Re: LuaJIT and Floating Point Determinism
- From: Paul Du Bois <dubois@xxxxxxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Thu, 23 Jan 2014 16:48:12 -0800
As one data point, we've made determinism work on a large, physics- and
math-heavy game. Only within a single platform, of course; but it runs on
360, PS3, Windows, OSX, and Linux. So the idea itself isn't completely out
of the question.
p
On Mon, Jan 13, 2014 at 9:03 PM, malkia <malkia@xxxxxxxxx> wrote:
> It's possible that the code generated by luajit would work the same
> way on different cpus.
>
> But it's a real problem (outside) luajit. For example read this:
>
> http://www-archive.xenproject.org/files/xensummit_germany09/AMD.pdf#page=15
> (This is a problem not only for games, but for virtual machines
> (moving from one host machine to another)).
>
> "SSE reciprocal and reciprocal-square root approximation
> instructions: AMD and Intel provide different numbers of significant
> bits – RCPPS, RCPSS, RSQRTPS, RSQRTSS.
>
> x87 instructions with infinite series implementations can return
> answers whose results differ in the units in the last place (ULP). –
> Trigonometric functions – ranscendental functions – Reciprocals"
>
> (I think luajit still uses some x87 - at least that's what vm_x86.dasc
> shows but I could be wrong).
>
> If you happen to use external library called by luajit, or something
> that changes rounding modes without your knowledge (directx used to be
> such offender), then you would have differences.
>
> Here is a fun cpuid detection code
> found here:
> http://randomascii.wordpress.com/2013/07/16/floating-point-determinism/
>
> // CPUID is for wimps:
> __m128 input = { -997.0f };
> input = _mm_rcp_ps(input);
> int platform = (input.m128_u32[0] >> 16) & 0xf;
> switch (platform)
> {
> case 0×0: printf(“Intel.\n”); break;
> case 0×7: printf(“AMD Bulldozer.\n”); break;
> case 0×8: printf(“AMD K8, Bobcat, Jaguar.\n”); break;
> default: printf(“Dunno\n”); break;
> }
>
> On Mon, Jan 13, 2014 at 11:32 AM, Alex <initrd.gz@xxxxxxxxx> wrote:
> >> It is not so hopeless
> >
> > I never said it was hopeless; I just wanted to be sure it wasn't. Also,
> > thanks for the library, I'll take a peek at it.
> >
> > --
> > Sincerely,
> > Alex Parrill
>
>
>
> --
> Dimiter "malkia" Stanev,
> ICQ: 21875894
> malkia@xxxxxxx
> malkia@xxxxxxxxx
>
>
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