float behaivior on ARMv8-A

N30dG christian.dreihsig@t-online.de
Sun May 1 17:46:00 GMT 2016


Hello,

forgive me if my question is a litte bit weak in content and linguistic. I'm
only a Hobbyist and english is not my nativ.

I'm trying to compile an App from Einstein@Home for AARCH64 using GCC.
Einstein@Home is a DC-Projekt using Boinc. The App mainly calculates FFT's
in single-precision using FFTW-lib. After the calculations are finished it
send's them back to the Server and validates them against the same Tasks
calculated by an different Host. Host's can vary over many different
CPU-Architectures.

The Problem is that Task's from my ARMv8-App-Version(tryed 32bit and 64bit)
are mostly above the threshold of the Validator. Compiling the App with the
same Settings for ARMv7 on the same Device delivers Valid results. I've
tryed this with many different GCC-versions(4.9.3; 5.3.0; 6.0.1),
lib-versions, with and without using of NEON. The result is always the same:
Compiling the App with -march=ARMv7-A gives Valid results, Compiling with
-march=ARMv8-A gives me mostly invalid results(not all).
I've talked to a Project-developer from the Einstein-Project and he say's me
that my ARMv8-results are slightly(not much) different from the ones
compiled for ARMv7. This occours on AARCH32 and AARCH64 Versions. There must
be somthink changed in the float-handling when compiled for ARMv8-A but I
really don't know?

Sorry for that squishy Question but I don't know where to start to debug
this.



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