kernel: speed up z_stack_space_get

The z_stack_space_get call currently checks for free space at the
top of the stack by checking each byte individually. This can
introduce significant runtime overhead for threads which have large,
mostly unused stacks. This change updates the check to first count
the free space by word, and then check the sub-word unused bytes.

Signed-off-by: Félix Turgeon <[email protected]>

Author: felixturgeonmeta

kernel/thread.c

@@ -853,7 +853,26 @@ int z_stack_space_get(const uint8_t *stack_start, size_t size, size_t *unused_pt
 		size -= 4;
 	}
 
-	for (size_t i = 0; i < size; i++) {
+	/* align to word boundary */
+	const unsigned long *checked_stack_word =
+		(const unsigned long *)ROUND_UP(checked_stack, sizeof(unsigned long));
+	size -= ((const uint8_t *)checked_stack_word - checked_stack);
+
+	/* compare using native machine word size */
+	size_t i;
+
+    for (i = 0; i < size / sizeof(unsigned long); i++) {
+        if (checked_stack_word[i] != (unsigned long)0xaaaaaaaaaaaaaaaa) {
+            break;
+        }
+    }
+    unused = i * sizeof(unsigned long);
+
+	/* Continue checking from last used word to find remaining unused bytes*/
+	size -= unused;
+	checked_stack = (const uint8_t *)checked_stack_word + unused;
+
+	for (i = 0; i < size; i++) {
 		if ((checked_stack[i]) == 0xaaU) {
 			unused++;
 		} else {