Implement closures

Implementation is pretty straightforward for now:
- find all names that get accessed from a nested function
- if any, create a closure object at function entry
- any time we set a name accessed from a nested function,
  update its value in the closure
- when evaluating a functiondef that needs a closure, attach
  the created closure to the created function object.

Closures are currently passed as an extra argument before any
python-level args, which I'm not convinced is the right strategy.
It's works out fine but it feels messy to say that functions
can have different C-level calling conventions.
It felt worse to include the closure as part of the python-level arg
passing.
Maybe it should be passed after all the other arguments?

Closures are currently just simple objects, on which we set and get
Python-level attributes.  The performance (which I haven't tested)
relies on attribute access being made fast through the hidden-class
inline caches.

There are a number of ways that this could be improved:
- be smarter about when we create the closure object, or when we
  update it.
- not create empty pass-through closures
- give the closures a pre-defined shape, since we know at irgen-time
  what names can get set.  could probably avoid the inline cache
  machinery and also have better code.

src/analysis/scoping_analysis.cpp

@@ -76,15 +76,9 @@ public:
 
     virtual ScopeInfo* getParent() { return parent; }
 
-    virtual bool createsClosure() {
-        assert(0);
-        return usage->referenced_from_nested.size() > 0;
-    }
+    virtual bool createsClosure() { return usage->referenced_from_nested.size() > 0; }
 
-    virtual bool takesClosure() {
-        assert(0);
-        return false;
-    }
+    virtual bool takesClosure() { return usage->got_from_closure.size() > 0; }
 
     virtual bool refersToGlobal(const std::string& name) {
         // HAX
@@ -143,7 +137,6 @@ public:
         return true;
     }
 
-    virtual bool visit_arguments(AST_arguments* node) { return false; }
     virtual bool visit_assert(AST_Assert* node) { return false; }
     virtual bool visit_assign(AST_Assign* node) { return false; }
     virtual bool visit_augassign(AST_AugAssign* node) { return false; }
@@ -201,8 +194,15 @@ public:
 
     virtual bool visit_classdef(AST_ClassDef* node) {
         if (node == orig_node) {
-            return false;
+            for (AST_stmt* s : node->body)
+                s->accept(this);
+            return true;
         } else {
+            for (auto* e : node->bases)
+                e->accept(this);
+            for (auto* e : node->decorator_list)
+                e->accept(this);
+
             doWrite(node->name);
             (*map)[node] = new ScopingAnalysis::ScopeNameUsage(node, cur);
             collect(node, map);
@@ -212,8 +212,21 @@ public:
 
     virtual bool visit_functiondef(AST_FunctionDef* node) {
         if (node == orig_node) {
-            return false;
+            for (AST_expr* e : node->args->args)
+                e->accept(this);
+            if (node->args->vararg.size())
+                doWrite(node->args->vararg);
+            if (node->args->kwarg.size())
+                doWrite(node->args->kwarg);
+            for (AST_stmt* s : node->body)
+                s->accept(this);
+            return true;
         } else {
+            for (auto* e : node->args->defaults)
+                e->accept(this);
+            for (auto* e : node->decorator_list)
+                e->accept(this);
+
             doWrite(node->name);
             (*map)[node] = new ScopingAnalysis::ScopeNameUsage(node, cur);
             collect(node, map);
@@ -278,6 +291,8 @@ void ScopingAnalysis::processNameUsages(ScopingAnalysis::NameUsageMap* usages) {
             if (usage->written.count(*it2))
                 continue;
 
+            std::vector<ScopeNameUsage*> intermediate_parents;
+
             ScopeNameUsage* parent = usage->parent;
             while (parent) {
                 if (parent->node->type == AST_TYPE::ClassDef) {
@@ -287,8 +302,15 @@ void ScopingAnalysis::processNameUsages(ScopingAnalysis::NameUsageMap* usages) {
                 } else if (parent->written.count(*it2)) {
                     usage->got_from_closure.insert(*it2);
                     parent->referenced_from_nested.insert(*it2);
+
+                    for (ScopeNameUsage* iparent : intermediate_parents) {
+                        iparent->referenced_from_nested.insert(*it2);
+                        iparent->got_from_closure.insert(*it2);
+                    }
+
                     break;
                 } else {
+                    intermediate_parents.push_back(parent);
                     parent = parent->parent;
                 }
             }
@@ -320,19 +342,6 @@ void ScopingAnalysis::processNameUsages(ScopingAnalysis::NameUsageMap* usages) {
 }
 
