diff --git a/iconvdata/iso-2022-cn-ext.c b/iconvdata/iso-2022-cn-ext.c
index df5b5df..92970a0 100644
--- a/iconvdata/iso-2022-cn-ext.c
+++ b/iconvdata/iso-2022-cn-ext.c
@@ -27,6 +27,7 @@
 #include "cns11643.h"
 #include "cns11643l1.h"
 #include "cns11643l2.h"
+#include <libc-internal.h>
 
 #include <assert.h>
 
@@ -394,6 +395,16 @@ enum
 #define MIN_NEEDED_OUTPUT	TO_LOOP_MIN_NEEDED_TO
 #define MAX_NEEDED_OUTPUT	TO_LOOP_MAX_NEEDED_TO
 #define LOOPFCT			TO_LOOP
+/* With GCC 5.3 when compiling with -Os the compiler emits a warning
+   that buf[0] and buf[1] may be used uninitialized.  This can only
+   happen in the case where tmpbuf[3] is used, and in that case the
+   write to the tmpbuf[1] and tmpbuf[2] was assured because
+   ucs4_to_cns11643 would have filled in those entries.  The difficulty
+   is in getting the compiler to see this logic because tmpbuf[0] is
+   involved in determining the code page and is the indicator that
+   tmpbuf[2] is initialized.  */
+DIAG_PUSH_NEEDS_COMMENT;
+DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 #define BODY \
   {									      \
     uint32_t ch;							      \
@@ -645,6 +656,7 @@ enum
     /* Now that we wrote the output increment the input pointer.  */	      \
     inptr += 4;								      \
   }
+DIAG_POP_NEEDS_COMMENT;
 #define EXTRA_LOOP_DECLS	, int *setp
 #define INIT_PARAMS		int set = (*setp >> 3) & CURRENT_MASK; \
 				int ann = (*setp >> 3) & ~CURRENT_MASK
diff --git a/include/libc-internal.h b/include/libc-internal.h
index 7a185bb..1a386e7 100644
--- a/include/libc-internal.h
+++ b/include/libc-internal.h
@@ -111,4 +111,19 @@ extern __typeof (__profile_frequency) __profile_frequency attribute_hidden;
 #define DIAG_IGNORE_NEEDS_COMMENT(version, option)	\
   _Pragma (_DIAG_STR (GCC diagnostic ignored option))
 
+/* Similar to DIAG_IGNORE_NEEDS_COMMENT the following macro ignores the
+   diagnostic OPTION but only if optimizations for size are enabled.
+   This is required because different warnings may be generated for
+   different optimization levels.  For example a key piece of code may
+   only generate a warning when compiled at -Os, but at -O2 you could
+   still want the warning to be enabled to catch errors.  In this case
+   you would use DIAG_IGNORE_Os_NEEDS_COMMENT to disable the warning
+   only for -Os.  */
+#ifdef __OPTIMIZE_SIZE__ 
+# define DIAG_IGNORE_Os_NEEDS_COMMENT(version, option)	\
+  _Pragma (_DIAG_STR (GCC diagnostic ignored option))
+#else
+# define DIAG_IGNORE_Os_NEEDS_COMMENT(version, option)
+#endif
+
 #endif /* _LIBC_INTERNAL  */
diff --git a/locale/weight.h b/locale/weight.h
index c99730c..1f61f01 100644
--- a/locale/weight.h
+++ b/locale/weight.h
@@ -61,9 +61,17 @@ findidx (const int32_t *table,
 	     already.  */
 	  size_t cnt;
 
+	  /* With GCC 5.3 when compiling with -Os the compiler warns
+	     that seq2.back_us, which becomes usrc, might be used
+	     uninitialized.  This can't be true because we pass a length
+	     of -1 for len at the same time which means that this loop
+	     never executes.  */
+	  DIAG_PUSH_NEEDS_COMMENT;
+	  DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 	  for (cnt = 0; cnt < nhere && cnt < len; ++cnt)
 	    if (cp[cnt] != usrc[cnt])
 	      break;
+	  DIAG_POP_NEEDS_COMMENT;
 
 	  if (cnt == nhere)
 	    {
diff --git a/locale/weightwc.h b/locale/weightwc.h
index ab26482..e42ce13 100644
--- a/locale/weightwc.h
+++ b/locale/weightwc.h
@@ -59,9 +59,17 @@ findidx (const int32_t *table,
 	     already.  */
 	  size_t cnt;
 
