diff --git a/CHANGES b/CHANGES
index 0f4fc9e183..5f96f3eacf 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,7 @@
+4869.	[bug]		Address some cases where NULL with zero length could
+			be passed to memmove which is undefined behaviour and
+			can lead to bad optimisation. [RT #46888]
+
 4868.	[func]		dnssec-keygen can no longer generate HMAC keys.
 			Use tsig-keygen instead. [RT #46404]
 
diff --git a/lib/dns/catz.c b/lib/dns/catz.c
index 84c1bea024..4d771bebb2 100644
--- a/lib/dns/catz.c
+++ b/lib/dns/catz.c
@@ -1002,7 +1002,7 @@ catz_process_version(dns_catz_zone_t *zone, dns_rdataset_t *value) {
 		result = ISC_R_BADNUMBER;
 		goto cleanup;
 	}
-	memcpy(t, rdatastr.data, rdatastr.length);
+	memmove(t, rdatastr.data, rdatastr.length);
 	t[rdatastr.length] = 0;
 	result = isc_parse_uint32(&tversion, t, 10);
 	if (result != ISC_R_SUCCESS) {
@@ -1097,7 +1097,7 @@ catz_process_masters(dns_catz_zone_t *zone, dns_ipkeylist_t *ipkl,
 			if (keyname == NULL)
 				return (ISC_R_NOMEMORY);
 			dns_name_init(keyname, 0);
-			memcpy(keycbuf, rdatastr.data, rdatastr.length);
+			memmove(keycbuf, rdatastr.data, rdatastr.length);
 			keycbuf[rdatastr.length] = 0;
 			result = dns_name_fromstring(keyname, keycbuf, 0, mctx);
 			if (result != ISC_R_SUCCESS) {
@@ -1126,8 +1126,8 @@ catz_process_masters(dns_catz_zone_t *zone, dns_ipkeylist_t *ipkl,
 			if (value->type == dns_rdatatype_txt)
 				ipkl->keys[i] = keyname;
 			else /* A/AAAA */
-				memcpy(&ipkl->addrs[i], &sockaddr,
-				       sizeof(isc_sockaddr_t));
+				memmove(&ipkl->addrs[i], &sockaddr,
+					sizeof(isc_sockaddr_t));
 		} else {
 			result = dns_ipkeylist_resize(mctx, ipkl,
 						      i+1);
@@ -1158,8 +1158,8 @@ catz_process_masters(dns_catz_zone_t *zone, dns_ipkeylist_t *ipkl,
 			if (value->type == dns_rdatatype_txt)
 				ipkl->keys[i] = keyname;
 			else /* A/AAAA */
-				memcpy(&ipkl->addrs[i], &sockaddr,
-				       sizeof(isc_sockaddr_t));
+				memmove(&ipkl->addrs[i], &sockaddr,
+					sizeof(isc_sockaddr_t));
 			ipkl->count++;
 		}
 		return (ISC_R_SUCCESS);
@@ -1251,7 +1251,8 @@ catz_process_apl(dns_catz_zone_t *zone, isc_buffer_t **aclbp,
 		result = dns_rdata_apl_current(&rdata_apl, &apl_ent);
 		RUNTIME_CHECK(result == ISC_R_SUCCESS);
 		memset(buf, 0, sizeof(buf));
-		memcpy(buf, apl_ent.data, apl_ent.length);
+		if (apl_ent.data != NULL && apl_ent.length > 0)
+			memmove(buf, apl_ent.data, apl_ent.length);
 		if (apl_ent.family == 1)
 			isc_netaddr_fromin(&addr, (struct in_addr*) buf);
 		else if (apl_ent.family == 2)
diff --git a/lib/dns/diff.c b/lib/dns/diff.c
index 75fdd8a34c..0c3d93169c 100644
--- a/lib/dns/diff.c
+++ b/lib/dns/diff.c
@@ -80,11 +80,16 @@ dns_difftuple_create(isc_mem_t *mctx,
 
 	t->ttl = ttl;
 
-	memmove(datap, rdata->data, rdata->length);
 	dns_rdata_init(&t->rdata);
 	dns_rdata_clone(rdata, &t->rdata);
-	t->rdata.data = datap;
-	datap += rdata->length;
+	if (rdata->data != NULL) {
+		memmove(datap, rdata->data, rdata->length);
+		t->rdata.data = datap;
+		datap += rdata->length;
+	} else {
+		t->rdata.data = NULL;
+		INSIST(rdata->length == 0);
+	}
 