 ScopeInfo* ScopingAnalysis::analyzeSubtree(AST* node) {
-#ifndef NDEBUG
-    std::vector<AST*> flattened;
-    flatten(parent_module->body, flattened, false);
-    bool found = 0;
-    for (AST* n : flattened) {
-        if (n == node) {
-            found = true;
-            break;
-        }
-    }
-    assert(found);
-#endif
-
     NameUsageMap usages;
     usages[node] = new ScopeNameUsage(node, NULL);
     NameCollectorVisitor::collect(node, &usages);

src/codegen/codegen.h

@@ -70,7 +70,7 @@ struct GlobalState {
     llvm::Type* llvm_flavor_type, *llvm_flavor_type_ptr;
     llvm::Type* llvm_opaque_type;
     llvm::Type* llvm_str_type_ptr;
-    llvm::Type* llvm_clfunction_type_ptr;
+    llvm::Type* llvm_clfunction_type_ptr, *llvm_closure_type_ptr;
     llvm::Type* llvm_module_type_ptr, *llvm_bool_type_ptr;
     llvm::Type* i1, *i8, *i8_ptr, *i32, *i64, *void_, *double_;
     llvm::Type* vector_ptr;

src/codegen/compvars.cpp

@@ -518,11 +518,24 @@ ConcreteCompilerVariable* UnknownType::nonzero(IREmitter& emitter, const OpInfo&
     return new ConcreteCompilerVariable(BOOL, rtn_val, true);
 }
 
-CompilerVariable* makeFunction(IREmitter& emitter, CLFunction* f) {
+CompilerVariable* makeFunction(IREmitter& emitter, CLFunction* f, CompilerVariable* closure) {
     // Unlike the CLFunction*, which can be shared between recompilations, the Box* around it
     // should be created anew every time the functiondef is encountered
-    llvm::Value* boxed
-        = emitter.getBuilder()->CreateCall(g.funcs.boxCLFunction, embedConstantPtr(f, g.llvm_clfunction_type_ptr));
+
+    llvm::Value* closure_v;
+    ConcreteCompilerVariable* converted = NULL;
+    if (closure) {
+        converted = closure->makeConverted(emitter, closure->getConcreteType());
+        closure_v = converted->getValue();
+    } else {
+        closure_v = embedConstantPtr(nullptr, g.llvm_closure_type_ptr);
+    }
+
+    llvm::Value* boxed = emitter.getBuilder()->CreateCall2(g.funcs.boxCLFunction,
+                                                           embedConstantPtr(f, g.llvm_clfunction_type_ptr), closure_v);
+
+    if (converted)
+        converted->decvref(emitter);
     return new ConcreteCompilerVariable(typeFromClass(function_cls), boxed, true);
 }
 
@@ -1147,6 +1160,30 @@ public:
 std::unordered_map<BoxedClass*, NormalObjectType*> NormalObjectType::made;
 ConcreteCompilerType* STR, *BOXED_INT, *BOXED_FLOAT, *BOXED_BOOL, *NONE;
 
+class ClosureType : public ConcreteCompilerType {
+public:
+    llvm::Type* llvmType() { return g.llvm_closure_type_ptr; }
+    std::string debugName() { return "closure"; }
+
+    CompilerVariable* getattr(IREmitter& emitter, const OpInfo& info, ConcreteCompilerVariable* var,
+                              const std::string* attr, bool cls_only) {
+        assert(!cls_only);
+        llvm::Value* bitcast = emitter.getBuilder()->CreateBitCast(var->getValue(), g.llvm_value_type_ptr);
+        return ConcreteCompilerVariable(UNKNOWN, bitcast, true).getattr(emitter, info, attr, cls_only);
+    }
+
+    void setattr(IREmitter& emitter, const OpInfo& info, ConcreteCompilerVariable* var, const std::string* attr,
+                 CompilerVariable* v) {
+        llvm::Value* bitcast = emitter.getBuilder()->CreateBitCast(var->getValue(), g.llvm_value_type_ptr);
+        ConcreteCompilerVariable(UNKNOWN, bitcast, true).setattr(emitter, info, attr, v);
+    }
+
+    virtual ConcreteCompilerType* getConcreteType() { return this; }
+    // Shouldn't call this:
+    virtual ConcreteCompilerType* getBoxType() { RELEASE_ASSERT(0, ""); }
+} _CLOSURE;
+ConcreteCompilerType* CLOSURE = &_CLOSURE;
+
 class StrConstantType : public ValuedCompilerType<std::string*> {
 public:
     std::string debugName() { return "str_constant"; }

src/codegen/compvars.h

@@ -29,7 +29,7 @@ class CompilerType;
 class IREmitter;
 
 extern ConcreteCompilerType* INT, *BOXED_INT, *FLOAT, *BOXED_FLOAT, *VOID, *UNKNOWN, *BOOL, *STR, *NONE, *LIST, *SLICE,
-    *MODULE, *DICT, *BOOL, *BOXED_BOOL, *BOXED_TUPLE, *SET;
+    *MODULE, *DICT, *BOOL, *BOXED_BOOL, *BOXED_TUPLE, *SET, *CLOSURE;
 extern CompilerType* UNDEF;
 
 class CompilerType {
@@ -316,7 +316,7 @@ ConcreteCompilerVariable* makeInt(int64_t);
 ConcreteCompilerVariable* makeFloat(double);
 ConcreteCompilerVariable* makeBool(bool);
 CompilerVariable* makeStr(std::string*);
-CompilerVariable* makeFunction(IREmitter& emitter, CLFunction*);
+CompilerVariable* makeFunction(IREmitter& emitter, CLFunction*, CompilerVariable* closure);
 ConcreteCompilerVariable* undefVariable();
 CompilerVariable* makeTuple(const std::vector<CompilerVariable*>& elts);
 

src/codegen/irgen.cpp

@@ -563,7 +563,8 @@ static void emitBBs(IRGenState* irstate, const char* bb_type, GuardList& out_gua
 
                 emitter->getBuilder()->SetInsertPoint(llvm_entry_blocks[source->cfg->getStartingBlock()]);
             }
-            generator->unpackArguments(arg_names, cf->spec->arg_types);
+
+            generator->doFunctionEntry(arg_names, cf->spec->arg_types);
 
             // Function-entry safepoint:
             // TODO might be more efficient to do post-call safepoints?
@@ -915,6 +916,9 @@ CompiledFunction* doCompile(SourceInfo* source, const OSREntryDescriptor* entry_
     ASSERT(nargs == spec->arg_types.size(), "%d %ld", nargs, spec->arg_types.size());
 
     std::vector<llvm::Type*> llvm_arg_types;
+    if (source->scoping->getScopeInfoForNode(source->ast)->takesClosure())
+        llvm_arg_types.push_back(g.llvm_closure_type_ptr);
+
     if (entry_descriptor == NULL) {
         for (int i = 0; i < nargs; i++) {
             if (i == 3) {

src/codegen/irgen/hooks.cpp

@@ -262,7 +262,7 @@ void compileAndRunModule(AST_Module* m, BoxedModule* bm) {
     _t.end();
 
     if (cf->is_interpreted)
-        interpretFunction(cf->func, 0, NULL, NULL, NULL, NULL);
+        interpretFunction(cf->func, 0, NULL, NULL, NULL, NULL, NULL);
     else
         ((void (*)())cf->code)();
 }

src/codegen/irgen/irgenerator.h

@@ -183,8 +183,9 @@ public:
 
     virtual ~IRGenerator() {}
 
-    virtual void unpackArguments(const std::vector<AST_expr*>& arg_names,
+    virtual void doFunctionEntry(const std::vector<AST_expr*>& arg_names,
                                  const std::vector<ConcreteCompilerType*>& arg_types) = 0;
+
     virtual void giveLocalSymbol(const std::string& name, CompilerVariable* var) = 0;
     virtual void copySymbolsFrom(SymbolTable* st) = 0;
     virtual void run(const CFGBlock* block) = 0;

src/codegen/llvm_interpreter.cpp

@@ -257,7 +257,7 @@ const LineInfo* getLineInfoForInterpretedFrame(void* frame_ptr) {
     }
 }
 
