[Local GC] Break EE dependency on GC's generation table and alloc lock in single-proc scenarios (#10065)

* Remove usage of the generation table from the EE by introducing an
EE-owned GC alloc context used for allocations on single-proc machines.

* Move the GC alloc lock to the EE side of the interface

* Repair the Windows ARM build

* Move the decision to use per-thread alloc contexts to the EE

* Rename the lock used by StartNoGCRegion and EndNoGCRegion to be more indicative of what it is protecting

* Address code review feedback 2 (enumerate the global alloc context as a part of GCToEEInterface)

* Code review feedback (3)

* Address code review feedback (move some GC-internal globals to gcimpl.h and gc.cpp)

* g_global_alloc_lock is a dword, not a qword - fixes a deadlock

* Move GlobalAllocLock to gchelpers.cpp and switch to preemptive mode when spinning

* Repair the Windows x86 build

1 files changed, 18 insertions, 17 deletions

diff --git a/src/vm/gcheaputilities.h b/src/vm/gcheaputilities.h
index 48481146b4..d5ce46c60d 100644
--- a/src/vm/gcheaputilities.h
+++ b/src/vm/gcheaputilities.h
@@ -20,6 +20,12 @@ GPTR_DECL(uint32_t,g_card_table);
 }
 #endif // !DACCESS_COMPILE
 
+// For single-proc machines, the EE will use a single, shared alloc context
+// for all allocations. In order to avoid extra indirections in assembly
+// allocation helpers, the EE owns the global allocation context and the
+// GC will update it when it needs to.
+extern "C" gc_alloc_context g_global_alloc_context;
+
 extern "C" uint32_t* g_card_bundle_table;
 extern "C" uint8_t* g_ephemeral_low;
 extern "C" uint8_t* g_ephemeral_high;
@@ -100,22 +106,6 @@ public:
             GetGCHeap()->WaitUntilGCComplete(bConsiderGCStart);
     }
 
-    // Returns true if we should be using allocation contexts, false otherwise.
-    inline static bool UseAllocationContexts()
-    {
-        WRAPPER_NO_CONTRACT;
-#ifdef FEATURE_REDHAWK
-        // SIMPLIFY:  only use allocation contexts
-        return true;
-#else
-#if defined(_TARGET_ARM_) || defined(FEATURE_PAL)
-        return true;
-#else
-        return ((IsServerHeap() ? true : (g_SystemInfo.dwNumberOfProcessors >= 2)));
-#endif 
-#endif 
-    }
-
     // Returns true if the held GC heap is a Server GC heap, false otherwise.
     inline static bool IsServerHeap()
     {
@@ -128,6 +118,18 @@ public:
 #endif // FEATURE_SVR_GC
     }
 
+    static bool UseThreadAllocationContexts()
+    {
+        // When running on a single-proc system, it's more efficient to use a single global
+        // allocation context for SOH allocations than to use one for every thread.
+#if defined(_TARGET_ARM_) || defined(FEATURE_PAL) || defined(FEATURE_REDHAWK)
+        return true;
+#else
+        return IsServerHeap() || ::GetCurrentProcessCpuCount() != 1;
+#endif
+
+    }
+
 #ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
 
     // Returns True if software write watch is currently enabled for the GC Heap,
@@ -192,7 +194,6 @@ public:
     }
 #endif // FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
 
-
 private:
     // This class should never be instantiated.
     GCHeapUtilities() = delete;