[Debian-med-packaging] Bug#819617: bcftools: FTBFS: test suite fails on several architectures

[Andreas Tille]

Hi Christian,

thanks for the hints.

On Sun, May 29, 2016 at 01:42:44PM +0200, Christian Seiler wrote:
> On Thu, 21 Jan 2016 22:29:16 -0500 "Aaron M. Ucko" <ucko at debian.org> wrote:
> > * On i386 and kfreebsd-i386, 57 other test cases failed with output
> >   differences (perhaps related to floating-point unit idiosyncracies --
> >   it normally works with double-extended precision internally, rather
> >   than rounding at every step).
> 
> I don't have anything to do with the package, but saw the bug
> referenced on debian-release, and know a bit about the x86 FPU.
> You can actually set the x86 FPU to use double precision internally
> (80bit long double calculations will not work as expected then,
> though, because the coprocessor will always truncate them to 64bit),
> by using:
> 
> static void set_fpu_mode()
> {
> #ifdef __i386__
>   unsigned short cw;
>   asm volatile ("fnstcw %0" : "=m" (cw));
>   cw = (cw & ~0x100) | 0x200;
>   asm volatile ("fldcw %0" :: "m" (cw));
> #endif
> }
> 
> (double extended precision is 0x300, double is 0x200 and single
> is 0x0, in case you want to change it back. There are other
> settings in the control word, so I would recommend touching only
> the bits 0x100 and 0x200.)
> 
> Other platforms (including x86_64) shouldn't have this trouble,
> because even if they support other floating point types, operations
> with the double data type are always done in double precision.
> 
> (Alternatively, you could use -msse2 as a compiler flag on new
> enough x86_32 processors, the compiler would then use the SIMD
> instructions instead of the older FPU, but that has the problem
> that the code then won't run on older processors, which are still
> supported in Debian at the moment.)

If you ask me the best solution would be to use -msse2 and just add some
code to throw an error if the machine has a to old FPU.  Typically
nobody will use those tools on old hardware anyway - so that's no
practical constraint.
 
> Hope that helps.

Yes, thanks for the hint

      Andreas. 




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