-Box* interpretFunction(llvm::Function* f, int nargs, Box* arg1, Box* arg2, Box* arg3, Box** args) {
+Box* interpretFunction(llvm::Function* f, int nargs, Box* closure, Box* arg1, Box* arg2, Box* arg3, Box** args) {
     assert(f);
 
 #ifdef TIME_INTERPRETS
@@ -280,18 +280,21 @@ Box* interpretFunction(llvm::Function* f, int nargs, Box* arg1, Box* arg2, Box*
     UnregisterHelper helper(frame_ptr);
 
     int arg_num = -1;
+    int closure_indicator = closure ? 1 : 0;
     for (llvm::Argument& arg : f->args()) {
         arg_num++;
 
-        if (arg_num == 0)
+        if (arg_num == 0 && closure)
+            symbols.insert(std::make_pair(static_cast<llvm::Value*>(&arg), Val(closure)));
+        else if (arg_num == 0 + closure_indicator)
             symbols.insert(std::make_pair(static_cast<llvm::Value*>(&arg), Val(arg1)));
-        else if (arg_num == 1)
+        else if (arg_num == 1 + closure_indicator)
             symbols.insert(std::make_pair(static_cast<llvm::Value*>(&arg), Val(arg2)));
-        else if (arg_num == 2)
+        else if (arg_num == 2 + closure_indicator)
             symbols.insert(std::make_pair(static_cast<llvm::Value*>(&arg), Val(arg3)));
         else {
-            assert(arg_num == 3);
-            assert(f->getArgumentList().size() == 4);
+            assert(arg_num == 3 + closure_indicator);
+            assert(f->getArgumentList().size() == 4 + closure_indicator);
             assert(f->getArgumentList().back().getType() == g.llvm_value_type_ptr->getPointerTo());
             symbols.insert(std::make_pair(static_cast<llvm::Value*>(&arg), Val((int64_t)args)));
             // printf("loading %%4 with %p\n", (void*)args);

src/codegen/llvm_interpreter.h

@@ -25,7 +25,7 @@ class Box;
 class GCVisitor;
 class LineInfo;
 
-Box* interpretFunction(llvm::Function* f, int nargs, Box* arg1, Box* arg2, Box* arg3, Box** args);
+Box* interpretFunction(llvm::Function* f, int nargs, Box* closure, Box* arg1, Box* arg2, Box* arg3, Box** args);
 
 void gatherInterpreterRoots(GCVisitor* visitor);
 const LineInfo* getLineInfoForInterpretedFrame(void* frame_ptr);

src/codegen/runtime_hooks.cpp

@@ -137,6 +137,9 @@ void initGlobalFuncs(GlobalState& g) {
     assert(vector_type);
     g.vector_ptr = vector_type->getPointerTo();
 
+    g.llvm_closure_type_ptr = g.stdlib_module->getTypeByName("class.pyston::BoxedClosure")->getPointerTo();
+    assert(g.llvm_closure_type_ptr);
+
 #define GET(N) g.funcs.N = getFunc((void*)N, STRINGIFY(N))
 
     g.funcs.printf = addFunc((void*)printf, g.i8_ptr, true);
@@ -161,6 +164,7 @@ void initGlobalFuncs(GlobalState& g) {
     GET(createList);
     GET(createDict);
     GET(createSlice);
+    GET(createClosure);
 
     GET(getattr);
     GET(setattr);

src/codegen/runtime_hooks.h

@@ -32,7 +32,7 @@ struct GlobalFuncs {
 
     llvm::Value* boxInt, *unboxInt, *boxFloat, *unboxFloat, *boxStringPtr, *boxCLFunction, *unboxCLFunction,
         *boxInstanceMethod, *boxBool, *unboxBool, *createTuple, *createDict, *createList, *createSlice,
-        *createUserClass;
+        *createUserClass, *createClosure;
     llvm::Value* getattr, *setattr, *print, *nonzero, *binop, *compare, *augbinop, *unboxedLen, *getitem, *getclsattr,
         *getGlobal, *setitem, *delitem, *unaryop, *import, *repr, *isinstance;
 

src/core/types.h

@@ -165,6 +165,7 @@ struct FunctionSpecialization {
         : rtn_type(rtn_type), arg_types(arg_types) {}
 };
 
