[OpenMP] Change initialization of __kmp_global

[Hahnfeld]

Summary

There's no need to initialize variables with static storage duration
because they're implicitly initialized to zero. See
https://en.cppreference.com/w/c/language/initialization#Implicit_initialization

I think that's already relied upon because the supplied 0 only sets
'kmp_time_global_t g_time;' in 'struct kmp_base_global'. The other fields
are not set in the code, but implicitly initialized by the compiler.

[Hahnfeld]

openmp/runtime/src/kmp_global.cpp

434

The other solution would be explicitly initializing all struct variables, not sure if that is preferred. Putting __kmp_global into the .bss section seems ok, it's initialized to zero by the loader.

[AndreyChurbanov]

LGTM

openmp/runtime/cmake/LibompHandleFlags.cmake

 # C++ compiler flags
 function(libomp_get_cxxflags cxxflags)
   set(flags_local)
   libomp_append(flags_local -fno-exceptions LIBOMP_HAVE_FNO_EXCEPTIONS_FLAG)
   libomp_append(flags_local -fno-rtti LIBOMP_HAVE_FNO_RTTI_FLAG)
   libomp_append(flags_local -Wno-class-memaccess LIBOMP_HAVE_WNO_CLASS_MEMACCESS_FLAG)
   libomp_append(flags_local -Wno-covered-switch-default LIBOMP_HAVE_WNO_COVERED_SWITCH_DEFAULT_FLAG)
   libomp_append(flags_local -Wno-frame-address LIBOMP_HAVE_WNO_FRAME_ADDRESS_FLAG)
-  libomp_append(flags_local -Wno-missing-braces LIBOMP_HAVE_WNO_MISSING_BRACES_FLAG)
   libomp_append(flags_local -Wno-strict-aliasing LIBOMP_HAVE_WNO_STRICT_ALIASING_FLAG)
   libomp_append(flags_local -Wstringop-overflow=0 LIBOMP_HAVE_WSTRINGOP_OVERFLOW_FLAG)
   libomp_append(flags_local -Wno-stringop-truncation LIBOMP_HAVE_WNO_STRINGOP_TRUNCATION_FLAG)
   libomp_append(flags_local -Wno-switch LIBOMP_HAVE_WNO_SWITCH_FLAG)
   libomp_append(flags_local -Wno-unused-but-set-variable LIBOMP_HAVE_WNO_UNUSED_BUT_SET_VARIABLE_FLAG)
   libomp_append(flags_local /GS LIBOMP_HAVE_GS_FLAG)
   libomp_append(flags_local /EHsc LIBOMP_HAVE_EHSC_FLAG)
   libomp_append(flags_local /Oy- LIBOMP_HAVE_OY__FLAG)

openmp/runtime/cmake/config-ix.cmake

 endfunction()
 
 # Checking CXX, Linker Flags
 check_cxx_compiler_flag(-fno-exceptions LIBOMP_HAVE_FNO_EXCEPTIONS_FLAG)
 check_cxx_compiler_flag(-fno-rtti LIBOMP_HAVE_FNO_RTTI_FLAG)
 check_cxx_compiler_flag(-Wno-class-memaccess LIBOMP_HAVE_WNO_CLASS_MEMACCESS_FLAG)
 check_cxx_compiler_flag(-Wno-covered-switch-default LIBOMP_HAVE_WNO_COVERED_SWITCH_DEFAULT_FLAG)
 check_cxx_compiler_flag(-Wno-frame-address LIBOMP_HAVE_WNO_FRAME_ADDRESS_FLAG)
-check_cxx_compiler_flag(-Wno-missing-braces LIBOMP_HAVE_WNO_MISSING_BRACES_FLAG)
 check_cxx_compiler_flag(-Wno-strict-aliasing LIBOMP_HAVE_WNO_STRICT_ALIASING_FLAG)
 check_cxx_compiler_flag(-Wstringop-overflow=0 LIBOMP_HAVE_WSTRINGOP_OVERFLOW_FLAG)
 check_cxx_compiler_flag(-Wno-stringop-truncation LIBOMP_HAVE_WNO_STRINGOP_TRUNCATION_FLAG)
 check_cxx_compiler_flag(-Wno-switch LIBOMP_HAVE_WNO_SWITCH_FLAG)
 check_cxx_compiler_flag(-Wno-unused-but-set-variable LIBOMP_HAVE_WNO_UNUSED_BUT_SET_VARIABLE_FLAG)
 check_cxx_compiler_flag(-msse2 LIBOMP_HAVE_MSSE2_FLAG)
 check_cxx_compiler_flag(-ftls-model=initial-exec LIBOMP_HAVE_FTLS_MODEL_FLAG)
 libomp_check_architecture_flag(-mmic LIBOMP_HAVE_MMIC_FLAG)

openmp/runtime/src/kmp_global.cpp

 volatile kmp_team_t *__kmp_team_pool = NULL;
 
 KMP_ALIGN_CACHE
 std::atomic<int> __kmp_thread_pool_active_nth = ATOMIC_VAR_INIT(0);
 
 /* -------------------------------------------------
  * GLOBAL/ROOT STATE */
 KMP_ALIGN_CACHE
-kmp_global_t __kmp_global = {{0}};
+kmp_global_t __kmp_global;
 
 /* ----------------------------------------------- */
 /* GLOBAL SYNCHRONIZATION LOCKS */
 /* TODO verify the need for these locks and if they need to be global */
 
 #if KMP_USE_INTERNODE_ALIGNMENT
 /* Multinode systems have larger cache line granularity which can cause
  * false sharing if the alignment is not large enough for these locks */