Allow natives to return structs and arrays.

Natives can now return enum structs and fixed-size arrays. The address
is passed as a hidden first parameter. For example:

    native Struct DoStuff(int x, int y);

Will have:

    params[0] = 3
    params[1] = the address of the output struct
    params[2] = x
    params[3] = y

The native MUST return params[1] on success, otherwise, behavior is
undefined.

It is recommended that when defining C++ definitions of SourcePawn
structs, to use `#pragma pack(push, 1)` and `#pragma pack(pop)`. In
addition, the only types that should appear are "cell_t" and "float". No
other type should be used for scalars. For arrays, "char" may be used,
but not as a scalar.

When using char arrays, the C++ definition must be careful to convert
the char size to a cell-aligned size. For example:

    enum struct MyStruct {
        int x;
        int y;
        char message[50];
    }

Should look like this in C++:

    #pragma pack(push, 1)
    struct MyStruct {
        cell_t x;
        cell_t y;
        char message[CharArraySize<50>::bytes];
    };
    #pragma pack(pop)

compiler/code-generator.cpp

@@ -1139,7 +1139,7 @@ CodeGenerator::EmitFieldAccessExpr(FieldAccessExpr* expr)
 
 void CodeGenerator::EmitCallExpr(CallExpr* call) {
     // If returning an array, push a hidden parameter.
-    if (call->fun()->return_array()) {
+    if (call->fun()->return_array() && !call->fun()->is_native()) {
         cell retsize = call->fun()->return_type()->CellStorageSize();
         assert(retsize);
 
@@ -1208,12 +1208,58 @@ void CodeGenerator::EmitCallExpr(CallExpr* call) {
         __ emit(OP_PUSH_PRI);
     }
 
-    EmitCall(call->fun(), (cell)argv.size());
+    cell_t hidden_args = 0;
+    if (call->fun()->return_array() && call->fun()->is_native()) {
+        EmitNativeCallHiddenArg(call);
+        hidden_args++;
+    }
+
+    EmitCall(call->fun(), (cell)argv.size() + hidden_args);
 
-    if (call->fun()->return_array())
+    if (call->fun()->return_array() && !call->fun()->is_native())
         __ emit(OP_POP_PRI);
 }
 
+void CodeGenerator::EmitNativeCallHiddenArg(CallExpr* call) {
+    TrackTempHeapAlloc(call, 1);
+
+    auto fun = call->fun();
+
+    ArrayData array;
+    BuildCompoundInitializer(fun->return_type(), nullptr, &array);
+
+    cell retsize = call->fun()->return_type()->CellStorageSize();
+    assert(retsize);
+
+    __ emit(OP_HEAP, retsize * sizeof(cell));
+
+    auto info = fun->return_array();
+    if (array.iv.empty()) {
+        __ emit(OP_CONST_PRI, 0);
+        __ emit(OP_FILL, retsize);
+    } else {
+        if (!info->iv_size) {
+            // No initializer, so we should have no data.
+            assert(array.data.empty());
+            assert(array.zeroes);
+
+            info->iv_size = (cell_t)array.iv.size();
+            info->dat_addr = data_.dat_address();
+            info->zeroes = array.zeroes;
+            data_.Add(std::move(array.iv));
+        }
+
+        cell dat_addr = info->dat_addr;
+        cell iv_size = info->iv_size;
+        assert(iv_size);
+        assert(info->zeroes);
+
+        __ emit(OP_INITARRAY_ALT, dat_addr, iv_size, 0, info->zeroes, 0);
+    }
+
+    __ emit(OP_PUSH_ALT);
+}
+
 void
 CodeGenerator::EmitDefaultArgExpr(DefaultArgExpr* expr)
 {

compiler/code-generator.h

@@ -90,6 +90,7 @@ class CodeGenerator final
     void EmitIndexExpr(IndexExpr* expr);
     void EmitFieldAccessExpr(FieldAccessExpr* expr);
     void EmitCallExpr(CallExpr* expr);
+    void EmitNativeCallHiddenArg(CallExpr* expr);
     void EmitDefaultArgExpr(DefaultArgExpr* expr);
     void EmitCallUserOpExpr(CallUserOpExpr* expr);
     void EmitNewArrayExpr(NewArrayExpr* expr);

compiler/messages.h

@@ -63,7 +63,7 @@ static const char* errmsg[] = {
     /*036*/ "empty statement\n",
     /*037*/ "invalid string (possibly non-terminated string)\n",
     /*038*/ "extra characters on line\n",
-    /*039*/ "unused39\n",
+    /*039*/ "natives can only return fixed-size arrays\n",
     /*040*/ "duplicate \"case\" label (value %d)\n",
     /*041*/ "invalid ellipsis, array size is not known\n",
     /*042*/ "invalid combination of class specifiers\n",
@@ -165,7 +165,7 @@ static const char* errmsg[] = {
     /*138*/ "const was specified twice\n",
     /*139*/ "could not find type \"%s\"\n",
     /*140*/ "function '%s' does not return a value\n",
-    /*141*/ "natives, forwards, and public functions cannot return arrays\n",
+    /*141*/ "forwards and public functions cannot return arrays\n",
     /*142*/ "unexpected array expression\n",
     /*143*/ "new-style declarations should not have \"new\"\n",
     /*144*/ "void cannot be used as a variable type\n",

compiler/name-resolution.cpp

@@ -790,7 +790,7 @@ bool FunctionDecl::Bind(SemaContext& outer_sc) {
             markusage(args_[0], uREAD);
     }
 
