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On 2017-10-15 02:58 PM, Dirk Laurie wrote:

2017-10-15 17:43 GMT+02:00 Soni L. <fakedme@gmail.com>:

I'm reading the Lua code, and it seems that Lua always checks the hash part,
even for tables that don't use it. Why is this?

That's not how I read the code. Here are the comments and return
statements of luaH_next.

   /* find original element */
   /* try first array part */
   /* a non-nil value? */
  ...
       return 1;
     }
   /* hash part */
   /* a non-nil value? */
  ...
       return 1;
...
   return 0;  /* no more elements */

It's quite possible to return without reaching the code dealing with
the hash part.
Look at luaH_get. e.g. t[1] always checks hash part which does have a cost. the only way to avoid that cost is to fill the array part: if you have 256 possible values, you need to fill t[1] to t[256]. and ofc t[0] always goes to the hash part so you also need to shift by one if 0 is a valid index. 

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