From 19cceceb780ab13104d5a4e44c373472ac5f430d Mon Sep 17 00:00:00 2001
From: Robie Basak <robie.basak@canonical.com>
Date: Fri, 13 Dec 2013 15:52:07 +0000
Subject: [PATCH] SERVER-12064 SERVER-12283 Use gcc atomic builtins if
 available

Switch to using gcc atomic builtins for atomic operations if using a new
enough gcc and clang and support is available. Otherwise, fall back to
the old behaviour of existing non-portable inline assembly to continue
to support builds on older versions of gcc.

Signed-off-by: Benety Goh <benety@mongodb.com>
---
 SConstruct                             | 24 ++++++++++++++++++++++++
 src/mongo/platform/atomic_intrinsics.h |  6 ++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/SConstruct b/SConstruct
index 2d7e9a2b4a5..5c7c664bd5e 100644
--- a/SConstruct
+++ b/SConstruct
@@ -1431,6 +1431,30 @@ def doConfigure(myenv):
         if haveUUThread:
             myenv.Append(CPPDEFINES=['MONGO_HAVE___THREAD'])
 
+    if using_gcc() or using_clang():
+        def CheckGCCAtomicBuiltins(context):
+            test_body = """
+            int main(int argc, char **argv) {
+                int a = 0;
+                int b = 0;
+                int c = 0;
+
+                __atomic_compare_exchange(&a, &b, &c, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
+                return 0;
+            }
+            """
+            context.Message('Checking for gcc atomic builtins... ')
+            ret = context.TryLink(textwrap.dedent(test_body), '.cpp')
+            context.Result(ret)
+            return ret
+        conf = Configure(myenv, help=False, custom_tests = {
+            'CheckGCCAtomicBuiltins': CheckGCCAtomicBuiltins,
+        })
+        haveGCCAtomicBuiltins = conf.CheckGCCAtomicBuiltins()
+        conf.Finish()
+        if haveGCCAtomicBuiltins:
+            conf.env.Append(CPPDEFINES=["HAVE_GCC_ATOMIC_BUILTINS"])
+
     conf = Configure(myenv)
     libdeps.setup_conftests(conf)
 
diff --git a/src/mongo/platform/atomic_intrinsics.h b/src/mongo/platform/atomic_intrinsics.h
index 23b7b6bce46..ec3da125e69 100644
--- a/src/mongo/platform/atomic_intrinsics.h
+++ b/src/mongo/platform/atomic_intrinsics.h
@@ -43,10 +43,12 @@
 #if defined(_WIN32)
 #include "mongo/platform/atomic_intrinsics_win32.h"
 #elif defined(__GNUC__)
-#if defined(__i386__) || defined(__x86_64__)
+#if defined(HAVE_GCC_ATOMIC_BUILTINS)
+#include "mongo/platform/atomic_intrinsics_gcc_generic.h"
+#elif defined(__i386__) || defined(__x86_64__)
 #include "mongo/platform/atomic_intrinsics_gcc_intel.h"
 #else
-#include "mongo/platform/atomic_intrinsics_gcc_generic.h"
+#error "Unsupported platform: no gcc atomic builtins or port available"
 #endif
 #else
 #error "Unsupported os/compiler family"