+class BoxedClosure;
 struct CompiledFunction {
 private:
 public:
@@ -176,6 +177,7 @@ public:
 
     union {
         Box* (*call)(Box*, Box*, Box*, Box**);
+        Box* (*closure_call)(BoxedClosure*, Box*, Box*, Box*, Box**);
         void* code;
     };
     llvm::Value* llvm_code; // the llvm callable.

src/gc/heap.cpp

@@ -422,7 +422,6 @@ static Block** freeChain(Block** head) {
 }
 
 void Heap::freeUnmarked() {
-    Timer _t("looking at the thread caches");
     thread_caches.forEachValue([this](ThreadBlockCache* cache) {
         for (int bidx = 0; bidx < NUM_BUCKETS; bidx++) {
             Block* h = cache->cache_free_heads[bidx];
@@ -452,7 +451,6 @@ void Heap::freeUnmarked() {
             }
         }
     });
-    _t.end();
 
     for (int bidx = 0; bidx < NUM_BUCKETS; bidx++) {
         Block** chain_end = freeChain(&heads[bidx]);

src/runtime/inline/link_forcer.cpp

@@ -57,6 +57,7 @@ void force() {
     FORCE(createList);
     FORCE(createSlice);
     FORCE(createUserClass);
+    FORCE(createClosure);
 
     FORCE(getattr);
     FORCE(setattr);

src/runtime/objmodel.cpp

@@ -795,6 +795,26 @@ Box* getattr_internal(Box* obj, const std::string& attr, bool check_cls, bool al
         }
     }
 
+
+    // TODO closures should get their own treatment, but now just piggy-back on the
+    // normal hidden-class IC logic.
+    // Can do better since we don't need to guard on the cls (always going to be closure)
+    if (obj->cls == closure_cls) {
+        BoxedClosure* closure = static_cast<BoxedClosure*>(obj);
+        if (closure->parent) {
+            if (rewrite_args) {
+                rewrite_args->obj = rewrite_args->obj.getAttr(offsetof(BoxedClosure, parent), -1);
+            }
+            if (rewrite_args2) {
+                rewrite_args2->obj
+                    = rewrite_args2->obj.getAttr(offsetof(BoxedClosure, parent), RewriterVarUsage2::Kill);
+            }
+            return getattr_internal(closure->parent, attr, false, false, rewrite_args, rewrite_args2);
+        }
+        raiseExcHelper(NameError, "free variable '%s' referenced before assignment in enclosing scope", attr.c_str());
+    }
+
+
     if (allow_custom) {
         // Don't need to pass icentry args, since we special-case __getattribtue__ and __getattr__ to use
         // invalidation rather than guards
@@ -1655,6 +1675,7 @@ Box* callFunc(BoxedFunction* func, CallRewriteArgs* rewrite_args, ArgPassSpec ar
     int num_output_args = f->numReceivedArgs();
     int num_passed_args = argspec.totalPassed();
 
+    BoxedClosure* closure = func->closure;
 
     if (argspec.has_starargs || argspec.has_kwargs || f->takes_kwargs)
         rewrite_args = NULL;
@@ -1690,14 +1711,21 @@ Box* callFunc(BoxedFunction* func, CallRewriteArgs* rewrite_args, ArgPassSpec ar
     }
 
     if (rewrite_args) {
+        int closure_indicator = closure ? 1 : 0;
+
         if (num_passed_args >= 1)
-            rewrite_args->arg1 = rewrite_args->arg1.move(0);
+            rewrite_args->arg1 = rewrite_args->arg1.move(0 + closure_indicator);
         if (num_passed_args >= 2)
-            rewrite_args->arg2 = rewrite_args->arg2.move(1);
+            rewrite_args->arg2 = rewrite_args->arg2.move(1 + closure_indicator);
         if (num_passed_args >= 3)
-            rewrite_args->arg3 = rewrite_args->arg3.move(2);
+            rewrite_args->arg3 = rewrite_args->arg3.move(2 + closure_indicator);
         if (num_passed_args >= 4)
-            rewrite_args->args = rewrite_args->args.move(3);
+            rewrite_args->args = rewrite_args->args.move(3 + closure_indicator);
+
+        // TODO this kind of embedded reference needs to be tracked by the GC somehow?
+        // Or maybe it's ok, since we've guarded on the function object?
+        if (closure)
+            rewrite_args->rewriter->loadConst(0, (intptr_t)closure);
 
