Re: lua internals debugging hints? : lua_number2integer() not working
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- Subject: Re: lua internals debugging hints? : lua_number2integer() not working
- From: Mike Pall <mikelu-0510@...>
- Date: 2005年10月21日 15:53:56 +0200
Hi,
Roberto Ierusalimschy wrote:
> > This will probably be due to something mucking about with global fpu
> > flags, probably dx I remember having issues with this sort of thing
> > before.
>
> Can you give more details about that problem?
This is a well known issue when mixing DirectX and other
floating-point code. This affects lots of programs and not
just Lua. It has been mentioned on the list several times.
Quoting:
http://msdn.microsoft.com/archive/en-us/directx9_c_dec_2004/directx/GamesForWindows/TopIssues.asp
"Direct3D will set the floating-point unit to single-precision,
round-to-nearest as part of initialization (unless the
D3DCREATE_FPU_PRESERVE flag is used, in which case the FP control
word is untouched)."
Summary: This is not a Lua problem. It's DirectX violating the
x86/x87 ABI which requires the FP control-word to be in a specific
state. Lua depends on the standard setting in several places.
The new thing in Lua 5.1 is that it's _guaranteed_ to break,
because the new number2int conversion trick fails. And this one
is used in lots of places (e.g.: x = t[1]).
With earlier Lua versions it failed only intermittently -- which
is worse, because you may never find out.
Maybe this fact deserves a special mention in the manual.
Bye,
Mike