py: Fix native functions so they run with their correct globals context.

Prior to this commit a function compiled with the native decorator
@micropython.native would not work correctly when accessing global
variables, because the globals dict was not being set upon function entry.

This commit fixes this problem by, upon function entry, setting as the
current globals dict the globals dict context the function was defined
within, as per normal Python semantics, and as bytecode does.  Upon
function exit the original globals dict is restored.

In order to restore the globals dict when an exception is raised the native
function must guard its internals with an nlr_push/nlr_pop pair.  Because
this push/pop is relatively expensive, in both C stack usage for the
nlr_buf_t and CPU execution time, the implementation here optimises things
as much as possible.  First, the compiler keeps track of whether a function
even needs to access global variables.  Using this information the native
emitter then generates three different kinds of code:

1. no globals used, no exception handlers: no nlr handling code and no
   setting of the globals dict.

2. globals used, no exception handlers: an nlr_buf_t is allocated on the
   C stack but it is not used if the globals dict is unchanged, saving
   execution time because nlr_push/nlr_pop don't need to run.

3. function has exception handlers, may use globals: an nlr_buf_t is
   allocated and nlr_push/nlr_pop are always called.

In the end, native functions that don't access globals and don't have
exception handlers will run more efficiently than those that do.

Fixes issue #1573.

7 files changed, 96 insertions, 22 deletions

diff --git a/py/compile.c b/py/compile.c
index 58cf7de1c2..d8e175bb6e 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -567,6 +567,11 @@ STATIC void close_over_variables_etc(compiler_t *comp, scope_t *this_scope, int
     }
     this_scope->num_def_pos_args = n_pos_defaults;
 
+    #if MICROPY_EMIT_NATIVE
+    // When creating a function/closure it will take a reference to the current globals
+    comp->scope_cur->scope_flags |= MP_SCOPE_FLAG_REFGLOBALS;
+    #endif
+
     // make closed over variables, if any
     // ensure they are closed over in the order defined in the outer scope (mainly to agree with CPython)
     int nfree = 0;
@@ -3304,6 +3309,12 @@ STATIC void scope_compute_things(scope_t *scope) {
         if (SCOPE_IS_FUNC_LIKE(scope->kind) && id->kind == ID_INFO_KIND_GLOBAL_IMPLICIT) {
             id->kind = ID_INFO_KIND_GLOBAL_EXPLICIT;
         }
+        #if MICROPY_EMIT_NATIVE
+        if (id->kind == ID_INFO_KIND_GLOBAL_EXPLICIT) {
+            // This function makes a reference to a global variable
+            scope->scope_flags |= MP_SCOPE_FLAG_REFGLOBALS;
+        }
+        #endif
         // params always count for 1 local, even if they are a cell
         if (id->kind == ID_INFO_KIND_LOCAL || (id->flags & ID_FLAG_IS_PARAM)) {
             id->local_num = scope->num_locals++;
diff --git a/py/emitnative.c b/py/emitnative.c
index 73899b9e90..eb402c06b0 100644
--- a/py/emitnative.c
+++ b/py/emitnative.c
@@ -75,7 +75,8 @@
 #define NLR_BUF_IDX_RET_VAL (1)
 
 // Whether the native/viper function needs to be wrapped in an exception handler
-#define NEED_GLOBAL_EXC_HANDLER(emit) ((emit)->scope->exc_stack_size > 0)
+#define NEED_GLOBAL_EXC_HANDLER(emit) ((emit)->scope->exc_stack_size > 0 \
+    || (!(emit)->do_viper_types && ((emit)->scope->scope_flags & MP_SCOPE_FLAG_REFGLOBALS)))
 
 // Whether registers can be used to store locals (only true if there are no
 // exception handlers, because otherwise an nlr_jump will restore registers to
@@ -928,30 +929,56 @@ STATIC void emit_native_global_exc_entry(emit_t *emit) {
         mp_uint_t start_label = *emit->label_slot + 2;
         mp_uint_t global_except_label = *emit->label_slot + 3;
 
