[Haskell-cafe] Possible floating point bug in GHC?

[Malcolm Wallace]

Interesting.  This could be the cause of a weird floating point bug  
that has been showing up in the ghc testsuite recently, specifically  
affecting MacOS/Intel (but not MacOS/ppc).
     http://darcs.haskell.org/testsuite/tests/ghc-regress/lib/Numeric/num009.hs

That test compares the result of the builtin floating point ops with  
the same ops imported via FFI.  The should not be different, but on  
Intel they sometimes are.

Regards,
     Malcolm

On 3 Apr 2009, at 18:58, Peter Verswyvelen wrote:

> For days I'm fighting against a weird bug.
>
> My Haskell code calls into a C function residing in a DLL (I'm on  
> Windows, the DLL is generated using Visual Studio). This C function  
> computes a floating point expression. However, the floating point  
> result is incorrect.
>
> I think I found the source of the problem: the C code expects that  
> all the Intel's x86's floating point register tag bits are set to 1,  
> but it seems the Haskell code does not preserve that.
>
> Since the x86 has all kinds of floating point weirdness - it is both  
> a stack based and register based system - so it is crucially  
> important that generated code plays nice. For example, when using  
> MMX one must always emit an EMMS instruction to clear these tag bits.
>
> If I manually clear these tags bits, my code works fine.
>
> Is this something other people encountered as well? I'm trying to  
> make a very simple test case to reproduce the behavior...
>
> I'm not sure if this is a visual C compiler bug, GHC bug, or  
> something I'm doing wrong...
>
> Is it possible to annotate a foreign imported C function to tell the  
> Haskell code generator the functioin is using floating point  
> registers somehow?