Re: Lua/LuaJIT2 performance on Android

[Mike Pall <mikelu-1104@...>]

Adam Strzelecki wrote:
> Mike, just wanted to ask whether support for VFP/NEON
> instruction somewhere close on roadmap?

The sponsorship currently only entails an ARMv5 soft-float port.
It may be extended later to add VFP support or new features of
ARMv6/ARMv7. But at this time I do not have a covenant for this.

Note that NEON isn't very useful here, because it only helps with
single-precision FP arithmetic. But Lua numbers are doubles.

> > For integer-heavy benchmarks, the ARM JIT compiler _will_ get into
> > the 10x-100x region. Some characteristics of these low-end devices
> > heavily penalize interpreters in comparison to native code.
> 
> Does it mean for soft-float devices integer internal
> representation needs/will be used instead of doubles (as on
> X86/X64) and NaN tagging to achieve this performance?

The ARM port uses the dual-number mode of LuaJIT. All Lua numbers
are represented either as integers or doubles internally. This is
transparent to the user of the VM. Except for the performance
effects, of course: e.g. 'for i=1,1e7 do end' runs much faster
than 'for i=1.5,1e7 do end'.

--Mike