[*] Harden weakly ordered systems: atomic stores (rare) and lock clears (common) should perform a full memory barrier (gross, but required) BEFORE the atomic store (allowing other threads to acquire)

Include/auROXTL/auAtomic.hpp

@@ -57,8 +57,8 @@
 //     AuAtomicUnset (ret: bool)
 //     AuAtomicAndUnsetBit (ret: T)
 //     -
-//     AuAtomicLoadWeak (tbd)
+//     AuAtomicLoadWeak
-//     (no weak store) (tbd)
+//     AuAtomicStoreWeak
 //     -
 //     AuAtomicLoad
 //     AuAtomicStore
@@ -66,8 +66,8 @@
 //     AuAtomicOrSetBit
 //     AuAtomicOr
 //     AuAtomicAnd
-//     AuAtomicAdd
+//     AuAtomicAdd (warn: returns post op atomic value)
-//     AuAtomicSub
+//     AuAtomicSub (warn: returns post op atomic value)
 
 template <class T>
 struct AuAtomicUtils
@@ -138,6 +138,8 @@ struct AuAtomicUtils
 
     //
     static T LoadWeak(T *in);
+
+    static void StoreWeak(T *in, T value);
 };
 
 #if defined(AURORA_COMPILER_MSVC)
@@ -419,6 +421,12 @@ inline auline T AuAtomicUtils<T>::LoadWeak(T *in)
     return AuAtomicUtils<T>::Load(in);
 }
 
+template <class T>
+inline void AuAtomicUtils<T>::StoreWeak(T *in, T val)
+{
+    *in = val;
+}
+
 #if defined(AURORA_COMPILER_CLANG)
     #define ATOMIC_PREFIX_HAX(name) __c11_ ## name
 #else
@@ -434,19 +442,11 @@ inline auline T AuAtomicUtils<T>::Load(T *in)
     }
     else
     {
-#if defined(AURORA_COMPILER_MSVC) && (defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
+#if defined(AURORA_COMPILER_MSVC)
-    const auto read = *in;
+    return *(volatile T*)in;
-    ::_ReadWriteBarrier();
-    return read;
-#elif defined(AURORA_COMPILER_MSVC)
-    ::MemoryBarrier(); // works on all legacy MSVC targets including AMD64, IA64, and POWER
-    return *in;
 #else
 
 #if defined(AURORA_COMPILER_CLANG)
-#if !(defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
-    __sync_synchronize(); // brute force on unknown archs. gcc-like compilers will accept this
-#endif
     if constexpr (AuIsSame_v<AuUInt8, T>)
     {
         return ATOMIC_PREFIX_HAX(atomic_load)((_Atomic(AuUInt8) *)(in), __ATOMIC_ACQUIRE);
@@ -501,12 +501,15 @@ inline auline void AuAtomicUtils<T>::Store(T *in, T val)
     else
     {
 #if defined(AURORA_COMPILER_MSVC) && (defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
-    *in = val;
+    *(volatile T*)in = val;
 #elif defined(AURORA_COMPILER_MSVC)
 #if 0
-    * in = val;
     ::MemoryBarrier();
+    *(volatile T *)in = val;
 #else
+    // assume /volatile:iso 
+    MemoryBarrier();
+
     if constexpr (AuIsSame_v<AuUInt8, T>)
     {
         ::InterlockedExchange8((CHAR volatile *)in, (CHAR)val);
@@ -543,8 +546,12 @@ inline auline void AuAtomicUtils<T>::Store(T *in, T val)
     {
         static_assert(AuIsVoid_v<T>, "T");
     }
+
 #endif
 #else
+    #if !(defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
+        __sync_synchronize();
+    #endif
     if constexpr (AuIsSame_v<AuUInt8, T>)
     {
         ATOMIC_PREFIX_HAX(atomic_store)((_Atomic(AuUInt8) *)(in), val, __ATOMIC_RELEASE);
@@ -581,9 +588,6 @@ inline auline void AuAtomicUtils<T>::Store(T *in, T val)
     {
         static_assert(AuIsVoid_v<T>, "T");
     }
-#if !(defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
-    __sync_synchronize();
-#endif
 #endif
     }
 }
@@ -593,17 +597,21 @@ inline auline
 void AuAtomicUtils<AuUInt8>::ClearU8Lock(AuUInt8 *in)
 {
 #if defined(AURORA_COMPILER_MSVC) && (defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
-    *in = 0;
+    *(volatile AuUInt8 *)in = 0;
-    ::_ReadWriteBarrier();
 #elif defined(AURORA_COMPILER_MSVC)
+    // assume /volatile:iso 
+    MemoryBarrier();
     ::_InterlockedExchange8((volatile char *)in, 0);
     // i think this will work on aarch64 and most risc architectures
     //InterlockedAndRelease((volatile LONG *)in, ~0xFF);
 #else
-    ATOMIC_PREFIX_HAX(atomic_store)((_Atomic(AuUInt8) *)(in), 0, __ATOMIC_RELEASE);
+    // Flush non-atomic operations within L1 or other store/load queue
 #if !(defined(AURORA_ARCH_X64) || defined(AURORA_ARCH_X86))
     __sync_synchronize();
 #endif
+    
+    // Clear the lock
+    ATOMIC_PREFIX_HAX(atomic_store)((_Atomic(AuUInt8) *)(in), 0, __ATOMIC_RELEASE);
 #endif
 }
 
@@ -687,6 +695,13 @@ T        AuAtomicLoadWeak(T *in)
     return AuAtomicUtils<T>::LoadWeak(in);
 }
 
+template <class T>
+auline
+void      AuAtomicStoreWeak(T *in, T value)
+{
+    AuAtomicUtils<T>::StoreWeak(in, value);
+}
+
 template <class T>
 auline
 void     AuAtomicClearU8Lock(T *in)
@@ -786,6 +801,13 @@ T        AuAtomicLoadWeak(volatile T *in)
     return AuAtomicUtils<T>::LoadWeak((T *)in);
 }
 
+template <class T>
+auline
+void     AuAtomicStoreWeak(volatile T *in, T value)
+{
+    AuAtomicUtils<T>::StoreWeak((T *)in, value);
+}
+
 template <class T>
 auline
 void     AuAtomicClearU8Lock(volatile T *in)