Re: ANN: LuaJIT 1.1.0 / Re: Lua x DirectX

["Framework Studios: Hugo" <hugo@...>]

If it 'only' means the FPU computes things differently internally, why does 
Lua start crashing as soon as DX sets this flag to single-precision? :-)

Using double-precision with DX means the driver will be setting/resetting 
the flag a lot (run-time) whilst this can be avoided simply by only using 
single-precision (compile-time). You make a good point DirectX itself would 
probably be fine on modern CPU-s if it wouldn't touch the flag at all, 
however this is not our choice; it simply is as it is.

A performance hit of 3.5% just for having some FPU flag in the correct state 
without even knowing if it actually really is used (SSE2) sounds like a very 
good reason not to use double-precision to me.

   Cheers,
       Hugo

P.S.: design-flaws -> check out DirectShow. ;-)

----- Original Message ----- 
From: "Andras Balogh" <andras.balogh@gmx.net>
To: "Lua list" <lua@bazar2.conectiva.com.br>
Sent: Thursday, March 16, 2006 4:51 PM
Subject: Re: ANN: LuaJIT 1.1.0 / Re: Lua x DirectX


> Having the FPU operate in full precision mode does not mean that you have 
> to send doubles to the GPU. I'm using 32 bit floats for geometry too. This 
> flag only means that internally, the FPU computes everything in 
> single/double precision. Besides, this 580FPS is an ill test case, normal 
> apps don't run at this rate. So it is probably limited by CPU, in which 
> case, I think that setting and resetting the FPU registers in every API 
> call probably hurts a bit (even then, 3.5% is nothing). If DirectX just 
> didn't touch it, it would probably be fine. I don't think there's any 
> speed difference internally. Modern FPUs do most instructions in one 
> cycle.. Sure, if you do software transforms (god forbid rasterizing), then 
> it _might_ give you an edge, but I just don't think it's worth the 
> trouble. Besides, DirectX was designed by humans too, it's not without 
> design flaws...
>
>
>
> andras
>
>
> On Thu, 16 Mar 2006 07:35:18 -0700, Framework Studios: Hugo 
> <hugo@framework-studios.com> wrote:
>
> > Well, one number I found on the decrease of Direct3D's speed with and 
> > without the FPU preserve flag:
> >
> > http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind0504b&L=directxdev&D=1&P=3121
> >
> > with: 560 fps
> > without: 580 fps
> >
> > However I think it is a bit beside the point to 'prove' this with 
> > numbers since DirectX more or less already chose single precision for us 
> > (for a good reason, I trust). Also it seems logical for a 3D API to be 
> > faster when using float-s in stead of double-s because twice the data 
> > can be pushed to the GPU with the same bandwidth / stored in VRAM. Isn't 
> > this the same for OpenGL?
> >
> > Looking at the performance of double vs float on modern CPU-s should be 
> > interesting though. Are double-s faster, slower or the same compared to 
> > float-s on 32-bit and 64-bit CPU architecture? What about the CPU-s 
> > people are actually using on average at the moment? (to sell games we 
> > need to look at what is average on the market, not only to what is 
> > top-notch :)
> >
> >         Cheers,
> >                 Hugo