[mono][aot] Prefer specific instances instead of gshared for methods...

[mono][aot] Prefer specific instances instead of gshared for methods containing static virtual calls. (#76033)

These calls cannot be resolved at compile time in gshared methods, so they cannot be inlined etc.
They are used in perf sensitive BCL code like SpanHelpers. To fix this, modify the AOT compiler
so in addition to the gshared versions, it emits specific instances of these methods if possible.
This only affects a small subset of gshared methods so it doesn't lead to a noticable code size increase.

Fixes https://github.com/dotnet/runtime/issues/75801.

src/mono/mono/mini/aot-compiler.c

@@ -396,6 +396,10 @@ typedef struct MonoAotCompile {
 	GList *profile_data;
 	GHashTable *profile_methods;
 	GHashTable *blob_hash;
+	/* Maps MonoMethod*->GPtrArray* */
+	GHashTable *gshared_instances;
+	/* Hash of gshared methods where specific instances are preferred */
+	GHashTable *prefer_instances;
 #ifdef EMIT_WIN32_UNWIND_INFO
 	GList *unwind_info_section_cache;
 #endif
@@ -4302,8 +4306,23 @@ mono_dedup_cache_method (MonoAotCompile *acfg, MonoMethod *method)
 	g_hash_table_insert (acfg->dedup_stats, stats_name, GUINT_TO_POINTER (count));
 }
 
+static gboolean
+is_open_method (MonoMethod *method)
+{
+	MonoGenericContext *context;
+
+	if (!method->is_inflated)
+		return FALSE;
+	context = mono_method_get_context (method);
+	if (context->class_inst && context->class_inst->is_open)
+		return TRUE;
+	if (context->method_inst && context->method_inst->is_open)
+		return TRUE;
+	return FALSE;
+}
+
 static void
-add_extra_method_with_depth (MonoAotCompile *acfg, MonoMethod *method, int depth)
+add_extra_method_full (MonoAotCompile *acfg, MonoMethod *method, gboolean prefer_gshared, int depth)
 {
 	ERROR_DECL (error);
 
@@ -4315,7 +4334,7 @@ add_extra_method_with_depth (MonoAotCompile *acfg, MonoMethod *method, int depth
 		return;
 	}
 
-	if (mono_method_is_generic_sharable_full (method, TRUE, TRUE, FALSE)) {
+	if (prefer_gshared && mono_method_is_generic_sharable_full (method, TRUE, TRUE, FALSE)) {
 		MonoMethod *orig = method;
 
 		method = mini_get_shared_method_full (method, SHARE_MODE_NONE, error);
@@ -4325,10 +4344,23 @@ add_extra_method_with_depth (MonoAotCompile *acfg, MonoMethod *method, int depth
 			return;
 		}
 
+		if (g_hash_table_lookup (acfg->prefer_instances, method))
+			/* Compile an instance as well */
+			add_extra_method_full (acfg, orig, FALSE, depth);
+
 		/* Add it to profile_methods so its not skipped later */
 		if (acfg->aot_opts.profile_only && g_hash_table_lookup (acfg->profile_methods, orig))
 			g_hash_table_insert (acfg->profile_methods, method, method);
-	} else if ((acfg->jit_opts & MONO_OPT_GSHAREDVT) && prefer_gsharedvt_method (acfg, method) && mono_method_is_generic_sharable_full (method, FALSE, FALSE, TRUE)) {
+
+		if (!is_open_method (orig) && !mono_method_is_generic_sharable_full (orig, TRUE, FALSE, FALSE)) {
+			GPtrArray *instances = g_hash_table_lookup (acfg->gshared_instances, method);
+			if (!instances) {
+				instances = g_ptr_array_new ();
+				g_hash_table_insert (acfg->gshared_instances, method, instances);
+			}
+			g_ptr_array_add (instances, orig);
+		}
+	} else if ((acfg->jit_opts & MONO_OPT_GSHAREDVT) && prefer_gshared && prefer_gsharedvt_method (acfg, method) && mono_method_is_generic_sharable_full (method, FALSE, FALSE, TRUE)) {
 		/* Use the gsharedvt version */
 		method = mini_get_shared_method_full (method, SHARE_MODE_GSHAREDVT, error);
 		mono_error_assert_ok (error);
@@ -4347,6 +4379,12 @@ add_extra_method_with_depth (MonoAotCompile *acfg, MonoMethod *method, int depth
 	add_method_full (acfg, method, TRUE, depth);
 }
 
