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. --- gcc/c-family/c-cppbuiltin.cc | 2 + gcc/doc/invoke.texi | 19 ++++++- gcc/tree-profile.cc | 99 +++++++++++++++++++++++++++++++++--- libgcc/libgcov.h | 10 ++-- 4 files changed, 114 insertions(+), 16 deletions(-) diff --git a/gcc/c-family/c-cppbuiltin.cc b/gcc/c-family/c-cppbuiltin.cc index cdf9850cb19e..e8576737fafb 100644 --- a/gcc/c-family/c-cppbuiltin.cc +++ b/gcc/c-family/c-cppbuiltin.cc @@ -1538,6 +1538,8 @@ c_cpp_builtins (cpp_reader *pfile) /* For libgcov. */ builtin_define_with_int_value ("__LIBGCC_VTABLE_USES_DESCRIPTORS__", TARGET_VTABLE_USES_DESCRIPTORS); + builtin_define_with_int_value ("__LIBGCC_HAVE_LIBATOMIC", + TARGET_HAVE_LIBATOMIC); } /* For use in assembly language. */ diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index de40f62e219c..8fe3c86ad419 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -16603,7 +16603,24 @@ while the second one prevents profile corruption by emitting thread-safe code. Using @samp{prefer-atomic} would be transformed either to @samp{atomic}, when supported by a target, or to @samp{single} otherwise. The GCC driver automatically selects @samp{prefer-atomic} when @option{-pthread} -is present in the command line. +is present in the command line, otherwise the default method is @samp{single}. + +If @samp{atomic} is selected, then the profile information is updated using +atomic operations on a best-effort basis. Ideally, the profile information is +updated through atomic operations in hardware. If the target platform does not +support the required atomic operations in hardware, however, @file{libatomic} +is available, then the profile information is updated through calls to +@file{libatomic}. If the target platform neither supports the required atomic +operations in hardware nor @file{libatomic}, then the profile information is +not atomically updated and a warning is issued. In this case, the obtained +profiling information may be corrupt for multi-threaded applications. + +For performance reasons, if 64-bit counters are used for the profiling +information and the target platform only supports 32-bit atomic operations in +hardware, then the performance critical profiling updates are done using two +32-bit atomic operations for each counter update. If a signal interrupts these +two operations updating a counter, then the profiling information may be in an +inconsistent state. @opindex fprofile-filter-files @item -fprofile-filter-files=@var{regex} diff --git a/gcc/tree-profile.cc b/gcc/tree-profile.cc index 24805ff905c7..12255f06f992 100644 --- a/gcc/tree-profile.cc +++ b/gcc/tree-profile.cc @@ -73,6 +73,41 @@ static GTY(()) tree ic_tuple_var; static GTY(()) tree ic_tuple_counters_field; static GTY(()) tree ic_tuple_callee_field; +/* Types of counter update methods. + + By default, the counter updates are done for a single threaded system + (COUNTER_UPDATE_SINGLE_THREAD). + + If the user selected atomic profile counter updates + (-fprofile-update=atomic), then the counter updates will be done atomically + on a best-effort basis. One of three methods to do the counter updates is + selected according to the target capabilities. + + Ideally, the counter updates are done through atomic operations in hardware + (COUNTER_UPDATE_ATOMIC_BUILTIN). + + If the target supports only 32-bit atomic increments and gcov_type_node is a + 64-bit integer type, then for the profile edge counters the increment is + performed through two separate 32-bit atomic increments + (COUNTER_UPDATE_ATOMIC_SPLIT or COUNTER_UPDATE_ATOMIC_PARTIAL). If the + target supports libatomic (TARGET_HAVE_LIBATOMIC), then other counter + updates are carried out by libatomic calls (COUNTER_UPDATE_ATOMIC_SPLIT). + If the target does not support libatomic, then the other counter updates are + not done atomically (COUNTER_UPDATE_ATOMIC_PARTIAL) and a warning is + issued. + + If the target does not support atomic operations in hardware, however, it + supports libatomic, then all updates are carried out by libatomic calls + (COUNTER_UPDATE_ATOMIC_BUILTIN). */ +enum counter_update_method { + COUNTER_UPDATE_SINGLE_THREAD, + COUNTER_UPDATE_ATOMIC_BUILTIN, + COUNTER_UPDATE_ATOMIC_SPLIT, + COUNTER_UPDATE_ATOMIC_PARTIAL +}; + +static counter_update_method counter_update = COUNTER_UPDATE_SINGLE_THREAD; + /* Do initialization work for the edge profiler. */ /* Add code: @@ -269,7 +304,8 @@ gen_counter_update (gimple_stmt_iterator *gsi, tree counter, tree result, tree one = build_int_cst (type, 1); tree relaxed = build_int_cst (integer_type_node, MEMMODEL_RELAXED); - if (flag_profile_update == PROFILE_UPDATE_ATOMIC) + if (counter_update == COUNTER_UPDATE_ATOMIC_BUILTIN || + (result && counter_update == COUNTER_UPDATE_ATOMIC_SPLIT)) { /* __atomic_fetch_add (&counter, 1, MEMMODEL_RELAXED); */ tree f = builtin_decl_explicit (TYPE_PRECISION (type) > 32 @@ -278,6 +314,38 @@ gen_counter_update (gimple_stmt_iterator *gsi, tree counter, tree result, gcall *call = gimple_build_call (f, 3, addr, one, relaxed); gen_assign_counter_update (gsi, call, f, result, name); } + else if (!result && (counter_update == COUNTER_UPDATE_ATOMIC_SPLIT || + counter_update == COUNTER_UPDATE_ATOMIC_PARTIAL)) + { + /* low = __atomic_add_fetch_4 (addr, 1, MEMMODEL_RELAXED); + high_inc = low == 0 ? 