[python] Fixed MicroPython emscripten bug.

Micropython did not work because of two problems:
 - the stack in ascending order on emscripten
 - if optimized, some variables may be stored in local JavaScript
variables, which breaks garbage collection.

Change-Id: Ib454e0c4d995e9f5f85370eea758526119b35773

python/Makefile

@@ -67,7 +67,6 @@ py_objs = $(addprefix python/src/py/,\
   runtime_utils.o \
   scheduler.o \
   nativeglue.o \
-  stackctrl.o \
   argcheck.o \
   warning.o \
   map.o \
@@ -179,8 +178,21 @@ port_objs += $(addprefix python/port/,\
   modkandinsky.o \
   modkandinsky_impl.o \
   mphalport.o \
+  stackctrl.o \
 )
 
+# Reduce optimization for the emscripten platform.
+# With optimization, register and stack variables might be held in a JavaScript
+# local variable, which breaks garbage collection. Indeed, these JavaScript
+# variables cannot be marked as root during garbage collection, which means that
+# the heap objects they depend on will likely be destroyed. When the Python
+# computing resumes, if necessary heap objects have been destroyed, the Python
+# program crashes.
+
+ifeq ($(PLATFORM),emscripten)
+$(py_objs): SFLAGS := $(subst -Os,-O0,$(SFLAGS))
+endif
+
 # QSTR generation
 
 python/port/genhdr/qstrdefs.generated.h: python/port/genhdr/qstrdefs.in.h

python/port/port.cpp

@@ -137,7 +137,9 @@ void MicroPython::registerScriptProvider(ScriptProvider * s) {
 }
 
 void gc_collect(void) {
-  assert(MP_STATE_THREAD(stack_top) != NULL);
+  void * python_stack_top = MP_STATE_THREAD(stack_top);
+  assert(python_stack_top != NULL);
+
   gc_collect_start();
 
   /* get the registers.
@@ -146,18 +148,19 @@ void gc_collect(void) {
   jmp_buf regs;
   setjmp(regs);
 
-  void **regs_ptr = (void**)(void*)&regs;
+  void **regs_ptr = (void**)&regs;
 
   /* On the device, the stack is stored in reverse order, but it might not be
    * the case on a computer. We thus have to take the absolute value of the
    * addresses difference. */
   size_t stackLength;
-  if ((uintptr_t)MP_STATE_THREAD(stack_top) > (uintptr_t)(&regs)) {
-    stackLength = (((uintptr_t)(MP_STATE_THREAD(stack_top)) - (uintptr_t)(&regs)) / sizeof(uintptr_t));
+  if ((uintptr_t)python_stack_top > (uintptr_t)&regs) {
+    stackLength = ((uintptr_t)python_stack_top - (uintptr_t)&regs) / sizeof(uintptr_t);
+    gc_collect_root(regs_ptr, stackLength);
   } else {
-    stackLength = (((uintptr_t)(&regs) - (uintptr_t)(MP_STATE_THREAD(stack_top))) / sizeof(uintptr_t));
+    stackLength = ((uintptr_t)(&regs) - (uintptr_t)python_stack_top) / sizeof(uintptr_t);
+    gc_collect_root((void **)python_stack_top, stackLength);
   }
-  gc_collect_root(regs_ptr, stackLength);
 
   gc_collect_end();
 }

python/port/stackctrl.c

@@ -0,0 +1,70 @@
+/*
+ * This file is part of the Micro Python project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Paul Sokolovsky
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "py/mpstate.h"
+#include "py/nlr.h"
+#include "py/obj.h"
+#include "py/runtime.h"
+#include "py/stackctrl.h"
+
+void mp_stack_ctrl_init(void) {
+    volatile int stack_dummy;
+    MP_STATE_THREAD(stack_top) = (char*)&stack_dummy;
+}
+
+void mp_stack_set_top(void *top) {
+    MP_STATE_THREAD(stack_top) = top;
+}
+
+mp_uint_t mp_stack_usage(void) {
+    /* Micropython's stackctrl.c assumes that the stack is in descending order,
+     * which is not always the case for us. */
+    volatile int stack_dummy;
+    if (MP_STATE_THREAD(stack_top) > (char*)&stack_dummy) {
+        return MP_STATE_THREAD(stack_top) - (char*)&stack_dummy;
+    } else {
+        return (char*)&stack_dummy - MP_STATE_THREAD(stack_top);
+    }
+}
+
+#if MICROPY_STACK_CHECK
+
+void mp_stack_set_limit(mp_uint_t limit) {
+    MP_STATE_THREAD(stack_limit) = limit;
+}
+
+void mp_exc_recursion_depth(void) {
+    nlr_raise(mp_obj_new_exception_arg1(&mp_type_RuntimeError,
+        MP_OBJ_NEW_QSTR(MP_QSTR_maximum_space_recursion_space_depth_space_exceeded)));
+}
+
+void mp_stack_check(void) {
+    if (mp_stack_usage() >= MP_STATE_THREAD(stack_limit)) {
+        mp_exc_recursion_depth();
+    }
+}
+
+#endif // MICROPY_STACK_CHECK