-        // Clear the unwind state
-        ASM_XOR_REG_REG(emit->as, REG_TEMP0, REG_TEMP0);
-        ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_UNWIND(emit), REG_TEMP0);
+        if (!emit->do_viper_types) {
+            // Set new globals
+            ASM_MOV_REG_LOCAL(emit->as, REG_ARG_1, offsetof(mp_code_state_t, fun_bc) / sizeof(uintptr_t));
+            ASM_LOAD_REG_REG_OFFSET(emit->as, REG_ARG_1, REG_ARG_1, offsetof(mp_obj_fun_bc_t, globals) / sizeof(uintptr_t));
+            emit_call(emit, MP_F_NATIVE_SWAP_GLOBALS);
 
-        // Put PC of start code block into REG_LOCAL_1
-        ASM_MOV_REG_PCREL(emit->as, REG_LOCAL_1, start_label);
+            // Save old globals (or NULL if globals didn't change)
+            ASM_MOV_LOCAL_REG(emit->as, offsetof(mp_code_state_t, old_globals) / sizeof(uintptr_t), REG_RET);
+        }
 
-        // Wrap everything in an nlr context
-        emit_native_label_assign(emit, nlr_label);
-        ASM_MOV_REG_LOCAL(emit->as, REG_LOCAL_2, LOCAL_IDX_EXC_HANDLER_UNWIND(emit));
-        emit_get_stack_pointer_to_reg_for_push(emit, REG_ARG_1, sizeof(nlr_buf_t) / sizeof(uintptr_t));
-        emit_call(emit, MP_F_NLR_PUSH);
-        ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_UNWIND(emit), REG_LOCAL_2);
-        ASM_JUMP_IF_REG_NONZERO(emit->as, REG_RET, global_except_label, true);
+        if (emit->scope->exc_stack_size == 0) {
+            // Optimisation: if globals didn't change don't push the nlr context
+            ASM_JUMP_IF_REG_ZERO(emit->as, REG_RET, start_label, false);
 
-        // Clear PC of current code block, and jump there to resume execution
-        ASM_XOR_REG_REG(emit->as, REG_TEMP0, REG_TEMP0);
-        ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_PC(emit), REG_TEMP0);
-        ASM_JUMP_REG(emit->as, REG_LOCAL_1);
+            // Wrap everything in an nlr context
+            emit_get_stack_pointer_to_reg_for_push(emit, REG_ARG_1, sizeof(nlr_buf_t) / sizeof(uintptr_t));
+            emit_call(emit, MP_F_NLR_PUSH);
+            ASM_JUMP_IF_REG_ZERO(emit->as, REG_RET, start_label, true);
+        } else {
+            // Clear the unwind state
+            ASM_XOR_REG_REG(emit->as, REG_TEMP0, REG_TEMP0);
+            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_UNWIND(emit), REG_TEMP0);
+
+            // Put PC of start code block into REG_LOCAL_1
+            ASM_MOV_REG_PCREL(emit->as, REG_LOCAL_1, start_label);
+
+            // Wrap everything in an nlr context
+            emit_native_label_assign(emit, nlr_label);
+            ASM_MOV_REG_LOCAL(emit->as, REG_LOCAL_2, LOCAL_IDX_EXC_HANDLER_UNWIND(emit));
+            emit_get_stack_pointer_to_reg_for_push(emit, REG_ARG_1, sizeof(nlr_buf_t) / sizeof(uintptr_t));
+            emit_call(emit, MP_F_NLR_PUSH);
+            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_UNWIND(emit), REG_LOCAL_2);
+            ASM_JUMP_IF_REG_NONZERO(emit->as, REG_RET, global_except_label, true);
+
+            // Clear PC of current code block, and jump there to resume execution
+            ASM_XOR_REG_REG(emit->as, REG_TEMP0, REG_TEMP0);
+            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_HANDLER_PC(emit), REG_TEMP0);
+            ASM_JUMP_REG(emit->as, REG_LOCAL_1);
+
+            // Global exception handler: check for valid exception handler
+            emit_native_label_assign(emit, global_except_label);
+            ASM_MOV_REG_LOCAL(emit->as, REG_LOCAL_1, LOCAL_IDX_EXC_HANDLER_PC(emit));
+            ASM_JUMP_IF_REG_NONZERO(emit->as, REG_LOCAL_1, nlr_label, false);
+        }
 
