Introduce a long lived section of the heap.

This adapts the allocation process to start from either end of the heap
when searching for free space. The default behavior is identical to the
existing behavior where it starts with the lowest block and looks higher.
Now it can also look from the highest block and lower depending on the
long_lived parameter to gc_alloc. As the heap fills, the two sections may
overlap. When they overlap, a collect may be triggered in order to keep
the long lived section compact. However, free space is always eligable
for each type of allocation.

By starting from either of the end of the heap we have ability to separate
short lived objects from long lived ones. This separation reduces heap
fragmentation because long lived objects are easy to densely pack.

Most objects are short lived initially but may be made long lived when
they are referenced by a type or module. This involves copying the
memory and then letting the collect phase free the old portion.

QSTR pools and chunks are always long lived because they are never freed.

The reallocation, collection and free processes are largely unchanged. They
simply also maintain an index to the highest free block as well as the lowest.
These indices are used to speed up the allocation search until the next collect.

In practice, this change may slightly slow down import statements with the
benefit that memory is much less fragmented afterwards. For example, a test
import into a 20k heap that leaves ~6k free previously had the largest
continuous free space of ~400 bytes. After this change, the largest continuous
free space is over 3400 bytes.

1 files changed, 5 insertions, 4 deletions

diff --git a/py/qstr.c b/py/qstr.c
index 95c9b6835..49b2d9daa 100644
--- a/py/qstr.c
+++ b/py/qstr.c
@@ -28,6 +28,7 @@
 #include <string.h>
 #include <stdio.h>
 
+#include "py/gc.h"
 #include "py/mpstate.h"
 #include "py/qstr.h"
 #include "py/gc.h"
@@ -143,7 +144,7 @@ STATIC qstr qstr_add(const byte *q_ptr) {
 
     // make sure we have room in the pool for a new qstr
     if (MP_STATE_VM(last_pool)->len >= MP_STATE_VM(last_pool)->alloc) {
-        qstr_pool_t *pool = m_new_obj_var_maybe(qstr_pool_t, const char*, MP_STATE_VM(last_pool)->alloc * 2);
+        qstr_pool_t *pool = m_new_ll_obj_var_maybe(qstr_pool_t, const char*, MP_STATE_VM(last_pool)->alloc * 2);
         if (pool == NULL) {
             QSTR_EXIT();
             m_malloc_fail(MP_STATE_VM(last_pool)->alloc * 2);
@@ -213,10 +214,10 @@ qstr qstr_from_strn(const char *str, size_t len) {
             if (al < MICROPY_ALLOC_QSTR_CHUNK_INIT) {
                 al = MICROPY_ALLOC_QSTR_CHUNK_INIT;
             }
-            MP_STATE_VM(qstr_last_chunk) = m_new_maybe(byte, al);
+            MP_STATE_VM(qstr_last_chunk) = m_new_ll_maybe(byte, al);
             if (MP_STATE_VM(qstr_last_chunk) == NULL) {
                 // failed to allocate a large chunk so try with exact size
-                MP_STATE_VM(qstr_last_chunk) = m_new_maybe(byte, n_bytes);
+                MP_STATE_VM(qstr_last_chunk) = m_new_ll_maybe(byte, n_bytes);
                 if (MP_STATE_VM(qstr_last_chunk) == NULL) {
                     QSTR_EXIT();
                     m_malloc_fail(n_bytes);
@@ -258,7 +259,7 @@ qstr qstr_build_end(byte *q_ptr) {
         mp_uint_t hash = qstr_compute_hash(Q_GET_DATA(q_ptr), len);
         Q_SET_HASH(q_ptr, hash);
         q_ptr[MICROPY_QSTR_BYTES_IN_HASH + MICROPY_QSTR_BYTES_IN_LEN + len] = '\0';
-        q = qstr_add(q_ptr);
+        q = qstr_add(gc_make_long_lived(q_ptr));
     } else {
         m_del(byte, q_ptr, Q_GET_ALLOC(q_ptr));
     }