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.

1 files changed, 11 insertions, 0 deletions

diff --git a/py/mpconfig.h b/py/mpconfig.h
index 7784367085..96cd8c651a 100644
--- a/py/mpconfig.h
+++ b/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