Improve ethread atomics based on GCC builtins

* Use of __atomic builtins when available.
* Improved configure test that checks for missing memory
  barrier in __sync_synchronize(). The old approach was to
  verify known working gcc versions and check gcc version at
  compile time. Besides not being very safe, the old approach
  often unnecessarily caused usage of the very expensive
  workaround.
* Introduced (no overhead) workaround for missing clobber in
  __sync_synchronize() when using buggy LLVM implementation of
  __sync_synchronize().
* Implement native memory barriers for ARM processors supporting
  the DMB instruction.
* Use of volatile store on Alpha as atomic set operation if no
  __atomic_store_n() is available (already used on x86/x86_64
  Sparc V9, PowerPC, and MIPS). Fallback used when not using
  volatile store is typically very expensive.
* Use volatile load on Alpha and ARM as atomic read operation
  if no __atomic_load_n() is available (already used on
  x86/x86_64 Sparc V9, PowerPC, and MIPS). Fallback when not
  using volatile load is typically very expensive.

1 files changed, 23 insertions, 2 deletions

diff --git a/erts/configure.in b/erts/configure.in
index 1676d3d216..09a04ca1a9 100644
--- a/erts/configure.in
+++ b/erts/configure.in
@@ -2,7 +2,7 @@ dnl Process this file with autoconf to produce a configure script. -*-m4-*-
 
 dnl %CopyrightBegin%
 dnl
-dnl Copyright Ericsson AB 1997-2014. All Rights Reserved.
+dnl Copyright Ericsson AB 1997-2015. All Rights Reserved.
 dnl
 dnl The contents of this file are subject to the Erlang Public License,
 dnl Version 1.1, (the "License"); you may not use this file except in
@@ -1123,10 +1123,31 @@ if test $ERTS_BUILD_SMP_EMU = yes; then
 
 	case "$ethr_have_native_atomics-$smp_require_native_atomics-$ethr_have_native_spinlock" in
 	  yes-*)
+		if test "$ethr_native_atomic_implementation" = "gcc_sync"; then
+		    test -f "$ERL_TOP/erts/CONF_INFO" ||
+			echo "" > "$ERL_TOP/erts/CONF_INFO"
+		    cat >> $ERL_TOP/erts/CONF_INFO <<EOF
+
+                 WARNING:
+                   Only gcc's __sync_* builtins available for
+                   atomic memory access. This will cause lots
+                   of expensive and unnecessary memory barrier
+                   instructions to be issued which will make
+                   the performance of the runtime system
+                   suffer. You are *strongly* advised to
+                   upgrade to a gcc version that supports the
+                   __atomic_* builtins (at least gcc version
+                   4.7) or build with libatomic_ops. See the
+                   "Atomic Memory Operations and the VM"
+		   chapter of \$ERL_TOP/HOWTO/INSTALL.md for
+                   more information.
+
+EOF
+		fi
 		;;
 
 	  no-yes-*)
-		AC_MSG_ERROR([No native atomic implementation found. See Configuring section in INSTALL.md for more information.])
+		AC_MSG_ERROR([No native atomic implementation found. See the \"Atomic Memory Operations and the VM\" chapter of \$ERL_TOP/HOWTO/INSTALL.md for more information.])
 		;;
 
 	  no-no-yes)