Gcc O3 compile optimization

Ideas and Requirements
1

Hi All. I’m trying to compile Geant4 with -O3 optimization flags on x86_64-v3, but I get the following warnings when I am compiling:

  [4%] Building CXX object source/CMakeFiles/G4intercoms.dir/intercoms/src/G4VGlobalFastSimulationManager.cc.o
/home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/intercoms/src/G4UImanager.cc: In member function ‘void G4UImanager::SetAlias(const char*)’:
/home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/intercoms/src/G4UImanager.cc:760:20: warning: ‘*(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type*)((char*)&aliasValue + offsetof(G4String, G4String::<unnamed>.std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::<unnamed>))’ may be used uninitialized [-Wmaybe-uninitialized]
  760 |   if(aliasValue[0] == '"')
/home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/intercoms/src/G4UImanager.cc:759:12: note: ‘aliasValue’ declared here
  759 |   G4String aliasValue = aLine.substr(i + 1, aLine.length() - (i + 1));
      |            ^~~~~~~~~~

and also

[  6%] Building CXX object source/CMakeFiles/G4analysis.dir/analysis/csv/src/G4CsvAnalysisManager.cc.o
In file included from /usr/include/c++/12.2.1/ios:40,
                 from /usr/include/c++/12.2.1/istream:38,
                 from /usr/include/c++/12.2.1/sstream:38,
                 from /usr/include/c++/12.2.1/complex:45,
                 from /home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/global/management/include/G4Types.hh:74,
                 from /home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/global/management/include/G4ios.hh:35,
                 from /home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/global/management/include/globals.hh:35,
                 from /home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/graphics_reps/include/G4AttCheck.hh:46,
                 from /home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/graphics_reps/src/G4AttCheck.cc:28:
In static member function ‘static std::char_traits<char>::char_type* std::char_traits<char>::copy(char_type*, const char_type*, std::size_t)’,
    inlined from ‘static void std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_S_copy(_CharT*, const _CharT*, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12.2.1/bits/basic_string.h:423:21,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Allocator>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_M_replace(size_type, size_type, const _CharT*, size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12.2.1/bits/basic_string.tcc:532:22,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12.2.1/bits/basic_string.h:1647:19,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12.2.1/bits/basic_string.h:815:28,
    inlined from ‘G4AttCheck::G4AttCheck(const std::vector<G4AttValue>*, const std::map<G4String, G4AttDef>*)’ at /home/filippo/aur/geant4-full/src/geant4-v11.1.1/source/graphics_reps/src/G4AttCheck.cc:69:35:
/usr/include/c++/12.2.1/bits/char_traits.h:431:56: warning: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’ accessing 9223372036854775810 or more bytes at offsets -4611686018427387902 and [-4611686018427387903, 4611686018427387904] may overlap up to 9223372036854775813 bytes at offset -3 [-Wrestrict]
  431 |         return static_cast<char_type*>(__builtin_memcpy(__s1, __s2, __n));
      |                                        ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~

The second warning looks quite important, and it occurs multiple times.
Would it be possible to start looking into using -O3 instead of O2 when compiling, as this would eventually reduce run times?

Cheers

2

We’re only testing GCC 12.1 at the moment, so as these look like warnings we probably should fix would you mind posting a full report to Bugzilla please: https://bugzilla-geant4.kek.jp

Due to some issues we’ve seen in other builds at O3 level, release build flags are being reviewed for the next release. This may be a drop to O2, or O2 with addition specific options. Part of this work will also be the impact of performance of these combinations in typical Geant4 applications.

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