recent KahanSum change causes a new test failure on ppc64le

[Eike Rathke]

Hi,

On Thursday, 2023-09-07 11:36:33 +0200, Stephan Bergmann wrote:

> On 9/7/23 08:25, Dan Horák wrote:
> > On Thu, 7 Sep 2023 08:16:28 +0200
> > Stephan Bergmann <sbergman at redhat.com> wrote:
> > > On 9/5/23 18:53, Dan Horák wrote:
> > > > seems the recent change [1] to KahanSum handling causes a test failure
> > > > on ppc64le.
> > > > 
> > > > Running scope as unit: -home-sharkcz-projects-libreoffice-workdir-CppunitTest-sc_statistical_functions_test.test:20230905152639:2378561.scope
> > > > [_RUN_____] StatisticalFunctionsTest::testStatisticalFormulasFODS
> > > > Testing load file:///home/sharkcz/projects/libreoffice//sc/qa/unit/data/functions/statistical/fods/KahanSum.fods:
> > > > /home/sharkcz/projects/libreoffice/sc/qa/unit/functions_test.cxx:49:StatisticalFunctionsTest::testStatisticalFormulasFODS
> > > > forced failure
> > > > - Testing file:///home/sharkcz/projects/libreoffice//sc/qa/unit/data/functions/statistical/fods/KahanSum.fods failed, Sheet2.A90 '=SUM(F3:F102)' result: 6.6, expected: 5
> > > 
> > > Interesting; I also saw that failure with my latest local build on macOS
> > > aarch64 against Clang trunk.  (But didn't debug it further and wrote it
> > > off as maybe some intermittent Clang trunk bug.)
> > > 
> > 
> > someone on IRC reported the same issue, also from macOS I believe
> 
> So, wild speculation, maybe it's a difference between x86-64 and all other
> architectures, given how that
> <https://gerrit.libreoffice.org/c/core/+/156253/> "Resolves: tdf#156985
> Treat adding two KahanSum differently" kept failing for (32-bit)
> <https://ci.libreoffice.org/job/gerrit_windows/> up until patch set 7. (And
> where patch set 8 reverted back to the original code for _WIN32, maybe in a
> misguided attempt to fix something that was seen failing on Windows 32-bit
> x86, but not known to (not) fail on Windows x86-64.)
> 
> Eike, any thoughts?

Hum.. seeing there's platform specific code in
sc/inc/arraysumfunctor.hxx executeFast() that for
#if defined(X86_64) || (defined(X86) && defined(_WIN32))
calls executeSSE2() (implemented in
sc/source/core/tool/arraysumSSE2.cxx) and for others executeUnrolled(),
I wonder if we never have X86 defined and thus the _WIN32 fails as well
with executeUnrolled() and that "unrolled pair-wise" algorithm has
a flaw.

It would be worth a try to simply call executeFast() only if SC_USE_SSE2
is defined, so the failing platforms skip executeUnrolled(), here
https://opengrok.libreoffice.org/xref/core/sc/inc/arraysumfunctor.hxx?r=7f15354c#89

Please report back if that helps and I'll prepare another patch.

  Eike

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