py: Introduce a Python stack for scoped allocation.

This patch introduces the MICROPY_ENABLE_PYSTACK option (disabled by
default) which enables a "Python stack" that allows to allocate and free
memory in a scoped, or Last-In-First-Out (LIFO) way, similar to alloca().

A new memory allocation API is introduced along with this Py-stack.  It
includes both "local" and "nonlocal" LIFO allocation.  Local allocation is
intended to be equivalent to using alloca(), whereby the same function must
free the memory.  Nonlocal allocation is where another function may free
the memory, so long as it's still LIFO.

Follow-up patches will convert all uses of alloca() and VLA to the new
scoped allocation API.  The old behaviour (using alloca()) will still be
available, but when MICROPY_ENABLE_PYSTACK is enabled then alloca() is no
longer required or used.

The benefits of enabling this option are (or will be once subsequent
patches are made to convert alloca()/VLA):
- Toolchains without alloca() can use this feature to obtain correct and
  efficient scoped memory allocation (compared to using the heap instead
  of alloca(), which is slower).
- Even if alloca() is available, enabling the Py-stack gives slightly more
  efficient use of stack space when calling nested Python functions, due to
  the way that compilers implement alloca().
- Enabling the Py-stack with the stackless mode allows for even more
  efficient stack usage, as well as retaining high performance (because the
  heap is no longer used to build and destroy stackless code states).
- With Py-stack and stackless enabled, Python-calling-Python is no longer
  recursive in the C mp_execute_bytecode function.

The micropython.pystack_use() function is included to measure usage of the
Python stack.

py/gc.c

@@ -320,11 +320,18 @@ void gc_collect_start(void) {
     #endif
     MP_STATE_MEM(gc_stack_overflow) = 0;
     MP_STATE_MEM(gc_sp) = MP_STATE_MEM(gc_stack);
+
     // Trace root pointers.  This relies on the root pointers being organised
     // correctly in the mp_state_ctx structure.  We scan nlr_top, dict_locals,
     // dict_globals, then the root pointer section of mp_state_vm.
     void **ptrs = (void**)(void*)&mp_state_ctx;
     gc_collect_root(ptrs, offsetof(mp_state_ctx_t, vm.qstr_last_chunk) / sizeof(void*));
+
+    #if MICROPY_ENABLE_PYSTACK
+    // Trace root pointers from the Python stack.
+    ptrs = (void**)(void*)MP_STATE_THREAD(pystack_start);
+    gc_collect_root(ptrs, (MP_STATE_THREAD(pystack_cur) - MP_STATE_THREAD(pystack_start)) / sizeof(void*));
+    #endif
 }
 
 void gc_collect_root(void **ptrs, size_t len) {

py/modmicropython.c

@@ -112,6 +112,13 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_0(mp_micropython_stack_use_obj, mp_micropython_st
 
 #endif // MICROPY_PY_MICROPYTHON_MEM_INFO
 
+#if MICROPY_ENABLE_PYSTACK
+STATIC mp_obj_t mp_micropython_pystack_use(void) {
+    return MP_OBJ_NEW_SMALL_INT(mp_pystack_usage());
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_0(mp_micropython_pystack_use_obj, mp_micropython_pystack_use);
+#endif
+
 #if MICROPY_ENABLE_GC
 STATIC mp_obj_t mp_micropython_heap_lock(void) {
     gc_lock();
@@ -167,6 +174,9 @@ STATIC const mp_rom_map_elem_t mp_module_micropython_globals_table[] = {
 #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF && (MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE == 0)
     { MP_ROM_QSTR(MP_QSTR_alloc_emergency_exception_buf), MP_ROM_PTR(&mp_alloc_emergency_exception_buf_obj) },
 #endif
+    #if MICROPY_ENABLE_PYSTACK
+    { MP_ROM_QSTR(MP_QSTR_pystack_use), MP_ROM_PTR(&mp_micropython_pystack_use_obj) },
+    #endif
     #if MICROPY_ENABLE_GC
     { MP_ROM_QSTR(MP_QSTR_heap_lock), MP_ROM_PTR(&mp_micropython_heap_lock_obj) },
     { MP_ROM_QSTR(MP_QSTR_heap_unlock), MP_ROM_PTR(&mp_micropython_heap_unlock_obj) },

py/modthread.c

@@ -165,6 +165,12 @@ STATIC void *thread_entry(void *args_in) {
     mp_stack_set_top(&ts + 1); // need to include ts in root-pointer scan
     mp_stack_set_limit(args->stack_size);
 
