Re: Question about accessing Lua tables and arrays faster

[Steven Johnson <steve@...>]

More results in the meantime:

I have created a version of iterations, where I use 

  rawset(t, i, rawget(t, i) + 1) 
instead of 

  t[i] = t[i] + 1 

to skip any __index() and __newindex() calls completely.


And according to my tests rawget/rawset slow down array operations significantely compared to a simple Lua's t[i] access without metatables. Basically, rawset and rawget a C functions exposed via a standrad Lua C library. Therefore each invocation of these functions introduces the almost the same overhead as in case of my own C-based vector implementation (and obtained timings confirm it). Using simple t[i] = t[i] + 1 seems to be way more efficient, because it happens completely on Lua side and does not require and Lua->C function invocations.


-Leo

Since you mention "well-known tricks" [1], did you do something like

  local rawget, rawset = rawget, rawset

before the loop? If not, what you perceived about table lookup before will show up in a different way (looking those functions up in the globals or environment, each and every time). For little one-off stuff you might never notice, but over the course of a million loops...

[1] - http://www.lua.org/gems/sample.pdf