-    if ((is_native_ || is_public_ || is_forward_) &&
+    if ((is_public_ || is_forward_) &&
         (return_type()->isArray() || return_type()->isEnumStruct()))
     {
         error(pos_, 141);

compiler/semantics.cpp

@@ -2522,6 +2522,24 @@ bool Semantics::CheckCompoundReturnStmt(ReturnStmt* stmt) {
     return true;
 }
 
+bool Semantics::CheckNativeCompoundReturn(FunctionDecl* info) {
+    auto rt = info->return_type();
+    if (auto root = rt->as<ArrayType>()) {
+        for (auto it = root; it; it = it->inner()->as<ArrayType>()) {
+            if (it->size() == 0) {
+                report(info, 39);
+                return false;
+            }
+        }
+    }
+
+    // For native calls, the implicit arg is first, not last.
+    auto rai = new FunctionDecl::ReturnArrayInfo;
+    rai->hidden_address = sizeof(cell_t) * 3;
+    info->set_return_array(rai);
+    return true;
+}
+
 bool Semantics::CheckAssertStmt(AssertStmt* stmt) {
     if (Expr* expr = AnalyzeForTest(stmt->expr())) {
         stmt->set_expr(expr);
@@ -2861,6 +2879,9 @@ bool Semantics::CheckFunctionDeclImpl(FunctionDecl* info) {
             report(info->pos(), 83);
             return false;
         }
+        auto rt = info->return_type();
+        if ((rt->isArray() || rt->isEnumStruct()) && !CheckNativeCompoundReturn(info))
+            return false;
         return true;
     }
 

compiler/semantics.h

@@ -184,6 +184,7 @@ class Semantics final
     bool CheckStaticAssertStmt(StaticAssertStmt* stmt);
     bool CheckReturnStmt(ReturnStmt* stmt);
     bool CheckCompoundReturnStmt(ReturnStmt* stmt);
+    bool CheckNativeCompoundReturn(FunctionDecl* info);
     bool CheckExprStmt(ExprStmt* stmt);
     bool CheckIfStmt(IfStmt* stmt);
     bool CheckConstDecl(ConstDecl* decl);

include/sp_typeutil.h

@@ -77,4 +77,10 @@ sp_ctof(cell_t val)
     return sp::FloatCellUnion(val).f32;
 }
 
+template <size_t Size>
+struct CharArraySize {
+    static constexpr size_t cells = (Size + sizeof(cell_t) - 1) / sizeof(cell_t);
+    static constexpr size_t bytes = cells * sizeof(cell_t);
+};
+
 #endif //_INCLUDE_SOURCEPAWN_VM_TYPEUTIL_H_

tests/shell.inc

@@ -52,3 +52,4 @@ native void copy_2d_array_to_callback(const int[] flat_array, int length, int st
 native void assert_eq(any a1, any a2);
 
 native void print_test_struct(TestStruct x);
+native TestStruct add_test_structs(TestStruct a, TestStruct b);

vm/shell/shell.cpp

@@ -327,6 +327,24 @@ static cell_t PrintTestStruct(IPluginContext* cx, const cell_t* params) {
   return 0;
 }
 
+static cell_t AddTestStructs(IPluginContext* cx, const cell_t* params) {
+  TestStruct* a;
+  TestStruct* b;
+  TestStruct* out;
+
+  int err;
+  if ((err = cx->LocalToPhysAddr(params[1], reinterpret_cast<cell_t**>(&out))) != SP_ERROR_NONE)
+    return cx->ThrowNativeErrorEx(err, "Could not read out argument");
+  if ((err = cx->LocalToPhysAddr(params[2], reinterpret_cast<cell_t**>(&a))) != SP_ERROR_NONE)
+    return cx->ThrowNativeErrorEx(err, "Could not read argument 1");
+  if ((err = cx->LocalToPhysAddr(params[3], reinterpret_cast<cell_t**>(&b))) != SP_ERROR_NONE)
+    return cx->ThrowNativeErrorEx(err, "Could not read argument 2");
+
+  out->x = a->x + b->x;
+  out->y = a->y + b->y;
+  return params[1];
+}
+
 class DynamicNative : public INativeCallback
 {
   public:
@@ -353,6 +371,16 @@ class DynamicNative : public INativeCallback
     uintptr_t refcount_ = 0;
 };
 
+#pragma pack(push, 1)
+struct LayoutVerifier {
+  char message[CharArraySize<50>::bytes];
+  int x;
+};
+#pragma pack(pop)
+
+static_assert(offsetof(LayoutVerifier, message) == 0);
+static_assert(offsetof(LayoutVerifier, x) == 52);
+
 static int Execute(const char* file)
 {
   char error[255];
@@ -386,6 +414,7 @@ static int Execute(const char* file)
   BindNative(rt, "assert_eq", AssertEq);
   BindNative(rt, "printf", Printf);
   BindNative(rt, "print_test_struct", PrintTestStruct);
+  BindNative(rt, "add_test_structs", AddTestStructs);
 
   IPluginFunction* fun = rt->GetFunctionByName("main");
   if (!fun)