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gatomic: introduce G_ATOMIC_LOCK_FREE
We clean up the detection of if we should do 'real' atomic operations or mutex-emulated ones with the introduction of a new (public) macro: G_ATOMIC_LOCK_FREE. If defined, our atomic operations are guaranteed to be done in hardware. We need to use __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 to determine if our compiler supports GCC-style atomic operations from the gatomic.h header because we might be building a program against GLib using a different set of compiler options (or a different compiler) than was used to build GLib itself. Unfortunately, this macro is not available on clang, so it has currently regressed to using the mutex emulation. A bug about that has been opened here: http://llvm.org/bugs/show_bug.cgi?id=11174
This commit is contained in:
Ryan Lortie
parent
c9b6c3c85a
commit
aba0f0c38b
63
configure.ac
63
configure.ac
@ -2407,6 +2407,47 @@ dnl ************************
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dnl *** g_atomic_* tests ***
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dnl ************************
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dnl We need to decide at configure time if GLib will use real atomic
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dnl operations ("lock free") or emulated ones with a mutex. This is
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dnl because we must put this information in glibconfig.h so we know if
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dnl it is safe or not to inline using compiler intrinsics directly from
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dnl the header.
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dnl
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dnl We also publish the information via G_ATOMIC_LOCK_FREE in case the
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dnl user is interested in knowing if they can use the atomic ops across
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dnl processes.
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dnl
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dnl We can currently support the atomic ops natively when building GLib
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dnl with recent versions of GCC or MSVC. MSVC doesn't run ./configure,
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dnl so we skip that case here and define G_ATOMIC_LOCK_FREE exactly when
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dnl we are using GCC.
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dnl
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dnl Note that the atomic ops are only available with GCC on x86 when
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dnl using -march=i486 or higher. If we detect that the atomic ops are
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dnl not available but would be available given the right flags, we want
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dnl to abort and advise the user to fix their CFLAGS. It's better to do
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dnl that then to silently fall back on emulated atomic ops just because
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dnl the user had the wrong build environment.
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dnl We may add other compilers here in the future...
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AC_MSG_CHECKING([for lock-free atomic intrinsics])
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AC_TRY_COMPILE([],
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[__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4;],
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[g_atomic_lock_free=yes],
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[g_atomic_lock_free=no])
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AC_MSG_RESULT($g_atomic_lock_free)
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if test "$g_atomic_lock_free" = "no"; then
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SAVE_CFLAGS="${CFLAGS}"
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CFLAGS="-march=i486"
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AC_TRY_COMPILE([],
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[__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4;],
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[AC_MSG_ERROR([GLib must be build with -march=i486 or later.])],
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[])
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CFLAGS="${SAVE_CFLAGS}"
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fi
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dnl We need a more robust approach here...
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case $host_cpu in
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i?86|x86_64|s390|s390x|arm*|crisv32*|etrax*)
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glib_memory_barrier_needed=no
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@ -2419,24 +2460,6 @@ dnl ************************
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;;
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esac
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glib_cv_gcc_has_builtin_atomic_operations=no
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if test x"$GCC" = xyes; then
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AC_MSG_CHECKING([whether GCC supports built-in atomic intrinsics])
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AC_TRY_LINK([],
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[int i;
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__sync_synchronize ();
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__sync_bool_compare_and_swap (&i, 0, 1);
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__sync_fetch_and_add (&i, 1);
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],
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[glib_cv_gcc_has_builtin_atomic_operations=yes],
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[glib_cv_gcc_has_builtin_atomic_operations=no])
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AC_MSG_RESULT($glib_cv_gcc_has_builtin_atomic_operations)
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fi
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AM_CONDITIONAL(HAVE_GCC_BUILTINS_FOR_ATOMIC_OPERATIONS,
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[test $glib_cv_gcc_has_builtin_atomic_operations = yes])
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dnl ************************
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dnl ** Check for futex(2) **
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dnl ************************
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@ -3085,9 +3108,9 @@ _______EOF
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echo >>$outfile
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echo "#define G_ATOMIC_OP_MEMORY_BARRIER_NEEDED 1" >>$outfile
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fi
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if test x"$g_gcc_atomic_ops" != xno; then
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if test x"$g_atomic_lock_free" = xyes; then
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echo >>$outfile
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echo "#define G_ATOMIC_OP_USE_GCC_BUILTINS 1" >>$outfile
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echo "#define G_ATOMIC_LOCK_FREE" >>$outfile
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fi
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echo >>$outfile
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g_bit_sizes="16 32 64"
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@ -1,6 +1,11 @@
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# This file makes most of the thread related macros look like
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# functions, which they really were, if possible easy.
