Fix invalid check on value types (UUM-79115).

The previously inserted null check on 'this' assumed, at least on amd64,
that the pointer could be dereferenced as a pointer sized value. This is
not true for value types which can be smaller than pointer sized,
especially if the memory adjacent to the value type is protected.

Rather than improving the null check, we can avoid it entirely for value
type methods as they should never have a NULL 'this' pointer.

Author: joncham

mono/mini/method-to-ir.c

@@ -4601,7 +4601,7 @@ inline_method (MonoCompile *cfg, MonoMethod *cmethod, MonoMethodSignature *fsig,
 	guint32 prev_cil_offset_to_bb_len;
 	MonoMethod *prev_current_method;
 	MonoGenericContext *prev_generic_context;
-	gboolean ret_var_set, prev_ret_var_set, prev_disable_inline, virtual_ = FALSE;
+	gboolean ret_var_set, prev_ret_var_set, prev_disable_inline, emit_check_this = FALSE;
 	MonoProfilerCoverageInfo *prev_coverage_info;
 
 	g_assert (cfg->exception_type == MONO_EXCEPTION_NONE);
@@ -4681,10 +4681,15 @@ inline_method (MonoCompile *cfg, MonoMethod *cmethod, MonoMethodSignature *fsig,
 	cfg->ret_var_set = FALSE;
 	cfg->inline_depth ++;
 
-	if (ip && *ip == CEE_CALLVIRT && !(cmethod->flags & METHOD_ATTRIBUTE_STATIC))
-		virtual_ = TRUE;
+	if (ip && *ip == CEE_CALLVIRT && !(cmethod->flags & METHOD_ATTRIBUTE_STATIC) &&
+		// Only perform a null check on 'this' arguments for methods on:
+		// 1. Reference types
+		// 2. System.ValueType and System.Enum as those will have a boxed 'this' argument.
+		// NOTE: klass->valuetype is FALSE for System.ValueType so we only need to check System.Enum.
+		(!m_class_is_valuetype(cmethod->klass) || m_class_is_enumtype(cmethod->klass)))
+		emit_check_this = TRUE;
 
-	costs = mono_method_to_ir (cfg, cmethod, sbblock, ebblock, rvar, sp, real_offset, virtual_);
+	costs = mono_method_to_ir (cfg, cmethod, sbblock, ebblock, rvar, sp, real_offset, emit_check_this);
 
 	ret_var_set = cfg->ret_var_set;
 
@@ -6116,7 +6121,7 @@ mono_opcode_decode (guchar *ip, guint op_size, MonoOpcodeEnum il_op, MonoOpcodeP
  * @return_var: if not NULL, the place where the return value is stored, used during inlining.   
  * @inline_args: if not NULL, contains the arguments to the inline call
  * @inline_offset: if not zero, the real offset from the inline call, or zero otherwise.
- * @is_virtual_call: whether this method is being called as a result of a call to callvirt
+ * @emit_check_this: whether this method needs to perform an explicit this null check.
  *
  * This method is used to turn ECMA IL into Mono's internal Linear IR
  * reprensetation.  It is used both for entire methods, as well as
@@ -6129,7 +6134,7 @@ mono_opcode_decode (guchar *ip, guint op_size, MonoOpcodeEnum il_op, MonoOpcodeP
 int
 mono_method_to_ir (MonoCompile *cfg, MonoMethod *method, MonoBasicBlock *start_bblock, MonoBasicBlock *end_bblock, 
 		   MonoInst *return_var, MonoInst **inline_args, 
-		   guint inline_offset, gboolean is_virtual_call)
+		   guint inline_offset, gboolean emit_check_this)
 {
 	ERROR_DECL (error);
 	// Buffer to hold parameters to mono_new_array, instead of varargs.
@@ -6603,7 +6608,7 @@ mono_method_to_ir (MonoCompile *cfg, MonoMethod *method, MonoBasicBlock *start_b
 	}
 
 	/* add a check for this != NULL to inlined methods */
-	if (is_virtual_call) {
+	if (emit_check_this) {
 		MonoInst *arg_ins;
 
 		NEW_ARGLOAD (cfg, arg_ins, 0);