Re: LuaJIT and Floating Point Determinism
- From: Alex <initrd.gz@xxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Mon, 13 Jan 2014 13:25:42 -0600
Thanks for the info Mike; I just wanted to be sure that it wasn't
completely hopeless before I begin considering and putting effort into the
idea.
On Mon, Jan 13, 2014 at 1:13 PM, Mike Pall <mike-1401@xxxxxxxxxx> wrote:
> Alex wrote:
> > The article states "you can’t use SSE or SSE2 for floating point, because
> > it’s too under-specified to be deterministic", though I find that
> somewhat
> > suspect, and LuaJIT seems to compile with SSE2 enabled by default.
>
> Yes, that statement is uninformed. SSE *is* the way to get
> deterministic IEEE-conformant behavior on x86.
>
> In other words: if you use LuaJIT 2.0 on a 32 bit x86 platform,
> you need to explicitly enable SSE in the Makefile. It's on by
> default for x64 (and in general for LuaJIT 2.1).
>
> > My main concern is whether the trace optimizations or trace recording
> will
> > affect the order of operations or other things that may affect the
> output.
>
> That shouldn't affect the outcome, except if you invoke undefined
> or unspecified behavior. E.g. converting an out-of-range double to
> an integer may yield different results.
>
> > I think I've also read somewhere that trig functions and square root
> > implementations vary as well.
>
> Yup, that's pretty hopeless. You'll get to enjoy the differences
> between hardware implementations, different math libraries etc.
> Basically forget the whole idea, if it needs to be cross-platform
> and you intend to use anything other than + - * /.
>
> Oh, and stay away from denormals and infinities, too. Although
> there are standard rules for that, many mobile and embedded
> platforms take a shortcut and give you 'interesting' results. No
> way to tell without trying on each hardware/software combination.
>
> --Mike
>
>
--
Sincerely,
Alex Parrill
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