         // We might have trouble if we have more output args than input args,
         // such as if we need more space to pass defaults.
@@ -1898,7 +1926,7 @@ Box* callFunc(BoxedFunction* func, CallRewriteArgs* rewrite_args, ArgPassSpec ar
 
     assert(chosen_cf->is_interpreted == (chosen_cf->code == NULL));
     if (chosen_cf->is_interpreted) {
-        return interpretFunction(chosen_cf->func, num_output_args, oarg1, oarg2, oarg3, oargs);
+        return interpretFunction(chosen_cf->func, num_output_args, func->closure, oarg1, oarg2, oarg3, oargs);
     } else {
         if (rewrite_args) {
             rewrite_args->rewriter->addDependenceOn(chosen_cf->dependent_callsites);
@@ -1908,7 +1936,11 @@ Box* callFunc(BoxedFunction* func, CallRewriteArgs* rewrite_args, ArgPassSpec ar
             rewrite_args->out_rtn = var;
             rewrite_args->out_success = true;
         }
-        return chosen_cf->call(oarg1, oarg2, oarg3, oargs);
+
+        if (closure)
+            return chosen_cf->closure_call(closure, oarg1, oarg2, oarg3, oargs);
+        else
+            return chosen_cf->call(oarg1, oarg2, oarg3, oargs);
     }
 }
 

src/runtime/objmodel.h

@@ -69,6 +69,7 @@ extern "C" void checkUnpackingLength(i64 expected, i64 given);
 extern "C" void assertNameDefined(bool b, const char* name);
 extern "C" void assertFail(BoxedModule* inModule, Box* msg);
 extern "C" bool isSubclass(BoxedClass* child, BoxedClass* parent);
+extern "C" BoxedClosure* createClosure(BoxedClosure* parent_closure);
 
 class BinopRewriteArgs;
 extern "C" Box* binopInternal(Box* lhs, Box* rhs, int op_type, bool inplace, BinopRewriteArgs* rewrite_args);

src/runtime/types.cpp

@@ -65,7 +65,7 @@ llvm::iterator_range<BoxIterator> Box::pyElements() {
 }
 
 extern "C" BoxedFunction::BoxedFunction(CLFunction* f)
-    : Box(&function_flavor, function_cls), f(f), ndefaults(0), defaults(NULL) {
+    : Box(&function_flavor, function_cls), f(f), closure(NULL), ndefaults(0), defaults(NULL) {
     if (f->source) {
         assert(f->source->ast);
         // this->giveAttr("__name__", boxString(&f->source->ast->name));
@@ -78,13 +78,15 @@ extern "C" BoxedFunction::BoxedFunction(CLFunction* f)
     assert(f->num_defaults == ndefaults);
 }
 
-extern "C" BoxedFunction::BoxedFunction(CLFunction* f, std::initializer_list<Box*> defaults)
-    : Box(&function_flavor, function_cls), f(f), ndefaults(0), defaults(NULL) {
-    // make sure to initialize defaults first, since the GC behavior is triggered by ndefaults,
-    // and a GC can happen within this constructor:
-    this->defaults = new (defaults.size()) GCdArray();
-    memcpy(this->defaults->elts, defaults.begin(), defaults.size() * sizeof(Box*));
-    this->ndefaults = defaults.size();
+extern "C" BoxedFunction::BoxedFunction(CLFunction* f, std::initializer_list<Box*> defaults, BoxedClosure* closure)
+    : Box(&function_flavor, function_cls), f(f), closure(closure), ndefaults(0), defaults(NULL) {
+    if (defaults.size()) {
+        // make sure to initialize defaults first, since the GC behavior is triggered by ndefaults,
+        // and a GC can happen within this constructor:
+        this->defaults = new (defaults.size()) GCdArray();
+        memcpy(this->defaults->elts, defaults.begin(), defaults.size() * sizeof(Box*));
+        this->ndefaults = defaults.size();
+    }
 
     if (f->source) {
         assert(f->source->ast);
@@ -104,6 +106,9 @@ extern "C" void functionGCHandler(GCVisitor* v, void* p) {
 
     BoxedFunction* f = (BoxedFunction*)p;
 
