如何使用 JIT 编译从捕获的 lambda 生成 C 指针回调？

Question

In C++11 and later converting a lambda expression with no captured variables is very simple, but for lambdas with captured variables to do it properly requires dynamic code generation as per this guys answer .

Basically what I figure will have to be dynamically generate is this. (Note the code here indicates my desired semantics not any real code)

UserData *userdata;
api_return_value callback(api_data arg) {
    return customized_callback(arg, userdata);
}

And I would generate this so I can bind the dynamically generated code to a corresponding function pointer callback defined in some api

api_return_value (*callback) (api_data);

Is there a reasonably clean and portable way to go about this using something like LLVM or NativeJIT? I'm stuck using a C api with no provided user data pointer so this seems to be my only alternative.

Answer 1

Rather than doing JIT code generation at runtime, you can "pre-create" a pool of (extern "C") functions at compile time and manage an allocation pool of them. Something like:

#define REP10(P, M)  M(P##0) M(P##1) M(P##2) M(P##3) M(P##4) M(P##5) M(P##6) M(P##7) M(P##8) M(P##9)
#define REP100(M) REP10(,M) REP10(1,M) REP10(2,M) REP10(3,M) REP10(4,M) REP10(5,M) REP10(6,M) REP10(7,M) REP10(8,M) REP10(9,M)

extern struct func_wrap_t {
    func_wrap_t              *next;
    extern "C" void          (*c_fn)();
    std::function<void()>    fn;
} func_wrap_table[];
#define FUNC_WRAP_INIT(M) { M ? func_wrap_table+M-1 : 0, func_wrap_cfunc##M },
#define FUNC_WRAP_DEF(M) extern "C" void func_wrap_cfunc##M() { func_wrap_table[M].fn(); }
REP100(FUNC_WRAP_DEF)
func_wrap_t func_wrap_table[] = { REP100(FUNC_WRAP_INIT) };
func_wrap_t *func_wrap_freelist = &func_wrap_table[99];

will create 100 such functions statically and link them up into a freelist. You can then write a wrapper RAII type that allocates them from this list and returns them when finished.