xapian-core 1.4.21: 20 tests fail on i686

[Olly Betts]

On Sun, Oct 23, 2022 at 03:11:07PM +0300, Vitaly Chikunov wrote:
> (Unable to register on bug tracker, so writing here.)
> While building package for ALT Linux I found that when tests are run on
> i686 (32-bit x86 flavour) with 'make check' for 1.4.21 release, they
> fail with:

I can only reproduce your problem if I configure with --disable-sse,
which is not a recommended configuration because on x86 without SSE, we
get 387 FP's excess precision inflicted on us.

There are hacks in the code to work around the worst problems that
causes (such as segfaults due to undefined behaviour caused by not being
able to reliably even use an FP comparison in a sort comparison function)
but more subtle problems remain and we don't recommend using
--disable-sse.

Fundamentally it's hard to order results correctly by an FP value when
the same equation evaluated with the same inputs in two places in the
code is able to give different answers depending which intermediate
results are spilled to memory.

If you're using GCC, -ffloat-store mostly avoids the problem, but at
a significant performance cost because FP values get written to memory
everywhere rather than carried in registers when possible, and that cost
is paid by everyone using the package on i686 when most of them have a
perfectly capable FP unit.

GCC's -fexcess-precision=standard option would provide a solution with
less overhead, but sadly it's only been implemented for C.

I'm not sure what the ALT Linux baseline for i686 is, but if you really
need to build binary packages which will run on processors without SSE,
I'd strongly recommend the approach describing in the last entry here:

https://trac.xapian.org/wiki/PackagingXapian

That way most x86 users get a build using SSE (i.e. one which works
properly), and the problems excess precision causes will only affect the
dwindling number of users who really have an x86 old enough not to have
SSE.

> (Empty lines removed.) On other architectures we build tests succeed
> (aarch64, > arm7hf, ppc64le, x86_64). Would appreciate help resolving
> this.

If you insist on using --disable-sse, the simplest solution is to not
run the testsuite.  (The purpose of the testsuite is to find bugs; an
effect of --disable-sse is essentially to introduce bugs...)

Cheers,
    Olly