diff --git a/sys/include/string_utils.h b/sys/include/string_utils.h
index 5c7da82a8d..004e284aaf 100644
--- a/sys/include/string_utils.h
+++ b/sys/include/string_utils.h
@@ -7,8 +7,7 @@
  */
 
 /**
- * @defgroup    sys_string_utils    Utility functions that are missing in
- *                                  `string.h`
+ * @defgroup    sys_string_utils    Utility functions that are missing in `string.h`
  * @ingroup     sys
  *
  * This header provides utility functions that the standard C libs `string.h`
@@ -92,6 +91,18 @@ static inline void explicit_bzero(void *dest, size_t n_bytes)
  */
 ssize_t strscpy(char *dest, const char *src, size_t count);
 
+/**
+ * @brief   Check if the entire buffer is filled with the same byte.
+ *
+ * @param[in]   data    The buffer to probe
+ * @param[in]   c       The byte to check of
+ * @param[in]   len     Size of the buffer
+ *
+ * @return NULL if the entire buffer is filled with @p c
+ * @return pointer to the first non-matching byte
+ */
+const void *memchk(const void *data, uint8_t c, size_t len);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/sys/libc/string.c b/sys/libc/string.c
index c3c1cc661a..7a8b4ba047 100644
--- a/sys/libc/string.c
+++ b/sys/libc/string.c
@@ -37,4 +37,17 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
         return -E2BIG;
     }
 }
+
+const void *memchk(const void *data, uint8_t c, size_t len)
+{
+    const uint8_t *end = (uint8_t *)data + len;
+    for (const uint8_t *d = data; d != end; ++d) {
+        if (c != *d) {
+            return d;
+        }
+    }
+
+    return NULL;
+}
+
 /** @} */
diff --git a/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c b/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c
index 5ac2708886..7498390e45 100644
--- a/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c
+++ b/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c
@@ -30,6 +30,7 @@
 #include "net/gnrc/pktbuf.h"
 #include "net/gnrc/nettype.h"
 #include "net/gnrc/pkt.h"
+#include "string_utils.h"
 
 #include "pktbuf_internal.h"
 #include "pktbuf_static.h"
@@ -61,18 +62,6 @@ static gnrc_pktsnip_t *_create_snip(gnrc_pktsnip_t *next, const void *data, size
                                     gnrc_nettype_t type);
 static void *_pktbuf_alloc(size_t size);
 
-static const void *mem_is_set(const void *data, uint8_t c, size_t len)
-{
-    const uint8_t *end = (uint8_t *)data + len;
-    for (const uint8_t *d = data; d != end; ++d) {
-        if (c != *d) {
-            return d;
-        }
-    }
-
-    return NULL;
-}
-
 static inline void _set_pktsnip(gnrc_pktsnip_t *pkt, gnrc_pktsnip_t *next,
                                 void *data, size_t size, gnrc_nettype_t type)
 {
@@ -440,7 +429,7 @@ static void *_pktbuf_alloc(size_t size)
 
     const void *mismatch;
     if (CONFIG_GNRC_PKTBUF_CHECK_USE_AFTER_FREE &&
-        (mismatch = mem_is_set(ptr + 1, CANARY, size - sizeof(_unused_t)))) {
+        (mismatch = memchk(ptr + 1, CANARY, size - sizeof(_unused_t)))) {
         printf("[%p] mismatch at offset %"PRIuPTR"/%u (ignoring %u initial bytes that were repurposed)\n",
                (void *)ptr, (uintptr_t)mismatch - (uintptr_t)ptr, (unsigned)size, (unsigned)sizeof(_unused_t));
 #ifdef MODULE_OD
diff --git a/tests/unittests/tests-core/tests-core-atomic.c b/tests/unittests/tests-core/tests-core-atomic.c
index 98f6f6cc84..7089f9edfa 100644
--- a/tests/unittests/tests-core/tests-core-atomic.c
+++ b/tests/unittests/tests-core/tests-core-atomic.c
@@ -26,11 +26,6 @@ static void test_atomic_flag(void)
     TEST_ASSERT_EQUAL_INT(0, atomic_flag_test_and_set(&flag));
 }
 
-/* Prevent compiler optimization for SAML1X because of gcc internal bug */
-#ifdef CPU_CORE_CORTEX_M23
-#pragma GCC push_options
-#pragma GCC optimize ("O0")
-#endif
 /* Test atomic_fetch_add */
 static void test_atomic_inc_positive(void)
 {
@@ -72,9 +67,7 @@ static void test_atomic_inc_rollover(void)
     TEST_ASSERT_EQUAL_INT(INT_MIN + 1, atomic_fetch_add(&res, 1));
     TEST_ASSERT_EQUAL_INT(INT_MIN + 2, atomic_load(&res));
 }
-#ifdef CPU_SAML1X
-#pragma GCC pop_options
-#endif
+
 /* Test atomic_fetch_sub */
 static void test_atomic_dec_negative(void)
 {
diff --git a/tests/unittests/tests-libc/tests-libc.c b/tests/unittests/tests-libc/tests-libc.c
index 3d4a714ed3..1f0a081919 100644
--- a/tests/unittests/tests-libc/tests-libc.c
+++ b/tests/unittests/tests-libc/tests-libc.c
@@ -29,10 +29,21 @@ static void test_libc_strscpy(void)
     TEST_ASSERT_EQUAL_INT(strscpy(buffer, "empty", 0), -E2BIG);
 }
 
+static void test_libc_memchk(void)
+{
+    char buffer[32];
+    memset(buffer, 0xff, sizeof(buffer));
+    TEST_ASSERT_NULL(memchk(buffer, 0xff, sizeof(buffer)));
+
+    buffer[5] = 5;
+    TEST_ASSERT(memchk(buffer, 0xff, sizeof(buffer)) == &buffer[5]);
+}
+
 Test *tests_libc_tests(void)
 {
     EMB_UNIT_TESTFIXTURES(fixtures) {
         new_TestFixture(test_libc_strscpy),
+        new_TestFixture(test_libc_memchk),
     };
 
     EMB_UNIT_TESTCALLER(libc_tests, NULL, NULL, fixtures);