Add function to measure stack high water mark (thanks @g3gg0)

Author: igrr

cores/esp8266/cont.h

@@ -60,6 +60,10 @@ void cont_yield(cont_t*);
 // return 1 if guard bytes were overwritten.
 int cont_check(cont_t* cont);
 
+// Go through stack and check how many bytes are most probably still unchanged
+// and thus weren't used by the user code. i.e. that stack space is free. (high water mark)
+int cont_get_free_stack(cont_t* cont);
+
 // Check if yield() may be called. Returns true if we are running inside
 // continuation stack
 bool cont_can_yield(cont_t* cont);

cores/esp8266/cont_util.c

@@ -30,6 +30,12 @@ void ICACHE_RAM_ATTR cont_init(cont_t* cont) {
     cont->stack_guard2 = CONT_STACKGUARD;
     cont->stack_end = cont->stack + (sizeof(cont->stack) / 4);
     cont->struct_start = (unsigned*) cont;
+    
+    // fill stack with magic values to check high water mark
+    for(int pos = 0; pos < sizeof(cont->stack) / 4; pos++)
+    {
+        cont->stack[pos] = CONT_STACKGUARD;
+    }
 }
 
 int ICACHE_RAM_ATTR cont_check(cont_t* cont) {
@@ -38,6 +44,19 @@ int ICACHE_RAM_ATTR cont_check(cont_t* cont) {
     return 0;
 }
 
+int ICACHE_RAM_ATTR cont_get_free_stack(cont_t* cont) {
+    uint32_t *head = cont->stack;
+    int freeWords = 0;
+
+    while(*head == CONT_STACKGUARD)
+    {
+        head++;
+        freeWords++;
+    }
+    
+    return freeWords * 4;
+}
+
 bool ICACHE_RAM_ATTR cont_can_yield(cont_t* cont) {
     return !ETS_INTR_WITHINISR() &&
            cont->pc_ret != 0 && cont->pc_yield == 0;