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2016-09-07 00:00:00 UTC
Fast lookups in JIT-compiled maps

This post shows a way of optimizing lookup performance in maps associating integer keys to arbitrary data.


Some time ago, I reimplemented the RSX GPU command processor in the emulator, Nucleus. This GPU is made of several engines, each bound at a specific index (0-7) of the command processor, and each index provides a MMIO register window (0x0-0x1FFC). Commands are 16-bit bitfields containing an index (3-bit) and MMIO offset (13-bit). Recent userland drivers always bound engines to the same indices and there was a limited number valid MMIO offsets, our command processor was just a big hardcoded switch-case mapping commands to corresponding emulator function.

However, older or custom drivers might bind engines at different indices making our compile-time switch-case useless. Ignoring wasted memory, a static array of 2^16 entries could be a fast solution. Nevertheless, 32-bit or 64-bit commands could have made this impossible. Since lookup times are critical, this yields the question, what's the fastest way of doing a lookup in a set of sparse commands -or sparse non-random integers- generated at runtime? Should we use huge static arrays? Should we use hash tables? Which data structure will optimize lookup time?

Jitter solves this by letting the compiler decide that.

TODO: More information soon.