[Arn64/Unix] Revise Volatile.T barriers (#12156)

* [Arm64] Revise Volatile<T> barriers

* Remove VolateStore from CopyValueClassUnchecked

* Replace MemoryBarrier() in CopyValueClassUnchecked

3 files changed, 66 insertions, 6 deletions

diff --git a/src/gc/env/gcenv.base.h b/src/gc/env/gcenv.base.h
index af1a3cd9f0..1e9406c296 100644
--- a/src/gc/env/gcenv.base.h
+++ b/src/gc/env/gcenv.base.h
@@ -351,7 +351,7 @@ typedef PTR_PTR_Object PTR_UNCHECKED_OBJECTREF;
 #if defined(__clang__)
 #if defined(_ARM_) || defined(_ARM64_)
 // This is functionally equivalent to the MemoryBarrier() macro used on ARM on Windows.
-#define VOLATILE_MEMORY_BARRIER() asm volatile ("dmb sy" : : : "memory")
+#define VOLATILE_MEMORY_BARRIER() asm volatile ("dmb ish" : : : "memory")
 #else
 //
 // For Clang, we prevent reordering by the compiler by inserting the following after a volatile
@@ -392,8 +392,22 @@ template<typename T>
 inline
 T VolatileLoad(T const * pt)
 {
+#if defined(_ARM64_) && defined(__clang__)
+    T val;
+    static const unsigned lockFreeAtomicSizeMask = (1 << 1) | (1 << 2) | (1 << 4) | (1 << 8);
+    if((1 << sizeof(T)) & lockFreeAtomicSizeMask)
+    {
+        __atomic_load((T volatile const *)pt, &val, __ATOMIC_ACQUIRE);
+    }
+    else
+    {
+        val = *(T volatile const *)pt;
+        asm volatile ("dmb ishld" : : : "memory");
+    }
+#else
     T val = *(T volatile const *)pt;
     VOLATILE_MEMORY_BARRIER();
+#endif
     return val;
 }
 
@@ -420,8 +434,21 @@ template<typename T>
 inline
 void VolatileStore(T* pt, T val)
 {
+#if defined(_ARM64_) && defined(__clang__)
+    static const unsigned lockFreeAtomicSizeMask = (1 << 1) | (1 << 2) | (1 << 4) | (1 << 8);
+    if((1 << sizeof(T)) & lockFreeAtomicSizeMask)
+    {
+        __atomic_store((T volatile *)pt, &val, __ATOMIC_RELEASE);
+    }
+    else
+    {
+        VOLATILE_MEMORY_BARRIER();
+        *(T volatile *)pt = val;
+    }
+#else
     VOLATILE_MEMORY_BARRIER();
     *(T volatile *)pt = val;
+#endif
 }
 
 extern GCSystemInfo g_SystemInfo;
diff --git a/src/inc/volatile.h b/src/inc/volatile.h
index 9531d98085..5aa0e50866 100644
--- a/src/inc/volatile.h
+++ b/src/inc/volatile.h
@@ -76,7 +76,7 @@
 #if defined(__GNUC__)
 #if defined(_ARM_) || defined(_ARM64_)
 // This is functionally equivalent to the MemoryBarrier() macro used on ARM on Windows.
-#define VOLATILE_MEMORY_BARRIER() asm volatile ("dmb sy" : : : "memory")
+#define VOLATILE_MEMORY_BARRIER() asm volatile ("dmb ish" : : : "memory")
 #else
 //
 // For GCC, we prevent reordering by the compiler by inserting the following after a volatile
@@ -120,8 +120,22 @@ T VolatileLoad(T const * pt)
     STATIC_CONTRACT_SUPPORTS_DAC_HOST_ONLY;
 
 #ifndef DACCESS_COMPILE
+#if defined(_ARM64_) && defined(__GNUC__)
+    T val;
+    static const unsigned lockFreeAtomicSizeMask = (1 << 1) | (1 << 2) | (1 << 4) | (1 << 8);
+    if((1 << sizeof(T)) & lockFreeAtomicSizeMask)
+    {
+        __atomic_load((T volatile const *)pt, &val, __ATOMIC_ACQUIRE);
+    }
+    else
+    {
+        val = *(T volatile const *)pt;
+        asm volatile ("dmb ishld" : : : "memory");
+    }
+#else
     T val = *(T volatile const *)pt;
     VOLATILE_MEMORY_BARRIER();
+#endif
 #else
     T val = *pt;
 #endif
@@ -166,8 +180,21 @@ void VolatileStore(T* pt, T val)
     STATIC_CONTRACT_SUPPORTS_DAC_HOST_ONLY;
 
 #ifndef DACCESS_COMPILE
+#if defined(_ARM64_) && defined(__GNUC__)
+    static const unsigned lockFreeAtomicSizeMask = (1 << 1) | (1 << 2) | (1 << 4) | (1 << 8);
+    if((1 << sizeof(T)) & lockFreeAtomicSizeMask)
+    {
+        __atomic_store((T volatile *)pt, &val, __ATOMIC_RELEASE);
+    }
+    else
+    {
+        VOLATILE_MEMORY_BARRIER();
+        *(T volatile *)pt = val;
+    }
+#else
     VOLATILE_MEMORY_BARRIER();
     *(T volatile *)pt = val;
+#endif
 #else
     *pt = val;
 #endif
diff --git a/src/vm/object.cpp b/src/vm/object.cpp
index 3e3f6d120a..15c3a457a9 100644
--- a/src/vm/object.cpp
+++ b/src/vm/object.cpp
@@ -1521,11 +1521,17 @@ void STDCALL CopyValueClassUnchecked(void* dest, void* src, MethodTable *pMT)
 
     _ASSERTE(!pMT->IsArray());  // bunch of assumptions about arrays wrong. 
 
+    // <TODO> @todo Only call MemoryBarrier() if needed.
+    // Reflection is a known use case where this is required.
+    // Unboxing is a use case where this should not be required.
+    // </TODO>
+    MemoryBarrier();
+
         // Copy the bulk of the data, and any non-GC refs. 
     switch (pMT->GetNumInstanceFieldBytes())
     {        
     case 1:
-        VolatileStore((UINT8*)dest, *(UINT8*)src);
+        *(UINT8*)dest = *(UINT8*)src;
         break;
 #ifndef ALIGN_ACCESS
         // we can hit an alignment fault if the value type has multiple 
@@ -1533,13 +1539,13 @@ void STDCALL CopyValueClassUnchecked(void* dest, void* src, MethodTable *pMT)
         // value class can be aligned to 4-byte boundaries, yet the 
         // NumInstanceFieldBytes is 8
     case 2:
-        VolatileStore((UINT16*)dest, *(UINT16*)src);
+        *(UINT16*)dest = *(UINT16*)src;
         break;
     case 4:
-        VolatileStore((UINT32*)dest, *(UINT32*)src);
+        *(UINT32*)dest = *(UINT32*)src;
         break;
     case 8:
-        VolatileStore((UINT64*)dest, *(UINT64*)src);
+        *(UINT64*)dest = *(UINT64*)src;
         break;
 #endif // !ALIGN_ACCESS
     default: