[Buildroot] [PATCH v2 18/18] erlang: support choosing atomic ops

[Frank Hunleth]

Erlang requires atomic ops support and provides several options for
fulfilling this need. Previously in Buildroot only the libatomic_ops
option was used, but with Erlang 18, the Erlang developers prefer
their own atomic ops implementation is used. This change makes it
possible to select between options.

Additionally, this fixes an autobuilder failure that occurs when using
libatomic_ops on aarch64. Erlang's native atomic ops implementation
works on aarch64.

Fixes:
http://autobuild.buildroot.net/results/0cd/0cd22eb74fa29e5a85bf897762e16ab3daf33962/

Signed-off-by: Frank Hunleth <fhunleth at troodon-software.com>
---
Changes v1->v2:
  - Add BR2_PACKAGE_ERLANG_ARCH_SUPPORTS and related changes
    (Thanks to Thomas)
  - Atomic ops selection is a choice now due to the potential for
    more than two options based on the Erlang documentation.
  - Line wrapping updates
  - Added note on Erlang choosing native atomic ops even if
    libatomic_ops is present.
  - Removed LIBS=-latomic_ops. The ./configure test for it still
    worked, and Erlang linked to it without errors. 

 package/erlang/Config.in | 55 ++++++++++++++++++++++++++++++++++++++++++------
 package/erlang/erlang.mk |  6 +++++-
 2 files changed, 54 insertions(+), 7 deletions(-)

diff --git a/package/erlang/Config.in b/package/erlang/Config.in
index 0ec01bb..2a3098c 100644
--- a/package/erlang/Config.in
+++ b/package/erlang/Config.in
@@ -1,14 +1,30 @@
-comment "erlang needs a toolchain w/ dynamic library"
-	depends on BR2_USE_MMU # fork()
-	depends on BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS
-	depends on BR2_STATIC_LIBS
+# Erlang requires platform support for atomic operations. See the
+# HOWTO/INSTALL.md file for the full description. A summary of the
+# options is as follows:
+#
+#   1. Erlang's atomic operations on 32/64-bit x86, 32/64-bit
+#      SPARC V9, 32-bit PowerPC, or 32-bit Tile. The INSTALL.md
+#      omits 32/64-bit ARM support, but those are supported as well.
+#   2. The API provided by Windows (N/A for Buildroot)
+#   3. Native atomic operations provided by the '__atomic_*' builtins
+#      with gcc 4.7 or later.
+#   4. libatomic_ops
+#   5. The availability of __sync_*() operations. Based on
+#      http://autobuild.buildroot.org/results/209/2090874eee165af3349cf2d597657fc1b4ca1012/build-end.log,
+#      it needs the 4-byte and 8-byte versions.
+#
+# Cases 3 and 5 do not appear to work on configurations that are not
+# included in 1 and 4 due to compiler errors.
+config BR2_PACKAGE_ERLANG_ARCH_SUPPORTS
+	bool
+	default y if BR2_i386 || BR2_x86_64 || BR2_powerpc || BR2_sparc_v9 || BR2_arm || BR2_aarch64 # case (1)
+	default y if BR2_PACKAGE_LIBATOMIC_OPS_ARCH_SUPPORTS # case (4)
 
 config BR2_PACKAGE_ERLANG
 	bool "erlang"
 	depends on BR2_USE_MMU # fork()
 	depends on !BR2_STATIC_LIBS
-	depends on BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS
-	select BR2_PACKAGE_LIBATOMIC_OPS
+	depends on BR2_PACKAGE_ERLANG_ARCH_SUPPORTS
 	help
 	  Erlang is a programming language used to build massively scalable
 	  soft real-time systems with requirements on high availability.
@@ -20,6 +36,28 @@ config BR2_PACKAGE_ERLANG
 
 if BR2_PACKAGE_ERLANG
 
+choice
+	prompt "atomic ops"
+	help
+	  Selects the atomic operations implementation used by the Erlang
+	  VM
+
+config BR2_PACKAGE_ERLANG_NATIVE_ATOMIC_OPS
+	bool "native atomic ops"
+	help
+	  Use Erlang's native atomic ops implementation. If unsure, select
+	  this choice.
+
+config BR2_PACKAGE_ERLANG_LIBATOMIC_OPS
+	bool "libatomic_ops"
+	depends on BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS
+	depends on !BR2_aarch64 # causes build failures
+	select BR2_PACKAGE_LIBATOMIC_OPS
+	help
+	  Use libatomic_ops to provide atomic ops support.
+
+endchoice
+
 config BR2_PACKAGE_ERLANG_SMP
 	bool "enable SMP support"
 	help
@@ -38,3 +76,8 @@ config BR2_PACKAGE_ERLANG_MEGACO
 	  enable it.
 
 endif # BR2_PACKAGE_ERLANG
+
+comment "erlang needs a toolchain w/ dynamic library, atomic_ops"
+	depends on BR2_USE_MMU # fork()
+	depends on BR2_STATIC_LIBS || !BR2_PACKAGE_ERLANG_ARCH_SUPPORTS
+
diff --git a/package/erlang/erlang.mk b/package/erlang/erlang.mk
index 638d2c2..b1c8e35 100644
--- a/package/erlang/erlang.mk
+++ b/package/erlang/erlang.mk
@@ -30,8 +30,12 @@ ERLANG_CONF_ENV += erl_xcomp_sysroot=$(STAGING_DIR)
 
 ERLANG_CONF_OPTS = --without-javac
 
+# Erlang uses its own native atomic ops implementation if support is
+# available. Override this if the user wants libatomic_ops.
+ifeq ($(BR2_PACKAGE_ERLANG_LIBATOMIC_OPS),y)
 ERLANG_DEPENDENCIES += libatomic_ops
-ERLANG_CONF_OPTS += --with-libatomic_ops=$(STAGING_DIR)/usr LIBS=-latomic_ops
+ERLANG_CONF_OPTS += --with-libatomic_ops=$(STAGING_DIR)/usr
+endif
 
 # erlang uses openssl for all things crypto. Since the host tools (such as
 # rebar) uses crypto, we need to build host-erlang with support for openssl.
-- 
2.5.0