atmel-samd: Fix sporadic "syntax errors"

The GC was deleting memory that was in use because its scan of the
stack missed the very top. Switching to _estack fixes this by relying
on the location from the linker.

Fixes #124

2 files changed, 4 insertions, 9 deletions

diff --git a/atmel-samd/main.c b/atmel-samd/main.c
index 9ed1c3cd1..6184d64eb 100644
--- a/atmel-samd/main.c
+++ b/atmel-samd/main.c
@@ -115,7 +115,6 @@ void init_flash_fs(void) {
     f_chdrive("/flash");
 }
 
-static char *stack_top;
 static char heap[16384];
 
 void reset_mp(void) {
@@ -519,12 +518,6 @@ int main(void) {
     // initialise the cpu and peripherals
     samd21_init();
 
-    int stack_dummy;
-    // Store the location of stack_dummy as an approximation for the top of the
-    // stack so the GC can account for objects that may be referenced by the
-    // stack between here and where gc_collect is called.
-    stack_top = (char*)&stack_dummy;
-
     // Stack limit should be less than real stack size, so we have a chance
     // to recover from limit hit.  (Limit is measured in bytes.)
     mp_stack_ctrl_init();
@@ -581,7 +574,7 @@ void gc_collect(void) {
     gc_collect_start();
     // This naively collects all object references from an approximate stack
     // range.
-    gc_collect_root(&dummy, ((mp_uint_t)stack_top - (mp_uint_t)&dummy) / sizeof(mp_uint_t));
+    gc_collect_root(&dummy, ((mp_uint_t)&_estack - (mp_uint_t)&dummy) / sizeof(mp_uint_t));
     gc_collect_end();
 }
 
diff --git a/py/gc.c b/py/gc.c
index 75fc82382..72eecd7ef 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -104,11 +104,13 @@
 
 #ifdef LOG_HEAP_ACTIVITY
 volatile uint32_t change_me;
-
+#pragma GCC push_options
+#pragma GCC optimize ("O0")
 void __attribute__ ((noinline)) gc_log_change(uint32_t start_block, uint32_t length) {
     change_me += start_block;
     change_me += length; // Break on this line.
 }
+#pragma GCC pop_options
 #endif
 
 // TODO waste less memory; currently requires that all entries in alloc_table have a corresponding block in pool