mp: Generalize function information retrieval

Generalizes MemFnInfo to be compatible with all function types.
Also adds type introspection for arguments, as well as helper templates for the common types supported by all partial specializations.

src/common/mp.h

@@ -6,21 +6,64 @@
 
 #pragma once
 
+#include <tuple>
 #include "common/common_types.h"
 
 namespace Dynarmic {
 namespace mp {
 
-template<typename MemFnT>
-struct MemFnInfo;
+/// Used to provide information about an arbitrary function.
+template <typename Function>
+struct FunctionInfo;
 
-/// This struct provides information about a member function pointer.
-template<typename T, typename ReturnT, typename ...Args>
-struct MemFnInfo<ReturnT (T::*)(Args...)> {
-    using class_type = T;
-    using return_type = ReturnT;
+/**
+ * Partial specialization for function types.
+ *
+ * This is used as the supporting base for all other specializations.
+ */
+template <typename R, typename... Args>
+struct FunctionInfo<R(Args...)>
+{
+    using return_type = R;
     static constexpr size_t args_count = sizeof...(Args);
+
+    template <size_t ParameterIndex>
+    struct Parameter
+    {
+        static_assert(args_count != 0 && ParameterIndex < args_count - 1, "Non-existent function parameter index");
+        using type = std::tuple_element_t<ParameterIndex, std::tuple<Args...>>;
+    };
 };
 
+/// Partial specialization for function pointers
+template <typename R, typename... Args>
+struct FunctionInfo<R(*)(Args...)> : public FunctionInfo<R(Args...)>
+{
+};
+
+/// Partial specialization for member function pointers.
+template <typename C, typename R, typename... Args>
+struct FunctionInfo<R(C::*)(Args...)> : public FunctionInfo<R(Args...)>
+{
+    using class_type = C;
+};
+
+/**
+ * Helper template for retrieving the type of a function parameter.
+ *
+ * @tparam Function       An arbitrary function type.
+ * @tparam ParameterIndex Zero-based index indicating which parameter to get the type of.
+ */
+template <typename Function, size_t ParameterIndex>
+using parameter_type_t = typename FunctionInfo<Function>::template Parameter<ParameterIndex>::type;
+
+/**
+ * Helper template for retrieving the return type of a function.
+ *
+ * @tparam Function The function type to get the return type of.
+ */
+template <typename Function>
+using return_type_t = typename FunctionInfo<Function>::return_type;
+
 } // namespace mp
 } // namespace Dynarmic

src/frontend/decoder/arm.h

@@ -22,7 +22,7 @@ namespace Arm {
 
 template <typename Visitor>
 struct ArmMatcher {
-    using CallRetT = typename mp::MemFnInfo<decltype(&Visitor::arm_UDF)>::return_type;
+    using CallRetT = mp::return_type_t<decltype(&Visitor::arm_UDF)>;
 
     ArmMatcher(const char* const name, u32 mask, u32 expect, std::function<CallRetT(Visitor&, u32)> fn)
             : name(name), mask(mask), expect(expect), fn(fn) {}

src/frontend/decoder/decoder_detail.h

@@ -127,8 +127,8 @@ public:
      */
     template<typename FnT>
     static auto GetMatcher(FnT fn, const char* const name, const char* const bitstring) {
-        using Visitor = typename mp::MemFnInfo<FnT>::class_type;
-        constexpr size_t args_count = mp::MemFnInfo<FnT>::args_count;
+        using Visitor = typename mp::FunctionInfo<FnT>::class_type;
+        constexpr size_t args_count = mp::FunctionInfo<FnT>::args_count;
         using Iota = std::make_index_sequence<args_count>;
 
         const auto mask_and_expect = GetMaskAndExpect(bitstring);

src/frontend/decoder/thumb16.h

@@ -20,7 +20,7 @@ namespace Arm {
 
 template <typename Visitor>
 struct Thumb16Matcher {
-    using CallRetT = typename mp::MemFnInfo<decltype(&Visitor::thumb16_UDF)>::return_type;
+    using CallRetT = mp::return_type_t<decltype(&Visitor::thumb16_UDF)>;
 
     Thumb16Matcher(const char* const name, u16 mask, u16 expect, std::function<CallRetT(Visitor&, u16)> fn)
             : name(name), mask(mask), expect(expect), fn(fn) {}

src/frontend/decoder/thumb32.h

@@ -20,7 +20,7 @@ namespace Arm {
 
 template <typename Visitor>
 struct Thumb32Matcher {
-    using CallRetT = typename mp::MemFnInfo<decltype(&Visitor::thumb32_UDF)>::return_type;
+    using CallRetT = mp::return_type_t<decltype(&Visitor::thumb32_UDF)>;
 
     Thumb32Matcher(const char* const name, u32 mask, u32 expect, std::function<CallRetT(Visitor&, u32)> fn)
             : name(name), mask(mask), expect(expect), fn(fn) {}

src/frontend/decoder/vfp2.h

@@ -20,7 +20,7 @@ namespace Arm {
 
 template <typename Visitor>
 struct VFP2Matcher {
-    using CallRetT = typename mp::MemFnInfo<decltype(&Visitor::vfp2_VADD)>::return_type;
+    using CallRetT = mp::return_type_t<decltype(&Visitor::vfp2_VADD)>;
 
     VFP2Matcher(const char* const name, u32 mask, u32 expect, std::function<CallRetT(Visitor&, u32)> fn)
             : name(name), mask(mask), expect(expect), fn(fn) {}