Package: g++-8 Version: 8.2.0-4 Severity: serious Justification: -O2 results in different double precision number result. Affects: 906708
This bug is found in package src:hepmc, which currently FTBFS on i386, arm64, ppc64el and s390x. https://buildd.debian.org/status/package.php?p=hepmc - arm64: difference is : 3.55271e-15 - i386: difference is : 1.77636e-15 - ppc64el: difference is : 3.55271e-15 - s390x: difference is : 3.55271e-15 Data Matrix: * amd64 architecture + GCC-8: -O2 : OK -O0 : OK + Clang-6.0 -O2 : OK -O0 : OK * ppc64el architecture + GCC-8: -O2 : FTBFS because floating point precision beyond tolerance -O0 : OK + Clang-6.0: -O2 : OK -O0 : OK + GCC-7: -O2 : FTBFS + GCC-6: -O2 : FTBFS + GCC-5: -O2 : FTBFS * i386, arm64, s390x not tested. * In testSimpleVector.cc, the expected result of v4.m2() and vector.m2() are both 2.4200000000000053 . * -O2 and -O0 can be specified in DEB_CXXFLAGS_MAINT_APPEND * default compiler can be switched by simply e.g. export CC=clang export CXX=clang++ in debian/rules I wanted to trace into testSimpleVector.cc with gdb to find out what's going wrong. However this bug only occurs with -O2 option with which gdb cannot trace the C++ code line by line. Clang-6 doesn't FTBFS with the same compiler flags, so I consider this bug is an RC bug of GCC. I totally forgot how to dump the detail about gcc's -O2 option. Hence I'm not sure if gcc -O2 used something like -ffast-math .
