Addressing OE linking issues when built with GCC (#167)

* Use C++17 inline statics

This leads to better codegen in GCC, and fixes some linking issues in OE.

* Detect GCC and OE combination and fall-back to lock based ABA.

* clangformat

src/ds/aba.h

@@ -7,7 +7,8 @@
  * implementable with double-word compare and exchange or with load-link
  * store conditional.
  *
- * We provide a lock based implementation.
+ * We provide a lock based implementation as a backup for other platforms
+ * without appropriate intrinsics.
  */
 namespace snmalloc
 {
@@ -16,7 +17,12 @@ namespace snmalloc
   // check this on other platforms using a thread_local.
   inline thread_local bool operation_in_flight = false;
 #endif
-#ifdef PLATFORM_IS_X86
+  // The !(defined(GCC_NOT_CLANG) && defined(OPEN_ENCLAVE)) is required as
+  // GCC is outputing a function call to libatomic, rather than just the x86
+  // instruction, this causes problems for linking later. For this case
+  // fall back to locked implementation.
+#if defined(PLATFORM_IS_X86) && \
+  !(defined(GCC_NOT_CLANG) && defined(OPEN_ENCLAVE))
   template<typename T, Construction c = RequiresInit>
   class ABA
   {

src/ds/defines.h

@@ -19,6 +19,10 @@
 #endif
 
 #if !defined(__clang__) && defined(__GNUC__)
+#  define GCC_NOT_CLANG
+#endif
+
+#ifdef GCC_NOT_CLANG
 #  if __GNUC__ >= 8
 #    define GCC_VERSION_EIGHT_PLUS
 #  endif

src/ds/helpers.h

@@ -13,18 +13,18 @@ namespace snmalloc
   template<class Object, Object init() noexcept>
   class Singleton
   {
+    inline static std::atomic_flag flag;
+    inline static std::atomic<bool> initialised = false;
+    inline static Object obj;
+
   public:
     /**
      * If argument is non-null, then it is assigned the value
      * true, if this is the first call to get.
      * At most one call will be first.
      */
-    inline static Object& get(bool* first = nullptr)
+    inline SNMALLOC_SLOW_PATH static Object& get(bool* first = nullptr)
     {
-      static std::atomic_flag flag;
-      static std::atomic<bool> initialised;
-      static Object obj;
-
       // If defined should be initially false;
       SNMALLOC_ASSERT(first == nullptr || *first == false);