+    #if MICROPY_ENABLE_PYSTACK
+    // TODO threading and pystack is not fully supported, for now just make a small stack
+    mp_obj_t mini_pystack[128];
+    mp_pystack_init(mini_pystack, &mini_pystack[128]);
+    #endif
+
     // set locals and globals from the calling context
     mp_locals_set(args->dict_locals);
     mp_globals_set(args->dict_globals);

py/mpconfig.h

@@ -441,6 +441,17 @@
 #define MICROPY_ENABLE_FINALISER (0)
 #endif
 
+// Whether to enable a separate allocator for the Python stack.
+// If enabled then the code must call mp_pystack_init before mp_init.
+#ifndef MICROPY_ENABLE_PYSTACK
+#define MICROPY_ENABLE_PYSTACK (0)
+#endif
+
+// Number of bytes that memory returned by mp_pystack_alloc will be aligned by.
+#ifndef MICROPY_PYSTACK_ALIGN
+#define MICROPY_PYSTACK_ALIGN (8)
+#endif
+
 // Whether to check C stack usage. C stack used for calling Python functions,
 // etc. Not checking means segfault on overflow.
 #ifndef MICROPY_STACK_CHECK

py/mpstate.h

@@ -224,6 +224,12 @@ typedef struct _mp_state_thread_t {
     #if MICROPY_STACK_CHECK
     size_t stack_limit;
     #endif
+
+    #if MICROPY_ENABLE_PYSTACK
+    uint8_t *pystack_start;
+    uint8_t *pystack_end;
+    uint8_t *pystack_cur;
+    #endif
 } mp_state_thread_t;
 
 // This structure combines the above 3 structures.

py/nlr.h

@@ -62,15 +62,32 @@ struct _nlr_buf_t {
 #if MICROPY_NLR_SETJMP
     jmp_buf jmpbuf;
 #endif
+
+    #if MICROPY_ENABLE_PYSTACK
+    void *pystack;
+    #endif
 };
 
+// Helper macros to save/restore the pystack state
+#if MICROPY_ENABLE_PYSTACK
+#define MP_NLR_SAVE_PYSTACK(nlr_buf) (nlr_buf)->pystack = MP_STATE_THREAD(pystack_cur)
+#define MP_NLR_RESTORE_PYSTACK(nlr_buf) MP_STATE_THREAD(pystack_cur) = (nlr_buf)->pystack
+#else
+#define MP_NLR_SAVE_PYSTACK(nlr_buf) (void)nlr_buf
+#define MP_NLR_RESTORE_PYSTACK(nlr_buf) (void)nlr_buf
+#endif
+
 #if MICROPY_NLR_SETJMP
 #include "py/mpstate.h"
 
 NORETURN void nlr_setjmp_jump(void *val);
 // nlr_push() must be defined as a macro, because "The stack context will be
 // invalidated if the function which called setjmp() returns."
-#define nlr_push(buf) ((buf)->prev = MP_STATE_THREAD(nlr_top), MP_STATE_THREAD(nlr_top) = (buf), setjmp((buf)->jmpbuf))
+#define nlr_push(buf) ( \
+    (buf)->prev = MP_STATE_THREAD(nlr_top), \
+    MP_NLR_SAVE_PYSTACK(buf), \
+    MP_STATE_THREAD(nlr_top) = (buf), \
+    setjmp((buf)->jmpbuf))
 #define nlr_pop() { MP_STATE_THREAD(nlr_top) = MP_STATE_THREAD(nlr_top)->prev; }
 #define nlr_jump(val) nlr_setjmp_jump(val)
 #else

py/nlrsetjmp.c

@@ -35,6 +35,7 @@ void nlr_setjmp_jump(void *val) {
         nlr_jump_fail(val);
     }
     top->ret_val = val;
+    MP_NLR_RESTORE_PYSTACK(top);
     *top_ptr = top->prev;
     longjmp(top->jmpbuf, 1);
 }

py/nlrthumb.c

@@ -82,6 +82,7 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
 __attribute__((used)) unsigned int nlr_push_tail(nlr_buf_t *nlr) {
     nlr_buf_t **top = &MP_STATE_THREAD(nlr_top);
     nlr->prev = *top;
+    MP_NLR_SAVE_PYSTACK(nlr);
     *top = nlr;
     return 0; // normal return
 }
@@ -99,6 +100,7 @@ NORETURN __attribute__((naked)) void nlr_jump(void *val) {
     }
 
     top->ret_val = val;
+    MP_NLR_RESTORE_PYSTACK(top);
     *top_ptr = top->prev;
 
