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gcov: Improve -fprofile-update=atomic
The code coverage support uses counters to determine which edges in the control flow graph were executed. If a counter overflows, then the code coverage information is invalid. Therefore the counter type should be a 64-bit integer. In multi-threaded applications, it is important that the counter increments are atomic. This is not the case by default. The user can enable atomic counter increments through the -fprofile-update=atomic and -fprofile-update=prefer-atomic options. If the target supports 64-bit atomic operations, then everything is fine. If not and -fprofile-update=prefer-atomic was chosen by the user, then non-atomic counter increments will be used. However, if the target does not support the required atomic operations and -fprofile-atomic=update was chosen by the user, then a warning was issued and as a forced fallback to non-atomic operations was done. This is probably not what a user wants. There is still hardware on the market which does not have atomic operations and is used for multi-threaded applications. A user which selects -fprofile-update=atomic wants consistent code coverage data and not random data. This patch removes the fallback to non-atomic operations for -fprofile-update=atomic the target platform supports libatomic. To mitigate potential performance issues an optimization for systems which only support 32-bit atomic operations is provided. Here, the edge counter increments are done like this: low = __atomic_add_fetch_4 (&counter.low, 1, MEMMODEL_RELAXED); high_inc = low == 0 ? 1 : 0; __atomic_add_fetch_4 (&counter.high, high_inc, MEMMODEL_RELAXED); In gimple_gen_time_profiler() this split operation cannot be used, since the updated counter value is also required. Here, a library call is emitted. This is not a performance issue since the update is only done if counters[0] == 0. gcc/c-family/ChangeLog: * c-cppbuiltin.cc (c_cpp_builtins): Define __LIBGCC_HAVE_LIBATOMIC for libgcov. gcc/ChangeLog: * doc/invoke.texi (-fprofile-update): Clarify default method. Document the atomic method behaviour. * tree-profile.cc (enum counter_update_method): New. (counter_update): Likewise. (gen_counter_update): Use counter_update_method. Split the atomic counter update in two 32-bit atomic operations if necessary. (tree_profiling): Select counter_update_method. libgcc/ChangeLog: * libgcov.h (GCOV_SUPPORTS_ATOMIC): Always define it. Set it also to 1, if __LIBGCC_HAVE_LIBATOMIC is defined.
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