icu: remove BR2_ARCH_HAS_ATOMICS dependency

The BR2_ARCH_HAS_ATOMICS was added because on ARC, atomic instructions
may not be provided by the architecture and therefore the compiler
does not provide the __sync_*() built-ins.

However, since then, icu was changed and is now able to use C++11
atomics, or even no atomic operations at all. In fact, icu will:

 * If possible, it will use C++11 atomics, which internally rely on
   the __atomic built-ins. These are available since gcc 4.7, and all
   architectures provide it. On some architectures, you *must* link
   with libatomic, on some other architectures, they are available
   built-in, but in all cases, linking against libatomic does not
   harm. Thanks to this, even ARC with no atomic support (which was
   the original reason for adding the BR2_ARCH_HAS_ATOMICS) dependency
   builds fine, provided -latomic is added to LIBS.

 * If C++11 atomics are not available, then it falls back to
   __sync_*() built-ins, which allows compilers older than 4.7 to be
   supported.

 * If really no atomic mechanism is available, then it falls back to a
   basic implementation based on a mutex.

Conclusion:

 - The BR2_ARCH_HAS_ATOMICS dependency is no longer needed.

 - We need to link with -latomic when gcc >= 4.7 is used.

Note that reverse dependencies of icu are also changed accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

2 files changed, 8 insertions, 2 deletions

diff --git a/package/icu/Config.in b/package/icu/Config.in
index 4d47c32540..d31c1fa3dc 100644
--- a/package/icu/Config.in
+++ b/package/icu/Config.in
@@ -7,7 +7,6 @@ config BR2_PACKAGE_ICU
 	# file, and it cannot easily be changed to generate FLAT
 	# format.
 	depends on !BR2_BINFMT_FLAT
-	depends on BR2_ARCH_HAS_ATOMICS
 	help
 	  International Components for Unicode.
 
@@ -28,6 +27,5 @@ endif
 
 comment "icu needs a toolchain w/ C++, wchar, threads"
 	depends on !BR2_BINFMT_FLAT
-	depends on BR2_ARCH_HAS_ATOMICS
 	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_USE_WCHAR || \
 		!BR2_TOOLCHAIN_HAS_THREADS
diff --git a/package/icu/icu.mk b/package/icu/icu.mk
index 5604504896..7bc7f938ec 100644
--- a/package/icu/icu.mk
+++ b/package/icu/icu.mk
@@ -17,6 +17,14 @@ ICU_CONF_OPTS = \
 	--with-cross-build=$(HOST_ICU_DIR)/source \
 	--disable-samples \
 	--disable-tests
+
+# With gcc >= 4.7, icu prefers to use C++11 atomics, which rely on the
+# __atomic builtins. On certain architectures, this requires linking
+# with libatomic.
+ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_4_7),y)
+ICU_CONF_ENV += LIBS="-latomic"
+endif
+
 HOST_ICU_CONF_OPTS = \
 	--disable-samples \
 	--disable-tests \