py/gc: Reduce code size by specialising VERIFY_MARK_AND_PUSH macro.

This macro is written out explicitly in the two locations that it is used
and then the code is optimised, opening possibilities for further
optimisations and reducing code size:

unix:            -48
minimal CROSS=1: -32
stm32:           -32

py/gc.c

@@ -203,24 +203,6 @@ bool gc_is_locked(void) {
 #endif
 #endif
 
-// ptr should be of type void*
-#define VERIFY_MARK_AND_PUSH(ptr) \
-    do { \
-        if (VERIFY_PTR(ptr)) { \
-            size_t _block = BLOCK_FROM_PTR(ptr); \
-            if (ATB_GET_KIND(_block) == AT_HEAD) { \
-                /* an unmarked head, mark it, and push it on gc stack */ \
-                TRACE_MARK(_block, ptr); \
-                ATB_HEAD_TO_MARK(_block); \
-                if (MP_STATE_MEM(gc_sp) < &MP_STATE_MEM(gc_stack)[MICROPY_ALLOC_GC_STACK_SIZE]) { \
-                    *MP_STATE_MEM(gc_sp)++ = _block; \
-                } else { \
-                    MP_STATE_MEM(gc_stack_overflow) = 1; \
-                } \
-            } \
-        } \
-    } while (0)
-
 STATIC void gc_drain_stack(void) {
     while (MP_STATE_MEM(gc_sp) > MP_STATE_MEM(gc_stack)) {
         // pop the next block off the stack
@@ -236,7 +218,20 @@ STATIC void gc_drain_stack(void) {
         void **ptrs = (void**)PTR_FROM_BLOCK(block);
         for (size_t i = n_blocks * BYTES_PER_BLOCK / sizeof(void*); i > 0; i--, ptrs++) {
             void *ptr = *ptrs;
-            VERIFY_MARK_AND_PUSH(ptr);
+            if (VERIFY_PTR(ptr)) {
+                // Mark and push this pointer
+                size_t childblock = BLOCK_FROM_PTR(ptr);
+                if (ATB_GET_KIND(childblock) == AT_HEAD) {
+                    // an unmarked head, mark it, and push it on gc stack
+                    TRACE_MARK(childblock, ptr);
+                    ATB_HEAD_TO_MARK(childblock);
+                    if (MP_STATE_MEM(gc_sp) < &MP_STATE_MEM(gc_stack)[MICROPY_ALLOC_GC_STACK_SIZE]) {
+                        *MP_STATE_MEM(gc_sp)++ = childblock;
+                    } else {
+                        MP_STATE_MEM(gc_stack_overflow) = 1;
+                    }
+                }
+            }
         }
     }
 }
@@ -337,8 +332,17 @@ void gc_collect_start(void) {
 void gc_collect_root(void **ptrs, size_t len) {
     for (size_t i = 0; i < len; i++) {
         void *ptr = ptrs[i];
-        VERIFY_MARK_AND_PUSH(ptr);
-        gc_drain_stack();
+        if (VERIFY_PTR(ptr)) {
+            // Mark and push this pointer
+            size_t block = BLOCK_FROM_PTR(ptr);
+            if (ATB_GET_KIND(block) == AT_HEAD) {
+                // an unmarked head, mark it, and push it on gc stack
+                TRACE_MARK(block, ptr);
+                ATB_HEAD_TO_MARK(block);
+                *MP_STATE_MEM(gc_sp)++ = block;
+                gc_drain_stack();
+            }
+        }
     }
 }