     __asm volatile (

py/nlrx64.c

@@ -91,6 +91,7 @@ unsigned int nlr_push(nlr_buf_t *nlr) {
 __attribute__((used)) unsigned int nlr_push_tail(nlr_buf_t *nlr) {
     nlr_buf_t **top = &MP_STATE_THREAD(nlr_top);
     nlr->prev = *top;
+    MP_NLR_SAVE_PYSTACK(nlr);
     *top = nlr;
     return 0; // normal return
 }
@@ -108,6 +109,7 @@ NORETURN void nlr_jump(void *val) {
     }
 
     top->ret_val = val;
+    MP_NLR_RESTORE_PYSTACK(top);
     *top_ptr = top->prev;
 
     __asm volatile (

py/nlrx86.c

@@ -73,6 +73,7 @@ unsigned int nlr_push(nlr_buf_t *nlr) {
 __attribute__((used)) unsigned int nlr_push_tail(nlr_buf_t *nlr) {
     nlr_buf_t **top = &MP_STATE_THREAD(nlr_top);
     nlr->prev = *top;
+    MP_NLR_SAVE_PYSTACK(nlr);
     *top = nlr;
     return 0; // normal return
 }
@@ -90,6 +91,7 @@ NORETURN void nlr_jump(void *val) {
     }
 
     top->ret_val = val;
+    MP_NLR_RESTORE_PYSTACK(top);
     *top_ptr = top->prev;
 
     __asm volatile (

py/nlrxtensa.c

@@ -58,6 +58,7 @@ unsigned int nlr_push(nlr_buf_t *nlr) {
 __attribute__((used)) unsigned int nlr_push_tail(nlr_buf_t *nlr) {
     nlr_buf_t **top = &MP_STATE_THREAD(nlr_top);
     nlr->prev = *top;
+    MP_NLR_SAVE_PYSTACK(nlr);
     *top = nlr;
     return 0; // normal return
 }
@@ -75,6 +76,7 @@ NORETURN void nlr_jump(void *val) {
     }
 
     top->ret_val = val;
+    MP_NLR_RESTORE_PYSTACK(top);
     *top_ptr = top->prev;
 
     __asm volatile (

py/py.mk

@@ -110,6 +110,7 @@ PY_O_BASENAME = \
 	nlrsetjmp.o \
 	malloc.o \
 	gc.o \
+	pystack.o \
 	qstr.o \
 	vstr.o \
 	mpprint.o \

py/pystack.c

@@ -0,0 +1,56 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2017 Damien P. George
+ *
+ * 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 <stdio.h>
+
+#include "py/runtime.h"
+
+#if MICROPY_ENABLE_PYSTACK
+
+void mp_pystack_init(void *start, void *end) {
+    MP_STATE_THREAD(pystack_start) = start;
+    MP_STATE_THREAD(pystack_end) = end;
+    MP_STATE_THREAD(pystack_cur) = start;
+}
+
+void *mp_pystack_alloc(size_t n_bytes) {
+    n_bytes = (n_bytes + (MICROPY_PYSTACK_ALIGN - 1)) & ~(MICROPY_PYSTACK_ALIGN - 1);
+    #if MP_PYSTACK_DEBUG
+    n_bytes += MICROPY_PYSTACK_ALIGN;
+    #endif
+    if (MP_STATE_THREAD(pystack_cur) + n_bytes > MP_STATE_THREAD(pystack_end)) {
+        // out of memory in the pystack
+        mp_raise_recursion_depth();
+    }
+    void *ptr = MP_STATE_THREAD(pystack_cur);
+    MP_STATE_THREAD(pystack_cur) += n_bytes;
+    #if MP_PYSTACK_DEBUG
+    *(size_t*)(MP_STATE_THREAD(pystack_cur) - MICROPY_PYSTACK_ALIGN) = n_bytes;
+    #endif
+    return ptr;
+}
+
+#endif

