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Optimisation Fine Tuning, 1.0 branch (2017.04.10.)

Attila Krasznahorkay requested to merge akraszna/atlasexternals:OptimisationFineTuning-1.0-20170410 into 1.0

This is to fix ATLINFR-1642.

Until now the compiler optimisation flags were simply left at their default values. CMake at the moment sets the following values for these for GCC:

CMAKE_CXX_FLAGS_RELEASE = -DNDEBUG -O3
CMAKE_CXX_FLAGS_RELWITHDEBINFO = -DNDEBUG -O2 -g
CMAKE_CXX_FLAGS_DEBUG = -g

It was the -O3 setting that was causing us trouble so far.

With this update the AtlasCompilerSettings.cmake file now explicitly sets these flags. So that by default we would use -O2 for Release mode as well.

In order to keep it possible to override these settings from the command line (for experimental builds for instance), I had to introduce 3 new cache variables. (There's a long explanation to why this was needed.) These are:

  • ATLAS_CXX_FLAGS_RELEASE
  • ATLAS_CXX_FLAGS_RELWITHDEBINFO
  • ATLAS_CXX_FLAGS_DEBUG

In order to use some custom value for the compiler optimisation instead of the new defaults, one has to set one of these from the command line.

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