diff --git a/config/ChangeLog b/config/ChangeLog
index 3a488a1ae35..ef58a91545b 100644
--- a/config/ChangeLog
+++ b/config/ChangeLog
@@ -1,3 +1,8 @@
+2018-05-10  Martin Liska  <mliska@suse.cz>
+
+	PR bootstrap/64914
+	* bootstrap-ubsan.mk: Define UBSAN_BOOTSTRAP.
+
 2018-05-09  Joshua Watt <jpewhacker@gmail.com>
 
         * ax_pthread.m4: Add file.
diff --git a/config/bootstrap-ubsan.mk b/config/bootstrap-ubsan.mk
index d2615e8f807..1d85fe1c836 100644
--- a/config/bootstrap-ubsan.mk
+++ b/config/bootstrap-ubsan.mk
@@ -1,8 +1,8 @@
 # This option enables -fsanitize=undefined for stage2 and stage3.
 
-STAGE2_CFLAGS += -fsanitize=undefined
-STAGE3_CFLAGS += -fsanitize=undefined
-POSTSTAGE1_LDFLAGS += -fsanitize=undefined -static-libubsan \
+STAGE2_CFLAGS += -fsanitize=undefined -DUBSAN_BOOTSTRAP
+STAGE3_CFLAGS += -fsanitize=undefined -DUBSAN_BOOTSTRAP
+POSTSTAGE1_LDFLAGS += -fsanitize=undefined -static-libubsan -DUBSAN_BOOTSTRAP \
 		      -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ \
 		      -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ubsan/ \
 		      -B$$r/prev-$(TARGET_SUBDIR)/libsanitizer/ubsan/.libs
diff --git a/libiberty/ChangeLog b/libiberty/ChangeLog
index a116d373100..b249226155c 100644
--- a/libiberty/ChangeLog
+++ b/libiberty/ChangeLog
@@ -1,3 +1,8 @@
+2018-05-10  Martin Liska  <mliska@suse.cz>
+
+	PR bootstrap/64914
+	* md5.c: Use strict alignment with UBSAN_BOOTSTRAP.
+
 2018-04-30  Daniel van Gerpen  <daniel@vangerpen.de>
 
 	* argv.c (expandargv): Fix memory leak for copied argv.
diff --git a/libiberty/md5.c b/libiberty/md5.c
index f106d2262ca..4dd5186a69a 100644
--- a/libiberty/md5.c
+++ b/libiberty/md5.c
@@ -231,7 +231,7 @@ md5_process_bytes (const void *buffer, size_t len, struct md5_ctx *ctx)
   /* Process available complete blocks.  */
   if (len > 64)
     {
-#if !_STRING_ARCH_unaligned
+#if !_STRING_ARCH_unaligned || defined UBSAN_BOOTSTRAP
 /* To check alignment gcc has an appropriate operator.  Other
    compilers don't.  */
 # if __GNUC__ >= 2