+	  /* With GCC 5.3 when compiling with -Os the compiler warns
+	     that seq2.back_us, which becomes usrc, might be used
+	     uninitialized.  This can't be true because we pass a length
+	     of -1 for len at the same time which means that this loop
+	     never executes.  */
+	  DIAG_PUSH_NEEDS_COMMENT;
+	  DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 	  for (cnt = 0; cnt < nhere && cnt < len; ++cnt)
 	    if (cp[cnt] != usrc[cnt])
 	      break;
+	  DIAG_POP_NEEDS_COMMENT;
 
 	  if (cnt == nhere)
 	    {
diff --git a/nptl_db/thread_dbP.h b/nptl_db/thread_dbP.h
index 39c3061..b53f1c1 100644
--- a/nptl_db/thread_dbP.h
+++ b/nptl_db/thread_dbP.h
@@ -160,10 +160,19 @@ extern ps_err_e td_mod_lookup (struct ps_prochandle *ps, const char *modname,
 		   SYM_##type##_FIELD_##field, \
 		   (psaddr_t) 0 + (idx), (ptr), &(var))
 
+/* With GCC 5.3 when compiling with -Os the compiler emits a warning
+   that slot may be used uninitialized.  This is never the case since
+   the dynamic loader initializes the slotinfo list and
+   dtv_slotinfo_list will point slot at the first entry.  Therefore
+   when DB_GET_FIELD_ADDRESS is called with a slot for ptr, the slot is
+   always initialized.  */
+DIAG_PUSH_NEEDS_COMMENT;
+DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 #define DB_GET_FIELD_ADDRESS(var, ta, ptr, type, field, idx) \
   ((var) = (ptr), _td_locate_field ((ta), (ta)->ta_field_##type##_##field, \
 				    SYM_##type##_FIELD_##field, \
 				    (psaddr_t) 0 + (idx), &(var)))
+DIAG_POP_NEEDS_COMMENT;
 
 extern td_err_e _td_locate_field (td_thragent_t *ta,
 				  db_desc_t desc, int descriptor_name,
diff --git a/resolv/res_send.c b/resolv/res_send.c
index 6d46bb2..4ec8c1a 100644
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -664,7 +664,7 @@ send_vc(res_state statp,
 	   a false-positive.
 	 */
 	DIAG_PUSH_NEEDS_COMMENT;
-	DIAG_IGNORE_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
+	DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 	int resplen;
 	DIAG_POP_NEEDS_COMMENT;
 	struct iovec iov[4];
@@ -930,7 +930,16 @@ reopen (res_state statp, int *terrno, int ns)
 		 * error message is received.  We can thus detect
 		 * the absence of a nameserver without timing out.
 		 */
+		/* With GCC 5.3 when compiling with -Os the compiler
+		   emits a warning that slen may be used uninitialized,
+		   but that is never true.  Both slen and
+		   EXT(statp).nssocks[ns] are initialized together or
+		   the function return -1 before control flow reaches
+		   the call to connect with slen.  */
+		DIAG_PUSH_NEEDS_COMMENT;
+		DIAG_IGNORE_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 		if (connect(EXT(statp).nssocks[ns], nsap, slen) < 0) {
+		DIAG_POP_NEEDS_COMMENT;
 			Aerror(statp, stderr, "connect(dg)", errno, nsap);
 			__res_iclose(statp, false);
 			return (0);
diff --git a/string/strcoll_l.c b/string/strcoll_l.c
index 4d1e3ab..5605dbf 100644
--- a/string/strcoll_l.c
+++ b/string/strcoll_l.c
@@ -24,6 +24,7 @@
 #include <stdint.h>
 #include <string.h>
 #include <sys/param.h>
+#include <libc-internal.h>
 
 #ifndef STRING_TYPE
 # define STRING_TYPE char
@@ -170,7 +171,19 @@ get_next_seq (coll_seq *seq, int nrules, const unsigned char *rulesets,
 	    }
 	}
 
+      /* With GCC 5.3 when compiling with -Os the compiler complains
+	 that idx, taken from seq->idx (seq1 or seq2 from STRCOLL) may
+	 be used uninitialized.  In general this can't possibly be true
+	 since seq1.idx and seq2.idx are initialized to zero in the
+	 outer function.  Only one case where seq->idx is restored from
+	 seq->save_idx might result in an uninitialized idx value, but
+	 it is guarded by a sequence of checks against backw_stop which
+	 ensures that seq->save_idx was saved to first and contains a
+	 valid value.  */
+      DIAG_PUSH_NEEDS_COMMENT;
+      DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
       len = weights[idx++];
+      DIAG_POP_NEEDS_COMMENT;
       /* Skip over indices of previous levels.  */
       for (int i = 0; i < pass; i++)
 	{
