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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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| c++tools | ||
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This directory contains the GNU Compiler Collection (GCC). The GNU Compiler Collection is free software. See the files whose names start with COPYING for copying permission. The manuals, and some of the runtime libraries, are under different terms; see the individual source files for details. The directory INSTALL contains copies of the installation information as HTML and plain text. The source of this information is gcc/doc/install.texi. The installation information includes details of what is included in the GCC sources and what files GCC installs. See the file gcc/doc/gcc.texi (together with other files that it includes) for usage and porting information. An online readable version of the manual is in the files gcc/doc/gcc.info*. See http://gcc.gnu.org/bugs/ for how to report bugs usefully. Copyright years on GCC source files may be listed using range notation, e.g., 1987-2012, indicating that every year in the range, inclusive, is a copyrightable year that could otherwise be listed individually.