1 : 0; + __atomic_add_fetch_4 (addr_high, high_inc, MEMMODEL_RELAXED); */ + tree zero32 = build_zero_cst (uint32_type_node); + tree one32 = build_one_cst (uint32_type_node); + tree addr_high = make_temp_ssa_name (TREE_TYPE (addr), NULL, name); + tree four = build_int_cst (size_type_node, 4); + gassign *assign1 = gimple_build_assign (addr_high, POINTER_PLUS_EXPR, + addr, four); + gsi_insert_after (gsi, assign1, GSI_NEW_STMT); + if (WORDS_BIG_ENDIAN) + std::swap (addr, addr_high); + tree f = builtin_decl_explicit (BUILT_IN_ATOMIC_ADD_FETCH_4); + gcall *call1 = gimple_build_call (f, 3, addr, one, relaxed); + tree low = make_temp_ssa_name (uint32_type_node, NULL, name); + gimple_call_set_lhs (call1, low); + gsi_insert_after (gsi, call1, GSI_NEW_STMT); + tree is_zero = make_temp_ssa_name (boolean_type_node, NULL, name); + gassign *assign2 = gimple_build_assign (is_zero, EQ_EXPR, low, + zero32); + gsi_insert_after (gsi, assign2, GSI_NEW_STMT); + tree high_inc = make_temp_ssa_name (uint32_type_node, NULL, name); + gassign *assign3 = gimple_build_assign (high_inc, COND_EXPR, + is_zero, one32, zero32); + gsi_insert_after (gsi, assign3, GSI_NEW_STMT); + gcall *call2 = gimple_build_call (f, 3, addr_high, high_inc, + relaxed); + gsi_insert_after (gsi, call2, GSI_NEW_STMT); + } else { tree tmp1 = make_temp_ssa_name (type, NULL, name); @@ -689,15 +757,20 @@ tree_profiling (void) struct cgraph_node *node; /* Verify whether we can utilize atomic update operations. */ - bool can_support_atomic = false; + bool can_support_atomic = TARGET_HAVE_LIBATOMIC; unsigned HOST_WIDE_INT gcov_type_size = tree_to_uhwi (TYPE_SIZE_UNIT (get_gcov_type ())); - if (gcov_type_size == 4) - can_support_atomic - = HAVE_sync_compare_and_swapsi || HAVE_atomic_compare_and_swapsi; - else if (gcov_type_size == 8) - can_support_atomic - = HAVE_sync_compare_and_swapdi || HAVE_atomic_compare_and_swapdi; + bool have_atomic_4 + = HAVE_sync_compare_and_swapsi || HAVE_atomic_compare_and_swapsi; + bool have_atomic_8 + = HAVE_sync_compare_and_swapdi || HAVE_atomic_compare_and_swapdi; + if (!can_support_atomic) + { + if (gcov_type_size == 4) + can_support_atomic = have_atomic_4; + else if (gcov_type_size == 8) + can_support_atomic = have_atomic_8; + } if (flag_profile_update == PROFILE_UPDATE_ATOMIC && !can_support_atomic) @@ -710,6 +783,16 @@ tree_profiling (void) flag_profile_update = can_support_atomic ? PROFILE_UPDATE_ATOMIC : PROFILE_UPDATE_SINGLE; + if (flag_profile_update == PROFILE_UPDATE_ATOMIC) + { + if (gcov_type_size == 8 && !have_atomic_8 && have_atomic_4) + counter_update = COUNTER_UPDATE_ATOMIC_SPLIT; + else + counter_update = COUNTER_UPDATE_ATOMIC_BUILTIN; + } + else if (gcov_type_size == 8 && have_atomic_4) + counter_update = COUNTER_UPDATE_ATOMIC_PARTIAL; + /* This is a small-ipa pass that gets called only once, from cgraphunit.cc:ipa_passes(). */ gcc_assert (symtab->state == IPA_SSA); diff --git a/libgcc/libgcov.h b/libgcc/libgcov.h index 763118ea5b52..d04c070d0cfa 100644 --- a/libgcc/libgcov.h +++ b/libgcc/libgcov.h @@ -95,18 +95,14 @@ typedef unsigned gcov_type_unsigned __attribute__ ((mode (QI))); #define GCOV_LOCKED_WITH_LOCKING 0 #endif -#ifndef GCOV_SUPPORTS_ATOMIC /* Detect whether target can support atomic update of profilers. */ -#if __SIZEOF_LONG_LONG__ == 4 && __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 -#define GCOV_SUPPORTS_ATOMIC 1 -#else -#if __SIZEOF_LONG_LONG__ == 8 && __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 +#if (__SIZEOF_LONG_LONG__ == 4 && __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4) || \ + (__SIZEOF_LONG_LONG__ == 8 && __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8) || \ + defined (__LIBGCC_HAVE_LIBATOMIC) #define GCOV_SUPPORTS_ATOMIC 1 #else #define GCOV_SUPPORTS_ATOMIC 0 #endif -#endif -#endif /* In libgcov we need these functions to be extern, so prefix them with __gcov. In libgcov they must also be hidden so that the instance in -- 2.35.3