-        // Global exception handler: check for valid exception handler
-        emit_native_label_assign(emit, global_except_label);
-        ASM_MOV_REG_LOCAL(emit->as, REG_LOCAL_1, LOCAL_IDX_EXC_HANDLER_PC(emit));
-        ASM_JUMP_IF_REG_NONZERO(emit->as, REG_LOCAL_1, nlr_label, false);
+        if (!emit->do_viper_types) {
+            // Restore old globals
+            ASM_MOV_REG_LOCAL(emit->as, REG_ARG_1, offsetof(mp_code_state_t, old_globals) / sizeof(uintptr_t));
+            emit_call(emit, MP_F_NATIVE_SWAP_GLOBALS);
+        }
 
         // Re-raise exception out to caller
         ASM_MOV_REG_LOCAL(emit->as, REG_ARG_1, LOCAL_IDX_EXC_VAL(emit));
@@ -967,10 +994,28 @@ STATIC void emit_native_global_exc_exit(emit_t *emit) {
     emit_native_label_assign(emit, emit->exit_label);
 
     if (NEED_GLOBAL_EXC_HANDLER(emit)) {
+        if (!emit->do_viper_types) {
+            // Get old globals
+            ASM_MOV_REG_LOCAL(emit->as, REG_ARG_1, offsetof(mp_code_state_t, old_globals) / sizeof(uintptr_t));
+
+            if (emit->scope->exc_stack_size == 0) {
+                // Optimisation: if globals didn't change then don't restore them and don't do nlr_pop
+                ASM_JUMP_IF_REG_ZERO(emit->as, REG_ARG_1, emit->exit_label + 1, false);
+            }
+
+            // Restore old globals
+            emit_call(emit, MP_F_NATIVE_SWAP_GLOBALS);
+        }
+
         // Pop the nlr context
         emit_call(emit, MP_F_NLR_POP);
         adjust_stack(emit, -(mp_int_t)(sizeof(nlr_buf_t) / sizeof(uintptr_t)));
 
+        if (emit->scope->exc_stack_size == 0) {
+            // Destination label for above optimisation
+            emit_native_label_assign(emit, emit->exit_label + 1);
+        }
+
         // Load return value
         ASM_MOV_REG_LOCAL(emit->as, REG_RET, LOCAL_IDX_RET_VAL(emit));
     }
diff --git a/py/emitnx86.c b/py/emitnx86.c
index 056c3f052d..a536b9851e 100644
--- a/py/emitnx86.c
+++ b/py/emitnx86.c
@@ -18,6 +18,7 @@
 STATIC byte mp_f_n_args[MP_F_NUMBER_OF] = {
     [MP_F_CONVERT_OBJ_TO_NATIVE] = 2,
     [MP_F_CONVERT_NATIVE_TO_OBJ] = 2,
+    [MP_F_NATIVE_SWAP_GLOBALS] = 1,
     [MP_F_LOAD_NAME] = 1,
     [MP_F_LOAD_GLOBAL] = 1,
     [MP_F_LOAD_BUILD_CLASS] = 0,
diff --git a/py/nativeglue.c b/py/nativeglue.c
index b87da6931e..7ff8273f9c 100644
--- a/py/nativeglue.c
+++ b/py/nativeglue.c
@@ -83,6 +83,20 @@ mp_obj_t mp_convert_native_to_obj(mp_uint_t val, mp_uint_t type) {
 
