test/uint: fiber

When optimization level increased we've found that
memset calls might be optimized, moreover there
were no check if calls themselves would override
the stack.

Lets rework and use number of calls instead. Strictly
speaking the test is still a bit fragile and precise
testing would rather require asm level code.

Repored-by: "Alexander V. Tikhonov" <avtikhon@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

test/unit/fiber.cc

+#include <unistd.h>
+
 #include "memory.h"
 #include "fiber.h"
 #include "unit.h"
 #include "trivia/util.h"
 
+static size_t stack_expand_limit;
 static struct fiber_attr default_attr;
 
+static unsigned long page_size;
+#define PAGE_4K 4096
+
 static int
 noop_f(va_list ap)
 {
@@ -49,24 +55,38 @@ cancel_dead_f(va_list ap)
 	return 0;
 }
 
-static size_t stack_expand_limit;
-
 static void NOINLINE
-stack_expand(void *ptr)
+stack_expand(unsigned long *ret, unsigned long nr_calls)
 {
-	char buf[2048];
-	memset(buf, 0x45, 2048);
-	ptrdiff_t stack_diff = (buf - (char *)ptr);
-	stack_diff = stack_diff >= 0 ? stack_diff : -stack_diff;
-	if (stack_diff < (ptrdiff_t)stack_expand_limit)
-		stack_expand(ptr);
+	char volatile fill[PAGE_4K];
+	char volatile *p;
+
+	memset((void *)fill, (unsigned char)nr_calls, sizeof(fill));
+	p = fill;
+	p[PAGE_4K / 2] = (unsigned char)nr_calls;
+
+	if (nr_calls != 0) {
+		stack_expand(ret, nr_calls-1);
+	} else {
+		*ret = (unsigned long)&fill[0];
+	}
 }
 
 static int
 test_stack_f(va_list ap)
 {
-	char s;
-	stack_expand(&s);
+	unsigned long ret = 0;
+	unsigned long ret_addr = (unsigned long)&ret;
+
+	/*
+	 * We can't just dirty the stack in precise
+	 * way without using assembly. Thus lets do
+	 * the following trick:
+	 *  - assume 8K will be enough to carry all
+	 *    arguments passed for all calls, still
+	 *    we might need to adjust this value
+	 */
+	stack_expand(&ret, (stack_expand_limit - 2 * page_size) / page_size);
 	return 0;
 }
 
@@ -193,6 +213,11 @@ main_f(va_list ap)
 
 int main()
 {
+	page_size = sysconf(_SC_PAGESIZE);
+
+	/* Page should be at least 4K */
+	assert(page_size >= PAGE_4K);
+
 	memory_init();
 	fiber_init(fiber_cxx_invoke);
 	fiber_attr_create(&default_attr);