Rabbit: A Compiler for Scheme/Chapter 10

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86 10. Performance Measurements RABBIT has provision for metering runtime usage, and for controlling whether certain options in the optimizer are used. The standard test case has been RABBIT compiling itself (!); by running both interpreted and compiled version of this task, some comparisons have been made. Two different compiled versions have also been tested, where the code was produced with or without using the optimizer.

The overall speed gain of unoptimized compiled code over interpreted code has been measured to be a factor of 25. The speed gain ratio excluding time for garbage collection was 17, and the garbage collection time ratio was 34. (The SCHEME interpreter does a lot of consing. The straight runtime ratio of 17 is roughly typical for standard LISP compilers on non-numeric code.) The overall speed ratio of optimized compiled code to unoptimized compiled code has been measured to be 1.2. The speed ratio excluding garbage collection was 1.37, and the garbage collection time ratio was 1.07. We conclude that the amount of consing was reduced very little, despite optimizations which may eliminate closures, because the phase-2 analysis of closures eliminated most consing from that source anyway. Eliminations of register shuffling because of substitutions of one variable for another were probably more significant.

Combining these figures yields an overall speed ratio for optimized compiled code over interpreted code of about 30.

Turning now to the analysis of compilation time, as opposed to running time, we have found that using the optimizer approximately doubles the cost of compilation. It might be possible to reduce this with a more clever optimizer; presently RABBIT wastes much time redoing certain analysis unnecessarily. The extra time needed by the optimizer excluding garbage collection is only half

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87 again the overall compilation time, but the garbage collection time triples, because the optimizer copies and recopies parts of the program.

There is also one error check which is very expensive; it checks every argument of a combination against every other argument to check for possible side-effect conflicts (this is the "liberal" analysis in EFFS-ANALYZE, and the testing done by CHECK-COMBINATION-PEFFS). Use of this error check increased compilation time by thirty percent.