浅析 Android 平台 mono执行机制 by郡墙

在 Android平台中采用Mono机制编译、处理C#语言编写的逻辑代码,编译之后本地存储IL指令。在游戏运行阶段存在代码动态编译的过程,原理为:利用 Unity3D引擎的 Mono jit机制将IL指令编译为机器可识别的汇编指令。

Mono 是什么?

Sponsored by Microsoft, Mono is an open source implementation of Microsoft's .NET Framework based on the ECMA standards for C# and the Common Language Runtime. A growing family of solutions and an active and enthusiastic contributing community is helping position Mono to become the leading choice for development of cross platform applications.

引用官方的介绍,这部分不是本文的重点,有兴趣的读者可以自行前往官网了解。

Mono 的执行流程

首先我们从 mono的入口函数开始分析

大致运行流程如下

<pre class="public-DraftStyleDefault-pre" spellcheck="false" style="box-sizing: border-box; font-family: monospace, monospace; font-size: 18px; margin: 0px auto 32px; color: rgb(90, 90, 91); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); text-decoration-style: initial; text-decoration-color: initial;"> Main() => mono_main_with_optinos() => mono_main() => mini_init() => mono_assembly_open() => main_thread_handler() => mini_cleanup() </pre>

其中 main_thread_handler 函数主要负责编译 & 处理 IL 指令,执行流程如下
main_thread_handler()
=> mono_jit_exec()
=> mono_assembly_get_image() 得倒 image 信息
=> mono_runtime_run_main()
=> mono_thread_set_main()
=> mono_assembly_set_main()
=> mono_runtime_exec_main()
....

mono_runtime_invoke 处理将要调用的方法,例如 ClassName::Main()。default_mono_runtime_invoke 函数实际调用 mono_jit_runtime_invoke 函数。mono_jit_runtime_invoke 函数调用来 mono_jit_compile_method_with_opt 实现编译目标函数的代码,调用mono_jit_compile_method 编译目标函数的 runtime wrapper (运行时的上层封装调用)。 runtime_invoke 函数调用编译之后的目标函数,其中 info->compiled_method 参数为编译之后目标函数代码在内存中的地址信息。
通过以上分析,mono_jit_compile_method_with_opt 函数为C#函数代码编译为目标机器指令的关键函数,我们来分析以下这部分的执行流程
mono_jit_compile_method_with_opt()
mono_jit_compile_method_inner()
mini_method_compile()
....

其中 mini_method_compile 函数在内部通过 Mono的 JIT 机制实现动态编译过程。
C#函数的执行过程
mono_runtime_invoke 函数实现与 mono \ object.c中,而实际调用 mono_jit_runtime_invoke 函数代码则在 mini.c 文件中。mono_runtime_invoke 函数负责实现 C#函数的代码编译和执行。

/**
 * mono_jit_runtime_invoke:
 * \param method: the method to invoke
 * \param obj: this pointer
 * \param params: array of parameter values.
 * \param exc: Set to the exception raised in the managed method.
 * \param error: error or caught exception object
 * If \p exc is NULL, \p error is thrown instead.
 * If coop is enabled, \p exc argument is ignored -
 * all exceptions are caught and propagated through \p error
 */
static MonoObject*
mono_jit_runtime_invoke (MonoMethod *method, void *obj, void **params, MonoObject **exc, MonoError *error)
{
    MonoMethod *invoke, *callee;
    MonoObject *(*runtime_invoke) (MonoObject *this_obj, void **params, MonoObject **exc, void* compiled_method);
    MonoDomain *domain = mono_domain_get ();
    MonoJitDomainInfo *domain_info;
    RuntimeInvokeInfo *info, *info2;
    MonoJitInfo *ji = NULL;
    gboolean callee_gsharedvt = FALSE;

    if (mono_use_interpreter)
        return mini_get_interp_callbacks ()->runtime_invoke (method, obj, params, exc, error);

    error_init (error);
    if (exc)
        *exc = NULL;

    if (obj == NULL && !(method->flags & METHOD_ATTRIBUTE_STATIC) && !method->string_ctor && (method->wrapper_type == 0)) {
        g_warning ("Ignoring invocation of an instance method on a NULL instance.\n");
        return NULL;
    }

    domain_info = domain_jit_info (domain);

    info = (RuntimeInvokeInfo *)mono_conc_hashtable_lookup (domain_info->runtime_invoke_hash, method);