 	ISC_LINK_INIT(&t->rdata, link);
 	ISC_LINK_INIT(t, link);
diff --git a/lib/dns/ipkeylist.c b/lib/dns/ipkeylist.c
index dadcfd6df7..be6d8a7b88 100644
--- a/lib/dns/ipkeylist.c
+++ b/lib/dns/ipkeylist.c
@@ -187,34 +187,42 @@ dns_ipkeylist_resize(isc_mem_t *mctx, dns_ipkeylist_t *ipkl, unsigned int n) {
 	if (labels == NULL)
 		goto nomemory;
 
-	memmove(addrs, ipkl->addrs, ipkl->allocated * sizeof(isc_sockaddr_t));
-	if (ipkl->addrs != NULL)
+	if (ipkl->addrs != NULL) {
+		memmove(addrs, ipkl->addrs,
+			ipkl->allocated * sizeof(isc_sockaddr_t));
 		isc_mem_put(mctx, ipkl->addrs,
 			    ipkl->allocated * sizeof(isc_sockaddr_t));
+	}
 	ipkl->addrs = addrs;
 	memset(&ipkl->addrs[ipkl->allocated], 0,
 	       (n - ipkl->allocated) * sizeof(isc_sockaddr_t));
 
-	memmove(dscps, ipkl->dscps, ipkl->allocated * sizeof(isc_dscp_t));
-	if (ipkl->dscps != NULL)
+	if (ipkl->dscps != NULL) {
+		memmove(dscps, ipkl->dscps,
+			ipkl->allocated * sizeof(isc_dscp_t));
 		isc_mem_put(mctx, ipkl->dscps,
 			    ipkl->allocated * sizeof(isc_dscp_t));
+	}
 	ipkl->dscps = dscps;
 	memset(&ipkl->dscps[ipkl->allocated], 0,
 	       (n - ipkl->allocated) * sizeof(isc_dscp_t));
 
-	memmove(keys, ipkl->keys, ipkl->allocated * sizeof(dns_name_t *));
-	if (ipkl->keys)
+	if (ipkl->keys) {
+		memmove(keys, ipkl->keys,
+			ipkl->allocated * sizeof(dns_name_t *));
 		isc_mem_put(mctx, ipkl->keys,
 			    ipkl->allocated * sizeof(dns_name_t *));
+	}
 	ipkl->keys = keys;
 	memset(&ipkl->keys[ipkl->allocated], 0,
 	       (n - ipkl->allocated) * sizeof(dns_name_t *));
 
-	memmove(labels, ipkl->labels, ipkl->allocated * sizeof(dns_name_t *));
-	if (ipkl->labels)
+	if (ipkl->labels != NULL) {
+		memmove(labels, ipkl->labels,
+			ipkl->allocated * sizeof(dns_name_t *));
 		isc_mem_put(mctx, ipkl->labels,
 			    ipkl->allocated * sizeof(dns_name_t *));
+	}
 	ipkl->labels = labels;
 	memset(&ipkl->labels[ipkl->allocated], 0,
 	       (n - ipkl->allocated) * sizeof(dns_name_t *));
diff --git a/lib/dns/resolver.c b/lib/dns/resolver.c
index 5b3cb3479f..40f10461ac 100644
--- a/lib/dns/resolver.c
+++ b/lib/dns/resolver.c
@@ -1810,8 +1810,8 @@ fctx_setretryinterval(fetchctx_t *fctx, unsigned int rtt) {
 	/*
 	 * Exponential backoff after the first few tries.
 	 */
-	if (fctx->restarts >= fctx->res->nonbackofftries)
-		us <<= (fctx->restarts - fctx->res->nonbackofftries - 1);
+	if (fctx->restarts > fctx->res->nonbackofftries)
+		us <<= (fctx->restarts - fctx->res->nonbackofftries);
 
 	/*
 	 * Add a fudge factor to the expected rtt based on the current
diff --git a/lib/dns/tests/dispatch_test.c b/lib/dns/tests/dispatch_test.c
index 9627e7e065..0b40256f86 100644
--- a/lib/dns/tests/dispatch_test.c
+++ b/lib/dns/tests/dispatch_test.c
@@ -157,11 +157,11 @@ nameserver(isc_task_t *task, isc_event_t *event) {
 	static unsigned char buf1[16];
 	static unsigned char buf2[16];
 
-	memcpy(buf1, ev->region.base, 12);
+	memmove(buf1, ev->region.base, 12);
 	memset(buf1 + 12, 0, 4);
 	buf1[2] |= 0x80;	/* qr=1 */
 
-	memcpy(buf2, ev->region.base, 12);
+	memmove(buf2, ev->region.base, 12);
 	memset(buf2 + 12, 1, 4);
 	buf2[2] |= 0x80;	/* qr=1 */