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Am 2017-04-04 18:10, schrieb Luiz Henrique de Figueiredo:
It's mostly the order the opcodes are listed in the enum defined in lopcodes.h. A switch whose cases are in increasing order should be easily handled by the C compiler, which can generated a simple jump table. More generally,if the cases form an interval [1,n] with n small, then a simple jump table works, even if the cases are not ordered.
And the compiler does so quite efficiently. After stumbling upon this old paper:
https://www.jilp.org/vol5/v5paper12.pdfI had a go at making the core vm loop of lua 5.3.4 into a direct threaded interpreter. The change itself was pretty easy, basically it involved turning the big switch statement in luaV_execute() into a jumptable and a goto. This did result in a speedup, but the results are less than spectacular. On the average, a speedup of about 3% has been gained across a selection of benchmarks from here: http://benchmarksgame.alioth.debian.org/u64q/lua.html (with output and gc turned off), but for some cases the modified interpreter was even a little bit slower than the vanilla lua interpreter. An interesting observation was that the distribution of speedups across the benchmarks were quite different on the 2 different processors I could test this on, although the average was the same.
So, in conclusion, this was an interesting exercise, but compilers and processors have advanced quite a bit since the paper was written, and so the results are not worth maintaining a custom code base.
Gunnar