+    if (f->closure)
+        v->visit(f->closure);
+
     // It's ok for f->defaults to be NULL here even if f->ndefaults isn't,
     // since we could be collecting from inside a BoxedFunction constructor
     if (f->ndefaults) {
@@ -130,8 +135,11 @@ std::string BoxedModule::name() {
     }
 }
 
-extern "C" Box* boxCLFunction(CLFunction* f) {
-    return new BoxedFunction(f);
+extern "C" Box* boxCLFunction(CLFunction* f, BoxedClosure* closure) {
+    if (closure)
+        assert(closure->cls == closure_cls);
+
+    return new BoxedFunction(f, {}, closure);
 }
 
 extern "C" CLFunction* unboxCLFunction(Box* b) {
@@ -245,9 +253,18 @@ extern "C" void conservativeGCHandler(GCVisitor* v, void* p) {
     v->visitPotentialRange(start, start + (size / sizeof(void*)));
 }
 
+extern "C" void closureGCHandler(GCVisitor* v, void* p) {
+    boxGCHandler(v, p);
+
+    BoxedClosure* c = (BoxedClosure*)v;
+    if (c->parent)
+        v->visit(c->parent);
+}
+
 extern "C" {
 BoxedClass* object_cls, *type_cls, *none_cls, *bool_cls, *int_cls, *float_cls, *str_cls, *function_cls,
-    *instancemethod_cls, *list_cls, *slice_cls, *module_cls, *dict_cls, *tuple_cls, *file_cls, *member_cls;
+    *instancemethod_cls, *list_cls, *slice_cls, *module_cls, *dict_cls, *tuple_cls, *file_cls, *member_cls,
+    *closure_cls;
 
 const ObjectFlavor object_flavor(&boxGCHandler, NULL);
 const ObjectFlavor type_flavor(&typeGCHandler, NULL);
@@ -265,6 +282,7 @@ const ObjectFlavor dict_flavor(&dictGCHandler, NULL);
 const ObjectFlavor tuple_flavor(&tupleGCHandler, NULL);
 const ObjectFlavor file_flavor(&boxGCHandler, NULL);
 const ObjectFlavor member_flavor(&boxGCHandler, NULL);
+const ObjectFlavor closure_flavor(&closureGCHandler, NULL);
 
 const AllocationKind untracked_kind(NULL, NULL);
 const AllocationKind hc_kind(&hcGCHandler, NULL);
@@ -356,6 +374,12 @@ extern "C" Box* createSlice(Box* start, Box* stop, Box* step) {
     return rtn;
 }
 
+extern "C" BoxedClosure* createClosure(BoxedClosure* parent_closure) {
+    if (parent_closure)
+        assert(parent_closure->cls == closure_cls);
+    return new BoxedClosure(parent_closure);
+}
+
 extern "C" Box* sliceNew(Box* cls, Box* start, Box* stop, Box** args) {
     RELEASE_ASSERT(cls == slice_cls, "");
     Box* step = args[0];
@@ -483,6 +507,7 @@ void setupRuntime() {
     file_cls = new BoxedClass(object_cls, 0, sizeof(BoxedFile), false);
     set_cls = new BoxedClass(object_cls, 0, sizeof(BoxedSet), false);
     member_cls = new BoxedClass(object_cls, 0, sizeof(BoxedMemberDescriptor), false);
+    closure_cls = new BoxedClass(object_cls, offsetof(BoxedClosure, attrs), sizeof(BoxedClosure), false);
 
     STR = typeFromClass(str_cls);
     BOXED_INT = typeFromClass(int_cls);
@@ -524,6 +549,9 @@ void setupRuntime() {
     member_cls->giveAttr("__name__", boxStrConstant("member"));
     member_cls->freeze();
 
+    closure_cls->giveAttr("__name__", boxStrConstant("closure"));
+    closure_cls->freeze();
+
     setupBool();
     setupInt();
     setupFloat();

src/runtime/types.h

@@ -27,6 +27,7 @@ class BoxedList;
 class BoxedDict;
 class BoxedTuple;
 class BoxedFile;
+class BoxedClosure;
 
 void setupInt();
 void teardownInt();
@@ -62,12 +63,13 @@ BoxedList* getSysPath();
 
