Re: Computed goto optimization of vanilla Lua

[Jean-Luc Jumpertz <jean-luc@...>]

Thanks for sharing this. It looks like a really interesting and easy-to-perform optimization (even if not new to everyone in the list ;).
I will definitely check how it performs in my Lua implementation on iOS.

Jean-Luc

> Le 3 févr. 2016 à 23:28, Nagaev Boris <bnagaev@gmail.com> a écrit :
> 
> Hi,
> 
> computed gotos are a feature of modern compilers [1]. They can be used
> as a faster replacement for switch-based VM [2]. Many programming
> languages have VMs implemented in computed gotos (at least, Ruby and
> Python).
> 
> The computed goto version is faster because of two reasons [2]:
> 
>  * The switch does a bit more per iteration because of bounds checking.
>  * The effects of hardware branch prediction.
> 
> I have applied this optimization to VM of Lua 5.3.2, file src/lvm.c
> [3]. It was very easy, because VM uses macros vmdispatch, vmcase and
> vmbreak. I have redefined these macros and created a dispatch table.
> 
> It passes Lua basic tests (path/lua -e"_U=true" all.lua) [4].
> 
> My benchmark [5] shows speedup of 1.12:
> 
> $ time ./src/lua.orig ~/lang-bench/f3/test.lua
> 499500
> 
> real    0m28.208s
> user    0m28.166s
> sys     0m0.004s
> 
> $ time ./src/lua ~/lang-bench/f3/test.lua
> 499500
> 
> real    0m25.066s
> user    0m25.030s
> sys     0m0.000s
> 
> 
> This commit is a draft, not a final contribution. If a compiler
> doesn't support computed gotos, the switch based implementation should
> be used, as before.
> 
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
> [2] http://eli.thegreenplace.net/2012/07/12/computed-goto-for-efficient-dispatch-tables
> [3] https://github.com/starius/lua/commit/b10eb122ad1e6f94e3ebf100179515ed66921756
> [4] http://www.lua.org/tests/#basic
> [5] https://github.com/starius/lang-bench/blob/master/f3/test.lua
> 
> -- 
> 
> 
> Best regards,
> Boris Nagaev
>