//mono_jit_runtime_invoke 函数会先通过查找列表,判断是否已创建对应的 info 信息,若不存在,则先进行编译并得倒 info信息

    if (!info) {
        if (mono_security_core_clr_enabled ()) {
            /*
             * This might be redundant since mono_class_vtable () already does this,
             * but keep it just in case for moonlight.
             */
            mono_class_setup_vtable (method->klass);
            if (mono_class_has_failure (method->klass)) {
                mono_error_set_for_class_failure (error, method->klass);
                if (exc)
                    *exc = (MonoObject*)mono_class_get_exception_for_failure (method->klass);
                return NULL;
            }
        }

        gpointer compiled_method;

        callee = method;
        if (method->klass->rank && (method->iflags & METHOD_IMPL_ATTRIBUTE_INTERNAL_CALL) &&
            (method->iflags & METHOD_IMPL_ATTRIBUTE_NATIVE)) {
            /*
             * Array Get/Set/Address methods. The JIT implements them using inline code
             * inside the runtime invoke wrappers, so no need to compile them.
             */
            if (mono_aot_only) {
                /*
                 * Call a wrapper, since the runtime invoke wrapper was not generated.
                 */
                MonoMethod *wrapper;

                wrapper = mono_marshal_get_array_accessor_wrapper (method);
                invoke = mono_marshal_get_runtime_invoke (wrapper, FALSE);
                callee = wrapper;
            } else {
                callee = NULL;
            }
        }

        if (callee) {
            compiled_method = mono_jit_compile_method (callee, error);
            if (!compiled_method) {
                g_assert (!mono_error_ok (error));
                return NULL;
            }

            if (mono_llvm_only) {
                ji = mini_jit_info_table_find (mono_domain_get (), (char *)mono_get_addr_from_ftnptr (compiled_method), NULL);
                callee_gsharedvt = mini_jit_info_is_gsharedvt (ji);
                if (callee_gsharedvt)
                    callee_gsharedvt = mini_is_gsharedvt_variable_signature (mono_method_signature (jinfo_get_method (ji)));
            }

            if (!callee_gsharedvt)
                compiled_method = mini_add_method_trampoline (callee, compiled_method, mono_method_needs_static_rgctx_invoke (callee, TRUE), FALSE);
        } else {
            compiled_method = NULL;
        }

        info = create_runtime_invoke_info (domain, method, compiled_method, callee_gsharedvt, error);
        if (!mono_error_ok (error))
            return NULL;

        mono_domain_lock (domain);
        info2 = (RuntimeInvokeInfo *)mono_conc_hashtable_insert (domain_info->runtime_invoke_hash, method, info);
        mono_domain_unlock (domain);
        if (info2) {
            g_free (info);
            info = info2;
        }
    }

    /*
     * We need this here because mono_marshal_get_runtime_invoke can place
     * the helper method in System.Object and not the target class.
     */
    if (!mono_runtime_class_init_full (info->vtable, error)) {
        if (exc)
            *exc = (MonoObject*) mono_error_convert_to_exception (error);
        return NULL;
    }

    /* If coop is enabled, and the caller didn't ask for the exception to be caught separately,
       we always catch the exception and propagate it through the MonoError */
    gboolean catchExcInMonoError =
        (exc == NULL) && mono_threads_is_coop_enabled ();
    MonoObject *invoke_exc = NULL;
    if (catchExcInMonoError)
        exc = &invoke_exc;

