GDB jumps to wrong lines in out of order fashion

Question

Application Setup : I've C++11 application consuming the following 3rd party libraries :

• boost 1.51.0
• cppnetlib 0.9.4
• jsoncpp 0.5.0

The application code relies on several in-house shared objects, all of them developed by my team (classical link time against those shared objects is carried out, no usage of dlopen etc.)

I'm using GCC 4.6.2 and the issue appears when using GDB 7.4 and 7.6.

OS - Red Hat Linux release 7.0 (Guinness) x86-64

The issue While hitting breakpoints within the shared objects code, and issuing gdb next command, sometimes GDB jumps backward to certain lines w/o any plausible reason (especially after exceptions are thrown, for those exceptions there suitable catch blocks)

Similar issues in the web are answered in something along the lines 'turn off any GCC optimization) but my GCC CL clearly doesn't use any optimization and asked to have debug information, pls note the -O0 & -g switches :

COLLECT_GCC_OPTIONS= '-D' '_DEBUG' '-O0' '-g' '-Wall' '-fmessage-length=0' '-v' '-fPIC' '-D' 'BOOST_ALL_DYN_LINK' '-D' 'BOOST_PARAMETER_MAX_ARITY=15' '-D' '_GLIBCXX_USE_NANOSLEEP' '-Wno-deprecated' '-std=c++0x' '-fvisibility=hidden' '-c' '-MMD' '-MP' '-MF' 'Debug_x64/AgentRegisterer.d' '-MT' 'Debug_x64/AgentRegisterer.d' '-MT' 'Debug_x64/AgentRegisterer.o' '-o' 'Debug_x64/AgentRegisterer.o' '-shared-libgcc' '-mtune=generic' '-march=x86-64'

Please also note as per Linux DSO best known methods, we have hidden visibility of symbols, only classes we would like to expose are being exposed (maybe this is related ???)

What should be the next steps in root causing this issue ?

Answer 1

I had a code producing wrong output, and when I tried to debug it with gdb, the lines were jumping arbitrarily. Finally I figured that it was not a gdb problem but a bug in g++: when -O3 was used the last line of a constructor was getting skipped. If I put a printf line after that line, the code would work fine! After changing CFLAGS from -O3 to -O0, the code gave the correct output. I was using c++11 with gcc-5.4.0

Answer 2

This sort of problem is usually GIGO -- gdb is just acting in the way that the compiler has instructed it to act. So, it's typically a compiler bug rather than a gdb bug. I've seen this happen even with -O0 compilations. The example that comes to mind is that some versions of g++ emitted the location of a variable's declaration when emitting a call to the variable's destructor. This lead to this odd jumping behavior in otherwise straight-line code.