Stack overflow checks are done when tracing is enabled

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

debug.c

@@ -26,6 +26,7 @@
 #include "cr.h"
 #include "libmill.h"
 #include "list.h"
+#include "stack.h"
 #include "utils.h"
 
 #include <assert.h>
@@ -199,6 +200,11 @@ void mill_trace_(const char *location, const char *format, ...) {
     if(mill_fast(mill_tracelevel <= 0))
         return;
 
+    /* TODO: At the moment, stack overflow checking is done only if tracing
+       is enabled. Think of a better way to do it. */
+    if(mill_running != &mill_main)
+        mill_checkstack(mill_running + 1);
+
     char buf[256];
 
     /* First print the timestamp. */

stack.c

@@ -42,6 +42,9 @@
 /* 8 bytes written to the bottom of the stack to guard for stack overflows. */
 #define MILL_STACK_GUARD 0xdeadbeefbadcafe0
 
+static volatile int mill_stack_unoptimisable1 = 1;
+static volatile void *mill_stack_unoptimisable2 = NULL;
+
 /* A stack of unused coroutine stacks. This allows for extra-fast allocation
    of a new stack. The FIFO nature of this structure minimises cache misses.
    When the stack is cached its mill_slist_item is placed on its top rather
@@ -69,9 +72,12 @@ void mill_freestack(void *stack) {
     ++mill_num_cached_stacks;
 }
 
-void mill_checkstack(void *stack, void *ptr) {
+void mill_checkstack(void *stack) {
+    int anchor[mill_stack_unoptimisable1];
+    mill_stack_unoptimisable2 = &anchor;
     char *bottom = ((char*) stack) - MILL_STACK_SIZE;
-    assert (ptr <= stack && ptr > (void*)(bottom + 8) &&
-        *((uint64_t*)bottom) == MILL_STACK_GUARD);
+    assert(((void*)&anchor) <= stack);
+    assert(((void*)&anchor) > (void*)(bottom + 8));
+    assert(*((uint64_t*)bottom) == MILL_STACK_GUARD);
 }
 

stack.h

@@ -32,8 +32,7 @@ void *mill_allocstack(void);
 /* Deallocates a stack. The argument is pointer to the top of the stack. */
 void mill_freestack(void *stack);
 
-/* Checks whether the supplied pointer is within the stack and whether the
-   stack itself is not corrupted. */
-void mill_checkstack(void *stack, void *ptr);
+/* Checks for the signs of stack overflow. */
+void mill_checkstack(void *stack);
 
 #endif