+static void
+add_extra_method_with_depth (MonoAotCompile *acfg, MonoMethod *method, int depth)
+{
+	add_extra_method_full (acfg, method, TRUE, depth);
+}
+
 static void
 add_extra_method (MonoAotCompile *acfg, MonoMethod *method)
 {
@@ -9209,6 +9247,22 @@ compile_method (MonoAotCompile *acfg, MonoMethod *method)
 			printf ("%s ### %d\n", mono_method_get_full_name (method), cfg->code_size);
 	}
 
+	if (cfg->prefer_instances) {
+		/*
+		 * Compile the original specific instances in addition to the gshared method
+		 * for performance reasons, since gshared methods cannot implement some
+		 * features like static virtual methods efficiently.
+		 */
+		g_hash_table_insert (acfg->prefer_instances, method, method);
+		GPtrArray *instances = g_hash_table_lookup (acfg->gshared_instances, method);
+		if (instances) {
+			for (guint i = 0; i < instances->len; ++i) {
+				MonoMethod *instance = (MonoMethod*)g_ptr_array_index (instances, i);
+				add_extra_method_full (acfg, instance, FALSE, 0);
+			}
+		}
+	}
+
 	/* Adds generic instances referenced by this method */
 	/*
 	 * The depth is used to avoid infinite loops when generic virtual recursion is
@@ -13631,6 +13685,8 @@ acfg_create (MonoAssembly *ass, guint32 jit_opts)
 	acfg->gsharedvt_in_signatures = g_hash_table_new ((GHashFunc)mono_signature_hash, (GEqualFunc)mono_metadata_signature_equal);
 	acfg->gsharedvt_out_signatures = g_hash_table_new ((GHashFunc)mono_signature_hash, (GEqualFunc)mono_metadata_signature_equal);
 	acfg->profile_methods = g_hash_table_new (NULL, NULL);
+	acfg->gshared_instances = g_hash_table_new (NULL, NULL);
+	acfg->prefer_instances = g_hash_table_new (NULL, NULL);
 	mono_os_mutex_init_recursive (&acfg->mutex);
 
 	init_got_info (&acfg->got_info);

src/mono/mono/mini/method-to-ir.c

@@ -7493,10 +7493,14 @@ mono_method_to_ir (MonoCompile *cfg, MonoMethod *method, MonoBasicBlock *start_b
 			if (constrained_class) {
 				if (m_method_is_static (cil_method) && mini_class_check_context_used (cfg, constrained_class)) {
 					/* get_constrained_method () doesn't work on the gparams used by generic sharing */
-					// FIXME: Other configurations
-					//if (!cfg->gsharedvt)
-					//	GENERIC_SHARING_FAILURE (CEE_CALL);
 					gshared_static_virtual = TRUE;
+					if (!cfg->gsharedvt)
+						/*
+						 * We can't resolve these calls at compile time, and they are used in
+						 * perf-sensitive code in the BCL, so ask the AOT compiler to try to use specific instances
+						 * instead of this gshared method.
+						 */
+						cfg->prefer_instances = TRUE;
 				} else {
 					cmethod = get_constrained_method (cfg, image, token, cil_method, constrained_class, generic_context);
 					CHECK_CFG_ERROR;

src/mono/mono/mini/mini.h

@@ -1492,6 +1492,7 @@ typedef struct {
 	guint            after_method_to_ir : 1;
 	guint            disable_inline_rgctx_fetch : 1;
 	guint            deopt : 1;
+	guint            prefer_instances : 1;
 	guint8           uses_simd_intrinsics;
 	int              r4_stack_type;
 	gpointer         debug_info;