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.wall...@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? >> > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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