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----- Original Message -----
From: Robert G. Jakabosky
Date: 6/27/2011 7:22 PM

On Monday 27, Joshua Jensen wrote:

It isn't just about typing. It's also about performance. When written
like:

          player.ammo.shells = player.ammo.shells - 1

Lua must:

* Look up player.
* Look up ammo.
* Look up shells.
* Subtract 1.
* Look up player.
* Look up ammo.
* Set via hash the shells key to be the result of the subtraction.

That is almost right, the last two lookups come before the 'substract'
operation.

Opcodes:
1 GETGLOBAL	0 -1     ; player
2 GETTABLE 	0 0 -2  ; "ammo"
3 GETGLOBAL	1 -1     ; player
4 GETTABLE 	1 1 -2  ; "ammo"
5 GETTABLE 	1 1 -3  ; "shells"
6 SUB     	        1 1 -4  ; - 1
7 SETTABLE 	0 -3 1  ; "shells" -
Ah, right. I wrote the majority of the email without a build of Lua available to me. I kind of knew I'd end up getting it wrong. 
When written as "player.ammo.shells -= 1", Lua must:

* Look up player.
* Look up ammo.
* Look up shells.
* Subtract 1.
* Directly store the result.

Hooray!  (I hope this is how the patch works, anyway...)

The new syntax saves two lookups(one global, one table) in this example.

Opcodes:
1 GETGLOBAL	0 -1     ; player
2 GETTABLE 	0 0 -2  ; "ammo"
3 GETTABLE 	1 0 -3  ; "shells"
4 SUB_EQ   	1 -4
5 SETTABLE 	0 -3 1  ; "shells" -

The last operation is still a normal "SETTABLE".  No special support for
modifying a table slot directly, doing so wouldn't work with tables/userdata
that have __index/__newindex metamethods.
Okay, I understand why the SETTABLE would still be used, although I certainly like the idea of the performance gained by not needing the SETTABLE. 

Thank you for clarifying.  I really should have checked first.

-Josh