Could you confirm whether the following two tests fail for you after your latest change?

libtbx.python "/Users/rjgildea/tmp/tst_bootstrap/tmp/modules/cctbx_project/cctbx/maptbx/boost_python/tst_maptbx.py" [FAIL]
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libtbx.python "/Users/rjgildea/tmp/tst_bootstrap/tmp/modules/cctbx_project/scitbx/lbfgs/tst_lbfgs_fem.py" [FAIL]
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I see those too. Tracking them is too much effort indeed. I followed the route already suggested by Marcin: break -ffast-math in finer options and find the one breaking our code. After a round-trip through Clang source code (I wish they had a proper documentation of *all* their command-line options as it exists for gcc), I came with a fix which keeps all useful optimisations enabled while the 3 failing tests do now pass.