luau-lang / luau

A fast, small, safe, gradually typed embeddable scripting language derived from Lua
https://luau.org
MIT License
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Fix mesh-normal-vector benchmark array access #1514

Closed zeux closed 1 week ago

zeux commented 1 week ago

mesh-normal-scalar correctly fills sequential values in the output for triangle cone function, but mesh-normal-vector accidentally reuses the loop index, which results in writes to every third index of the array (1, 4, etc.).

This is both slower (as the table turns into a hash map), and incorrect, especially as we have a scalar version of the benchmark that does the right thing.

Note: there's a bunch of inefficiencies in the benchmark code that I have not fixed (around field access mostly, e.g. writing to v.n and then immediately reading it again). These are not ideal for performance, but they can be valuable to keep as is because this redundancy is common in real-world code, and it would be nice to see codegen optimizations eliminating most of that overhead. This one, however, is a straight up bug, and sparse arrays should not really be the thing this benchmark hits.