 #if MICROPY_EMIT_NATIVE
 
+mp_obj_dict_t *mp_native_swap_globals(mp_obj_dict_t *new_globals) {
+    if (new_globals == NULL) {
+        // Globals were the originally the same so don't restore them
+        return NULL;
+    }
+    mp_obj_dict_t *old_globals = mp_globals_get();
+    if (old_globals == new_globals) {
+        // Don't set globals if they are the same, and return NULL to indicate this
+        return NULL;
+    }
+    mp_globals_set(new_globals);
+    return old_globals;
+}
+
 // wrapper that accepts n_args and n_kw in one argument
 // (native emitter can only pass at most 3 arguments to a function)
 mp_obj_t mp_native_call_function_n_kw(mp_obj_t fun_in, size_t n_args_kw, const mp_obj_t *args) {
@@ -127,6 +141,7 @@ STATIC mp_obj_t mp_native_iternext(mp_obj_iter_buf_t *iter) {
 void *const mp_fun_table[MP_F_NUMBER_OF] = {
     mp_convert_obj_to_native,
     mp_convert_native_to_obj,
+    mp_native_swap_globals,
     mp_load_name,
     mp_load_global,
     mp_load_build_class,
diff --git a/py/runtime.h b/py/runtime.h
index ad65f3f46d..99a2204aaf 100644
--- a/py/runtime.h
+++ b/py/runtime.h
@@ -168,6 +168,7 @@ NORETURN void mp_raise_recursion_depth(void);
 // helper functions for native/viper code
 mp_uint_t mp_convert_obj_to_native(mp_obj_t obj, mp_uint_t type);
 mp_obj_t mp_convert_native_to_obj(mp_uint_t val, mp_uint_t type);
+mp_obj_dict_t *mp_native_swap_globals(mp_obj_dict_t *new_globals);
 mp_obj_t mp_native_call_function_n_kw(mp_obj_t fun_in, size_t n_args_kw, const mp_obj_t *args);
 void mp_native_raise(mp_obj_t o);
 
diff --git a/py/runtime0.h b/py/runtime0.h
index 2e89de9f41..b47a10ea22 100644
--- a/py/runtime0.h
+++ b/py/runtime0.h
@@ -31,6 +31,7 @@
 #define MP_SCOPE_FLAG_VARKEYWORDS  (0x02)
 #define MP_SCOPE_FLAG_GENERATOR    (0x04)
 #define MP_SCOPE_FLAG_DEFKWARGS    (0x08)
+#define MP_SCOPE_FLAG_REFGLOBALS   (0x10) // used only if native emitter enabled
 
 // types for native (viper) function signature
 #define MP_NATIVE_TYPE_OBJ  (0x00)
@@ -145,6 +146,7 @@ typedef enum {
 typedef enum {
     MP_F_CONVERT_OBJ_TO_NATIVE = 0,
     MP_F_CONVERT_NATIVE_TO_OBJ,
+    MP_F_NATIVE_SWAP_GLOBALS,
     MP_F_LOAD_NAME,
     MP_F_LOAD_GLOBAL,
     MP_F_LOAD_BUILD_CLASS,
diff --git a/tests/run-tests b/tests/run-tests
index 07d3268119..8c087f9f58 100755
--- a/tests/run-tests
+++ b/tests/run-tests
@@ -376,7 +376,6 @@ def run_tests(pyb, tests, args, base_path="."):
         skip_tests.add('micropython/schedule.py') # native code doesn't check pending events
         skip_tests.add('stress/gc_trace.py') # requires yield
         skip_tests.add('stress/recursive_gen.py') # requires yield
-        skip_tests.add('extmod/vfs_userfs.py') # because native doesn't properly handle globals across different modules
 
     for test_file in tests:
         test_file = test_file.replace('\\', '/')