    /* The wrappers expect this to be initialized to NULL */
    if (exc)
        *exc = NULL;

#ifdef MONO_ARCH_DYN_CALL_SUPPORTED
    if (info->dyn_call_info) {
        MonoMethodSignature *sig = mono_method_signature (method);
        gpointer *args;
        static RuntimeInvokeDynamicFunction dyn_runtime_invoke;
        int i, pindex, buf_size;
        guint8 *buf;
        guint8 retval [256];

        if (!dyn_runtime_invoke) {
            invoke = mono_marshal_get_runtime_invoke_dynamic ();
            dyn_runtime_invoke = (RuntimeInvokeDynamicFunction)mono_jit_compile_method (invoke, error);
            if (!mono_error_ok (error))
                return NULL;
        }

        /* Convert the arguments to the format expected by start_dyn_call () */
        args = (void **)g_alloca ((sig->param_count + sig->hasthis) * sizeof (gpointer));
        pindex = 0;
        if (sig->hasthis)
            args [pindex ++] = &obj;
        for (i = 0; i < sig->param_count; ++i) {
            MonoType *t = sig->params [i];

            if (t->byref) {
                args [pindex ++] = &params [i];
            } else if (MONO_TYPE_IS_REFERENCE (t) || t->type == MONO_TYPE_PTR) {
                args [pindex ++] = &params [i];
            } else {
                args [pindex ++] = params [i];
            }
        }

        //printf ("M: %s\n", mono_method_full_name (method, TRUE));

        buf_size = mono_arch_dyn_call_get_buf_size (info->dyn_call_info);
        buf = g_alloca (buf_size);
        g_assert (buf);

        mono_arch_start_dyn_call (info->dyn_call_info, (gpointer**)args, retval, buf);

        dyn_runtime_invoke (buf, exc, info->compiled_method);
        mono_arch_finish_dyn_call (info->dyn_call_info, buf);

        if (catchExcInMonoError && *exc != NULL) {
            mono_error_set_exception_instance (error, (MonoException*) *exc);
            return NULL;
        }

        if (info->ret_box_class)
            return mono_value_box_checked (domain, info->ret_box_class, retval, error);
        else
            return *(MonoObject**)retval;
    }
#endif

    MonoObject *result;

    if (mono_llvm_only) {
        result = mono_llvmonly_runtime_invoke (method, info, obj, params, exc, error);
        if (!is_ok (error))
            return NULL;
    } else {
        runtime_invoke = (MonoObject *(*)(MonoObject *, void **, MonoObject **, void *))info->runtime_invoke;

        result = runtime_invoke ((MonoObject *)obj, params, exc, info->compiled_method);
    }
    if (catchExcInMonoError && *exc != NULL)
        mono_error_set_exception_instance (error, (MonoException*) *exc);
    return result;
}

编译完成(或之前已经编译相同的 info 信息)后,则调用 info 自身的 runtime_invoke 函数实现对真正函数的调用。其中 runtime_invoke 函数的指针赋值。
compiled_method 变量存储目标函数编译之后所生成代码的内存地址。在之前的代码中,mono_jit_runtime_invoke 函数的第一个参数为 MonoMethod *method,在 MonoMethod 结构体中存储来需要编译和执行的 C# 函数的详细信息,包括:方法名、方法所属类的信息等。即根据 mono_jit_runtime_invoke 入口参数 method 可获取 compiled_method 变量存储的函数名,通过结构分析可以看到,在整个编译和调用过程中, MonoMethod 函数都是相关关键的结构,其结构在 class_internal.h 文件中定义。
。。。
1)name 存储 C# 函数名信息
2)klass 为 C# 函数所属类的相关信息。
通过 mono_jit_runtime_invoke 函数可知道 C# 函数的调用过程,以及函数地址、函数名、函数所属类等。

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