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

Peter Verswyvelen bugfact at gmail.com
Fri Apr 3 15:31:46 EDT 2009

Well this situation can indeed not occur on PowerPCs since these CPUs just
have floating point registers, not some weird dual stack sometimes /
registers sometimes architecture.
But in my case the bug is consistent, not from time to time.

So I'll try to reduce this to a small reproducible test case, maybe
including the assembly generated by the VC++ compiler.

On Fri, Apr 3, 2009 at 9:02 PM, Malcolm Wallace <
Malcolm.Wallace at cs.york.ac.uk> wrote:

> 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?
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