Last time, I talked about the performance drawbacks of bootstrapping a compiler using an interpreter, and how this was one of the factors that led me to bootstrap via SBCL. But Pachuco does contain a simple metacircular interpreter, used to support its Common-Lisp-style macro system. So I thought it would be interesting to run the compiler under the interpreter, and see how long it actually took.

The changes to support this are modest; their focus is support for passing command line arguments down into the program running under the interpreter. But this isn't of much general use, so the changes are off in a branch.

In the Pachuco build process, stage1 is the process of bootstrapping the compiler under SBCL, i.e. running the compiler under SBCL. Here is a timed run of stage1:

$ rm build/stage1.s

$ time make build/stage1.s

mkdir -p build

scripts/compile -C scripts/sbcl-wrapper -s -o build/stage1 language/util.pco language/expander.pco language/interpreter.pco compiler/mach.pco compiler/mach-x86_64.pco compiler/compiler.pco compiler/codegen.pco compiler/stack-traditional.pco compiler/stack-no-fp.pco compiler/codegen-x86_64.pco compiler/driver.pco compiler/drivermain.pco


real    0m6.736s

user    0m5.604s

sys     0m1.106s

Now here is the equivalent running under the interpreter — the compiler running under the interpreter running under SBCL.

$ time make build/stage1i.s

$ mkdir -p build

scripts/sbcl-wrapper interpret runtime/runtime.pco runtime/runtime2.pco runtime/gc.pco language/util.pco language/expander.pco language/interpreter.pco compiler/mach.pco compiler/mach-x86_64.pco compiler/compiler.pco compiler/codegen.pco compiler/stack-traditional.pco compiler/stack-no-fp.pco compiler/codegen-x86_64.pco compiler/driver.pco compiler/drivermain.pco -- compile runtime/runtime.pco runtime/runtime2.pco runtime/gc.pco language/util.pco language/expander.pco language/interpreter.pco compiler/mach.pco compiler/mach-x86_64.pco compiler/compiler.pco compiler/codegen.pco compiler/stack-traditional.pco compiler/stack-no-fp.pco compiler/codegen-x86_64.pco compiler/driver.pco compiler/drivermain.pco >build/stage1i.s


real    14m9.997s

user    13m30.174s

sys     0m12.979s

So the compiler runs 150 times more slowly under the interpreter.

We can do the same experiment, but using Pachuco rather than SBCL. stage2 is the process of building the compiler using the result of stage1, i.e. building Pachuco with itself:

$ rm build/stage3.s

$ time make build/stage3.s

mkdir -p build

scripts/compile -C build/stage2 -s -o build/stage3 language/util.pco language/expander.pco language/interpreter.pco compiler/mach.pco compiler/mach-x86_64.pco compiler/compiler.pco compiler/codegen.pco compiler/stack-traditional.pco compiler/stack-no-fp.pco compiler/codegen-x86_64.pco compiler/driver.pco compiler/drivermain.pco


real    0m2.034s

user    0m1.633s

sys     0m0.362s

And now the equivalent with the interpreter in the mix. That is, the Pachuco-compiled interpreter running the compiler compiling itself. Got that?

$ time make build/stage2i.s

mkdir -p build

build/stage1 interpret runtime/runtime.pco runtime/runtime2.pco runtime/gc.pco language/util.pco language/expander.pco language/interpreter.pco compiler/mach.pco compiler/mach-x86_64.pco compiler/compiler.pco compiler/codegen.pco compiler/stack-traditional.pco compiler/stack-no-fp.pco compiler/codegen-x86_64.pco compiler/driver.pco compiler/drivermain.pco -- compile runtime/runtime.pco runtime/runtime2.pco runtime/gc.pco language/util.pco language/expander.pco language/interpreter.pco compiler/mach.pco compiler/mach-x86_64.pco compiler/compiler.pco compiler/codegen.pco compiler/stack-traditional.pco compiler/stack-no-fp.pco compiler/codegen-x86_64.pco compiler/driver.pco compiler/drivermain.pco >build/stage2i.s


real    5m2.325s

user    5m0.765s

sys     0m0.926s

Almost exactly the same slowdown as under SBCL: 150 times.

(You might also notice that the Pachuco-compiled cases are about 3 times as fast as under SBCL. But this is not an apples-to-apples comparison. First, SBCL reads Lisp source code and has to compile it before running it, whereas the Pachuco cases involve the native executables produced by the Pachuco compiler (though this start-up cost for SBCL should be negligible in the long-running cases). Secondly, I have SBCL configured to prioritize safety and debuggability rather than optimizing performance during compilation. Thirdly, Pachuco currently cuts corners for the sake of simplicity in ways that help its performance.)

So the interpreter does indeed introduce a slow down of a couple of orders of magnitude. The interpreter could quite easily be made more efficient, and performance improvements could be valuable: the performance of the interpreter is of practical important due to its role in the implementation of the macro system. Because Pachuco features a small core language, on top of which many control structures and language features are implemented using macros, a significant fraction of the time taken to compile a program is actually spent performing macro expansion, and thus in the interpreter. So some low-hanging fruit for speeding up compilation could be in the interpreter! But at this stage it seems unlikely to be worth sacrificing the simplicity and brevity of the interpreter.