diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index b71072a7bc..e3232f1173 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -301,7 +301,7 @@ unit:gcc:stretch:amd64:
 gcc:sid:amd64:
   variables:
     CC: gcc
-    CFLAGS: "-Wall -Wextra -O2 -g"
+    CFLAGS: "-Wall -Wextra -O3 -g"
     EXTRA_CONFIGURE: "--with-libidn2"
     RUN_MAKE_INSTALL: 1
   <<: *debian_sid_amd64_image
@@ -324,7 +324,7 @@ unit:gcc:sid:amd64:
 gcc:sid:i386:
   variables:
     CC: gcc
-    CFLAGS: "-Wall -Wextra -O2 -g"
+    CFLAGS: "-Wall -Wextra -O3 -g"
     EXTRA_CONFIGURE: "--with-libidn2 --without-python"
   <<: *debian_sid_i386_image
   <<: *build_job
diff --git a/bin/tests/system/dlzexternal/driver.c b/bin/tests/system/dlzexternal/driver.c
index 2e59a13e77..310220b1b7 100644
--- a/bin/tests/system/dlzexternal/driver.c
+++ b/bin/tests/system/dlzexternal/driver.c
@@ -166,7 +166,7 @@ del_name(struct dlz_example_data *state, struct record *list,
 
 static isc_result_t
 fmt_address(isc_sockaddr_t *addr, char *buffer, size_t size) {
-	char addr_buf[100];
+	char addr_buf[INET6_ADDRSTRLEN];
 	const char *ret;
 	uint16_t port = 0;
 
diff --git a/lib/dns/tests/private_test.c b/lib/dns/tests/private_test.c
index e7e2d47dc9..e8e642d1ba 100644
--- a/lib/dns/tests/private_test.c
+++ b/lib/dns/tests/private_test.c
@@ -107,7 +107,7 @@ make_nsec3(nsec3_testcase_t *testcase, dns_rdata_t *private,
 	/* for simplicity, we're using a maximum salt length of 4 */
 	salt = htonl(testcase->salt);
 	sp = (unsigned char *) &salt;
-	while (*sp == '\0' && slen > 0) {
+	while (slen > 0 && *sp == '\0') {
 		slen--;
 		sp++;
 	}
diff --git a/lib/isc/task.c b/lib/isc/task.c
index 9af6914af4..7bfce3be63 100644
--- a/lib/isc/task.c
+++ b/lib/isc/task.c
@@ -1380,7 +1380,7 @@ isc_taskmgr_create(isc_mem_t *mctx, unsigned int workers,
 		RUNTIME_CHECK(isc_thread_create(run, &manager->queues[i],
 						&manager->queues[i].thread)
 			      == ISC_R_SUCCESS);
-		char name[16];
+		char name[21];
 		snprintf(name, sizeof(name), "isc-worker%04u", i);
 		isc_thread_setname(manager->queues[i].thread, name);
 	}