py/gc: Add hook to run code during time consuming GC operations.

This makes it possible for cooperative multitasking systems to keep running
event loops during garbage collector operations.

For example, this can be used to ensure that a motor control loop runs
approximately each 5 ms.  Without this hook, the loop time can jump to
about 15 ms.

Addresses #3475.

Signed-off-by: Laurens Valk <laurens@pybricks.com>

py/gc.c

@@ -213,6 +213,7 @@ STATIC void gc_mark_subtree(size_t block) {
     // Start with the block passed in the argument.
     size_t sp = 0;
     for (;;) {
+        MICROPY_GC_HOOK_LOOP
         // work out number of consecutive blocks in the chain starting with this one
         size_t n_blocks = 0;
         do {
@@ -222,6 +223,7 @@ STATIC void gc_mark_subtree(size_t block) {
         // check this block's children
         void **ptrs = (void **)PTR_FROM_BLOCK(block);
         for (size_t i = n_blocks * BYTES_PER_BLOCK / sizeof(void *); i > 0; i--, ptrs++) {
+            MICROPY_GC_HOOK_LOOP
             void *ptr = *ptrs;
             if (VERIFY_PTR(ptr)) {
                 // Mark and push this pointer
@@ -255,6 +257,7 @@ STATIC void gc_deal_with_stack_overflow(void) {
 
         // scan entire memory looking for blocks which have been marked but not their children
         for (size_t block = 0; block < MP_STATE_MEM(gc_alloc_table_byte_len) * BLOCKS_PER_ATB; block++) {
+            MICROPY_GC_HOOK_LOOP
             // trace (again) if mark bit set
             if (ATB_GET_KIND(block) == AT_MARK) {
                 gc_mark_subtree(block);
@@ -270,6 +273,7 @@ STATIC void gc_sweep(void) {
     // free unmarked heads and their tails
     int free_tail = 0;
     for (size_t block = 0; block < MP_STATE_MEM(gc_alloc_table_byte_len) * BLOCKS_PER_ATB; block++) {
+        MICROPY_GC_HOOK_LOOP
         switch (ATB_GET_KIND(block)) {
             case AT_HEAD:
                 #if MICROPY_ENABLE_FINALISER
@@ -354,6 +358,7 @@ static void *gc_get_ptr(void **ptrs, int i) {
 
 void gc_collect_root(void **ptrs, size_t len) {
     for (size_t i = 0; i < len; i++) {
+        MICROPY_GC_HOOK_LOOP
         void *ptr = gc_get_ptr(ptrs, i);
         if (VERIFY_PTR(ptr)) {
             size_t block = BLOCK_FROM_PTR(ptr);

py/mpconfig.h

@@ -602,6 +602,11 @@
 #define MICROPY_ENABLE_GC (0)
 #endif
 
+// Hook to run code during time consuming garbage collector operations
+#ifndef MICROPY_GC_HOOK_LOOP
+#define MICROPY_GC_HOOK_LOOP
+#endif
+
 // Whether to enable finalisers in the garbage collector (ie call __del__)
 #ifndef MICROPY_ENABLE_FINALISER
 #define MICROPY_ENABLE_FINALISER (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)