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<MACRO>
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<NAME>G_ATOMIC_LOCK_FREE</NAME>
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#define G_ATOMIC_LOCK_FREE
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</MACRO>
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# default thread implementation
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<MACRO>
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@ -806,6 +806,9 @@ g_async_queue_sort_unlocked
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<SECTION>
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<TITLE>Atomic Operations</TITLE>
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<FILE>atomic_operations</FILE>
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G_ATOMIC_LOCK_FREE
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<SUBSECTION>
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g_atomic_int_get
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g_atomic_int_set
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g_atomic_int_inc
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@ -50,22 +50,6 @@
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* hardware memory barrier. Acquire and release or producer and
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* consumer barrier semantics are not available through this API.
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*
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* On GCC, these macros are implemented using GCC intrinsic operations.
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* On non-GCC compilers they will evaluate to function calls to
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* functions implemented by GLib.
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*
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* If GLib itself was compiled with GCC then these functions will again
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* be implemented by the GCC intrinsics. On Windows without GCC, the
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* interlocked API is used to implement the functions.
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*
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* With non-GCC compilers on non-Windows systems, the functions are
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* currently incapable of implementing true atomic operations --
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* instead, they fallback to holding a global lock while performing the
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* operation. This provides atomicity between the threads of one
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* process, but not between separate processes. For this reason, one
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* should exercise caution when attempting to use these options on
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* shared memory regions.
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*
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* It is very important that all accesses to a particular integer or
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* pointer be performed using only this API and that different sizes of
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* operation are not mixed or used on overlapping memory regions. Never
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@ -82,6 +66,19 @@
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* perform the operations normally and then release the lock.
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**/
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/**
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* G_ATOMIC_LOCK_FREE:
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*
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* This macro is defined if the atomic operations of GLib are
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* implemented using real hardware atomic operations. This means that
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* the GLib atomic API can be used between processes and safely mixed
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* with other (hardware) atomic APIs.
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*
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* If this macro is not defined, the atomic operations may be
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* emulated using a mutex. In that case, the GLib atomic operations are
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* only atomic relative to themselves and within a single process.
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**/
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/* NOTE CAREFULLY:
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*
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* This file is the lowest-level part of GLib.
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@ -93,12 +90,13 @@
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* without risking recursion.
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*/
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#ifdef G_ATOMIC_OP_USE_GCC_BUILTINS
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#ifdef G_ATOMIC_LOCK_FREE
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#ifndef __GNUC__
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#error Using GCC builtin atomic ops, but not compiling with GCC?
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#endif
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/* if G_ATOMIC_LOCK_FREE was defined by ./configure then we MUST
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* implement the atomic operations in a lock-free manner.
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*/
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#if defined (__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
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/**
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* g_atomic_int_get:
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* @atomic: a pointer to a #gint or #guint
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@ -612,9 +610,17 @@ gsize
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return InterlockedXor (atomic, val);
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#endif
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}
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#else
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/* This error occurs when ./configure decided that we should be capable
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* of lock-free atomics but we find at compile-time that we are not.
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*/
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#error G_ATOMIC_LOCK_FREE defined, but incapable of lock-free atomics.
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#endif /* defined (__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4) */
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#else /* G_ATOMIC_LOCK_FREE */
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/* We are not permitted to call into any GLib functions from here, so we
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* can not use GMutex.
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*
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@ -70,7 +70,7 @@ gint g_atomic_int_exchange_and_add (volatile gint *a
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G_END_DECLS
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#if defined(__GNUC__) && defined(G_ATOMIC_OP_USE_GCC_BUILTINS)
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#if defined(G_ATOMIC_LOCK_FREE) && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
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#define g_atomic_int_get(atomic) \
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(G_GNUC_EXTENSION ({ \
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@ -177,7 +177,7 @@ G_END_DECLS
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(gsize) __sync_fetch_and_xor ((atomic), (val)); \
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}))
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#else /* defined(__GNUC__) && defined(G_ATOMIC_OP_USE_GCC_BUILTINS) */
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#else /* defined(G_ATOMIC_LOCK_FREE) && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4) */
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#define g_atomic_int_get(atomic) \
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(g_atomic_int_get ((gint *) (atomic)))
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@ -207,6 +207,8 @@ typedef unsigned __int64 guintptr;
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#define G_THREADS_ENABLED
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#define G_THREADS_IMPL_WIN32
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#define G_ATOMIC_LOCK_FREE
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#define GINT16_TO_LE(val) ((gint16) (val))
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#define GUINT16_TO_LE(val) ((guint16) (val))
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#define GINT16_TO_BE(val) ((gint16) GUINT16_SWAP_LE_BE (val))
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