 extern "C" {
 extern BoxedClass* object_cls, *type_cls, *bool_cls, *int_cls, *float_cls, *str_cls, *function_cls, *none_cls,
-    *instancemethod_cls, *list_cls, *slice_cls, *module_cls, *dict_cls, *tuple_cls, *file_cls, *xrange_cls, *member_cls;
+    *instancemethod_cls, *list_cls, *slice_cls, *module_cls, *dict_cls, *tuple_cls, *file_cls, *xrange_cls, *member_cls,
+    *closure_cls;
 }
 extern "C" {
 extern const ObjectFlavor object_flavor, type_flavor, bool_flavor, int_flavor, float_flavor, str_flavor,
     function_flavor, none_flavor, instancemethod_flavor, list_flavor, slice_flavor, module_flavor, dict_flavor,
-    tuple_flavor, file_flavor, xrange_flavor, member_flavor;
+    tuple_flavor, file_flavor, xrange_flavor, member_flavor, closure_flavor;
 }
 extern "C" { extern Box* None, *NotImplemented, *True, *False; }
 extern "C" {
@@ -86,7 +88,7 @@ Box* boxString(const std::string& s);
 extern "C" BoxedString* boxStrConstant(const char* chars);
 extern "C" void listAppendInternal(Box* self, Box* v);
 extern "C" void listAppendArrayInternal(Box* self, Box** v, int nelts);
-extern "C" Box* boxCLFunction(CLFunction* f);
+extern "C" Box* boxCLFunction(CLFunction* f, BoxedClosure* closure);
 extern "C" CLFunction* unboxCLFunction(Box* b);
 extern "C" Box* createUserClass(std::string* name, Box* base, BoxedModule* parent_module);
 extern "C" double unboxFloat(Box* b);
@@ -272,12 +274,13 @@ class BoxedFunction : public Box {
 public:
     HCAttrs attrs;
     CLFunction* f;
+    BoxedClosure* closure;
 
     int ndefaults;
     GCdArray* defaults;
 
     BoxedFunction(CLFunction* f);
-    BoxedFunction(CLFunction* f, std::initializer_list<Box*> defaults);
+    BoxedFunction(CLFunction* f, std::initializer_list<Box*> defaults, BoxedClosure* closure = NULL);
 };
 
 class BoxedModule : public Box {
@@ -307,6 +310,15 @@ public:
     BoxedMemberDescriptor(MemberType type, int offset) : Box(&member_flavor, member_cls), type(type), offset(offset) {}
 };
 
+// TODO is there any particular reason to make this a Box, ie a python-level object?
+class BoxedClosure : public Box {
+public:
+    HCAttrs attrs;
+    BoxedClosure* parent;
+
+    BoxedClosure(BoxedClosure* parent) : Box(&closure_flavor, closure_cls), parent(parent) {}
+};
+
 extern "C" void boxGCHandler(GCVisitor* v, void* p);
 
 Box* exceptionNew1(BoxedClass* cls);

test/tests/closure_test.py

+# closure tests
+
+# simple closure:
+def make_adder(x):
+    def g(y):
+        return x + y
+    return g
+
+a = make_adder(1)
+print a(5)
+print map(a, range(5))
+
+def make_adder2(x2):
+    # f takes a closure since it needs to get a reference to x2 to pass to g
+    def f():
+        def g(y2):
+            return x2 + y2
+        return g
+
+    r = f()
+    print r(1)
+    x2 += 1
+    print r(1)
+    return r
+
+a = make_adder2(2)
+print a(5)
+print map(a, range(5))
+
+def make_addr3(x3):
+    # this function doesn't take a closure:
+    def f1():
+        return 2
+    f1()
+
+    def g(y3):
+        return x3 + y3
+
+    # this function does a different closure
+    def f2():
+        print f1()
+    f2()
+    return g
+print make_addr3(10)(2)
+
+def bad_addr3(_x):
+    if 0:
+        x3 = _
+
+    def g(y3):
+        return x3 + y3
+    return g
+print bad_addr3(1)(2)

test/tests/closure_varargs_test.py

+# expected: fail
+# - varargs, kwarg
+
+# Regression test: make sure that args and kw get properly treated as potentially-saved-in-closure
+
+def f1(*args):
+    def inner():
+        return args
+    return inner
+
+print f1()()
+print f1(1, 2, 3, "a")()
+
+def f2(**kw):
+    def inner():
+        return kw
+    return inner
+
+print f2()()
+print f2(a=1)()