Bug in configure script for PPC64 machines

[Torbjorn Granlund]

If you too get spurious FAILs, then I think I might have figured out a
possible explanation.

Back in 2005, I isolated a problem with randomly clobbered registers
under the Linux kernel when running binaries for the 32-bit but using
the full powerpc64 instruction set.

I don't run GNU/Linux on any ppc64 machines I have control over, so I
cannot determine which kernels have this bug, even if I had the time
and enjoyed building Linux kernels.

I know the bug is in (Debian's version of) 2.6.11.

You can check if this is indeed the problem you're running into by
going into test/devel and then do,

  make try
  ./try mpn_mul_1

If that fails randomly, i.e. after a varying number of iterations,
you've almost certainly run into the kernel bug.

I reported this back in June 2005.

Some people might think configure should disable ppc64 instructions
when it detects a Linux kernel.  I suppose that would be a good thing,
at least if one can tell which kernels have the bug and which ones
don't.

Disabling important features in GMP to work around bugs in kernels and
compilers is in general a bad idea, unless it is disabled just for
problem kernels and compilers.  If we do it too broadly, we'll end up
with an empty envelope before long.

-- 
Torbjörn