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 weirdnesshttp://www.informit.com/articles/article.aspx?p=770362 - 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 instructionhttp://msdn.microsoft.com/en-us/library/590b9ks9(VS.80).aspxto 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?