py/pystack.h

@@ -0,0 +1,123 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2017 Damien P. George
+ *
+ * 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.
+ */
+#ifndef MICROPY_INCLUDED_PY_PYSTACK_H
+#define MICROPY_INCLUDED_PY_PYSTACK_H
+
+#include "py/mpstate.h"
+
+// Enable this debugging option to check that the amount of memory freed is
+// consistent with amounts that were previously allocated.
+#define MP_PYSTACK_DEBUG (0)
+
+#if MICROPY_ENABLE_PYSTACK
+
+void mp_pystack_init(void *start, void *end);
+void *mp_pystack_alloc(size_t n_bytes);
+
+// This function can free multiple continuous blocks at once: just pass the
+// pointer to the block that was allocated first and it and all subsequently
+// allocated blocks will be freed.
+static inline void mp_pystack_free(void *ptr) {
+    assert((uint8_t*)ptr >= MP_STATE_THREAD(pystack_start));
+    assert((uint8_t*)ptr <= MP_STATE_THREAD(pystack_cur));
+    #if MP_PYSTACK_DEBUG
+    size_t n_bytes_to_free = MP_STATE_THREAD(pystack_cur) - (uint8_t*)ptr;
+    size_t n_bytes = *(size_t*)(MP_STATE_THREAD(pystack_cur) - MICROPY_PYSTACK_ALIGN);
+    while (n_bytes < n_bytes_to_free) {
+        n_bytes += *(size_t*)(MP_STATE_THREAD(pystack_cur) - n_bytes - MICROPY_PYSTACK_ALIGN);
+    }
+    if (n_bytes != n_bytes_to_free) {
+        mp_printf(&mp_plat_print, "mp_pystack_free() failed: %u != %u\n", (uint)n_bytes_to_free,
+            (uint)*(size_t*)(MP_STATE_THREAD(pystack_cur) - MICROPY_PYSTACK_ALIGN));
+        assert(0);
+    }
+    #endif
+    MP_STATE_THREAD(pystack_cur) = (uint8_t*)ptr;
+}
+
+static inline void mp_pystack_realloc(void *ptr, size_t n_bytes) {
+    mp_pystack_free(ptr);
+    mp_pystack_alloc(n_bytes);
+}
+
+static inline size_t mp_pystack_usage(void) {
+    return MP_STATE_THREAD(pystack_cur) - MP_STATE_THREAD(pystack_start);
+}
+
+static inline size_t mp_pystack_limit(void) {
+    return MP_STATE_THREAD(pystack_end) - MP_STATE_THREAD(pystack_start);
+}
+
+#endif
+
+#if !MICROPY_ENABLE_PYSTACK
+
+#define mp_local_alloc(n_bytes) alloca(n_bytes)
+
+static inline void mp_local_free(void *ptr) {
+    (void)ptr;
+}
+
+static inline void *mp_nonlocal_alloc(size_t n_bytes) {
+    return m_new(uint8_t, n_bytes);
+}
+
+static inline void *mp_nonlocal_realloc(void *ptr, size_t old_n_bytes, size_t new_n_bytes) {
+    return m_renew(uint8_t, ptr, old_n_bytes, new_n_bytes);
+}
+
+static inline void mp_nonlocal_free(void *ptr, size_t n_bytes) {
+    m_del(uint8_t, ptr, n_bytes);
+}
+
+#else
+
+static inline void *mp_local_alloc(size_t n_bytes) {
+    return mp_pystack_alloc(n_bytes);
+}
+
+static inline void mp_local_free(void *ptr) {
+    mp_pystack_free(ptr);
+}
+
+static inline void *mp_nonlocal_alloc(size_t n_bytes) {
+    return mp_pystack_alloc(n_bytes);
+}
+
+static inline void *mp_nonlocal_realloc(void *ptr, size_t old_n_bytes, size_t new_n_bytes) {
+    (void)old_n_bytes;
+    mp_pystack_realloc(ptr, new_n_bytes);
+    return ptr;
+}
+
+static inline void mp_nonlocal_free(void *ptr, size_t n_bytes) {
+    (void)n_bytes;
+    mp_pystack_free(ptr);
+}
+
+#endif
+
+#endif // MICROPY_INCLUDED_PY_PYSTACK_H

py/runtime.c

@@ -1457,7 +1457,7 @@ NORETURN void mp_raise_NotImplementedError(const char *msg) {
     mp_raise_msg(&mp_type_NotImplementedError, msg);
 }
 
-#if MICROPY_STACK_CHECK
+#if MICROPY_STACK_CHECK || MICROPY_ENABLE_PYSTACK
 NORETURN void mp_raise_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)));

py/runtime.h

@@ -27,6 +27,7 @@
 #define MICROPY_INCLUDED_PY_RUNTIME_H
 
 #include "py/mpstate.h"
+#include "py/pystack.h"
 
 typedef enum {
     MP_VM_RETURN_NORMAL,