Re: lua internals debugging hints? : lua_number2integer() not working

[Mike Pall <mikelu-0510@...>]

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