance of most Lisp programs. When generating a call from one microcoded
function to another, the microcompiler can generate a particularly fast form of
function call termed a ‘micro-micro’ call. In the micro-micro function call, the
call and return are only one microinstruction each and are pipelined using the
delayed-branch mechanism of the Lambda micromachine. The overhead of calling
and returning from a function is only 200 nanoseconds. Argument transmission
is also efficient under microcompilation.
To produce optimal code the microcompiler requires special declarations
in much the same manner as do Lisps running on conventional architectures.
Microcode-to-microcode (micro-micro) calls can only be produced using appro-
priate declarations. First, the microcompiler needs assurance that the target
function will indeed be microcoded and will reside in the control store when it is
called. Second, the micro-micro call pushes return addresses on a special stack,
256-words deep, in the Lambda micromachine. Deeply recursing functions must
arrange to check this stack for overflow occasionally—this process is controlled by
declarations. It is interesting to note, however, that all of these benchmarks were
run without overflow checks and without microstack overflow.
There are several language constructs that cannot appear in microcompiled
functions. Currently these include &REST arguments, CATCH/THROW control