On 9/25/18 12:21 AM, Alexander Monakov wrote: > Hello, > > Here's the promised "libgcov summary"; sorry about the delay.
Thank you Alexander, I take it as productive discussion starting point. > > So libgcov has a bit unusual design where: > > - on the one hand, the library is static-only, has PIC code and may be > linked > into shared libraries, > > - almost all gcov symbols have "hidden" visibility so they don't participate > in dynamic linking > > - on the other hand, the __gcov_master symbol deliberately has default > visibility, presumably with the intention that a running program has > exactly > one instance of this symbol, although the exact motivation is unclear to > me. The only usage I see right now is support of __gcov_reset, __gcov_dump function. Which in my opinion should cover all loaded DSOs in an executable. > > This latter point does not reliably work as intended though: there are > scenarios > where a dynamically linked program will have multiple __gcov_masters anyway: > > - via repeated dlopen(RTLD_LOCAL) with main executable not linked against > libgcov > or not exporting libgcov symbols (as in PR 83879) Here we have a work-around: --dynamic-list-data. > - when shared libraries have version scripts that hide their __gcov_master > - when -Bsymbolic is in effect > > > Additionally, indirect call profiling symbols are not hidden either, and that > leads to extra complications. Since there are multiple symbols, during dynamic > linking they may be partially interposed. PR 84107 demonstrates how this leads > to libgcov segfaulting in a fairly simple and legitimate program. For this one, we have a working work-around: https://gcc.gnu.org/ml/gcc-patches/2018-09/msg00961.html > > Bottom line: static linking code with default-visibility symbols > into shared libraries is problematic. > > So one strategy is to ensure all gcov symbols have hidden visibility. That > would > isolate gcov instances in each shared library loaded in the program, and each > library would have the responsibility to write out its counters when unloaded. > Also, __gcov_dump would dump only the counters specific to the current > library. > > I may be missing something here so it might be nice to unearth why exactly > __gcov_master is intended to be global. > > Another strategy is to introduce libgcov.so and have it host either all > libgcov > symbols or just those that by design are required to exist once in the > program. > > When talking to Richi at the Cauldron I got the impression he'd question if > shared libgcov is worth the cost, e.g. would it make any easier for users to > mix two libraries, one linked against older libgcov, and another with a newer > (something that doesn't work at all now, but would be nice to support if I > understood Richard correctly). > > Alexander > Note that I'm fan of the shared library. I actually prepared working patch for that. So my strategy would be to first install the suggested patch: https://gcc.gnu.org/ml/gcc-patches/2018-09/msg00961.html and then we Richi is fine, we can also add the shared library patch. Martin
