[2.26 COMMITTED] Silence -O3 -Wall warning in malloc/hooks.c with GCC 7 [BZ #22052]

[2.26 COMMITTED] Silence -O3 -Wall warning in malloc/hooks.c with GCC 7 [BZ #22052]

[Florian Weimer]

From: H.J. Lu <hjl.tools@gmail.com>

realloc_check has

  unsigned char *magic_p;
...
  __libc_lock_lock (main_arena.mutex);
  const mchunkptr oldp = mem2chunk_check (oldmem, &magic_p);
  __libc_lock_unlock (main_arena.mutex);
  if (!oldp)
    malloc_printerr ("realloc(): invalid pointer");
...
  if (newmem == NULL)
    *magic_p ^= 0xFF;

with

static void malloc_printerr(const char *str) __attribute__ ((noreturn));

GCC 7 -O3 warns

hooks.c: In function ‘realloc_check’:
hooks.c:352:14: error: ‘magic_p’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     *magic_p ^= 0xFF;

due to the GCC bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82090

This patch silences GCC 7 by using DIAG_IGNORE_NEEDS_COMMENT.

	[BZ #22052]
	* malloc/hooks.c (realloc_check): Use DIAG_IGNORE_NEEDS_COMMENT
	to silence -O3 -Wall warning with GCC 7.

(cherry picked from commit 8e57c9432a2b68c8a1e7f4df28f0e8c7acc04753)

2017-10-15  H.J. Lu  <hongjiu.lu@intel.com>

	[BZ #22052]
	* malloc/hooks.c (realloc_check): Use DIAG_IGNORE_NEEDS_COMMENT
	to silence -O3 -Wall warning with GCC 7.

diff --git a/NEWS b/NEWS
index 895e2bcbf3..8810b57cd9 100644
--- a/NEWS
+++ b/NEWS
@@ -63,6 +63,7 @@ The following bugs are resolved with this release:
     occur with -O3
   [21987] Fix sparc32 bits/long-double.h
   [22051] libc: zero terminator in the middle of glibc's .eh_frame
+  [22052] malloc failed to compile with GCC 7 and -O3
   [22078] nss_files performance issue in hosts multi mode
   [22093] x86: Add x86_64 to x86-64 HWCAP
   [22095] resolv: Fix memory leak with OOM during resolv.conf parsing
diff --git a/malloc/hooks.c b/malloc/hooks.c
index 4398c0a017..2c6cebc889 100644
--- a/malloc/hooks.c
+++ b/malloc/hooks.c
@@ -347,11 +347,18 @@ realloc_check (void *oldmem, size_t bytes, const void *caller)
       newmem = _int_realloc (&main_arena, oldp, oldsize, nb);
     }
 
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (7, 0)
+  /* GCC 7 warns about magic_p may be used uninitialized.  But we never
+     reach here if magic_p is uninitialized.  */
+  DIAG_IGNORE_NEEDS_COMMENT (7, "-Wmaybe-uninitialized");
+#endif
   /* mem2chunk_check changed the magic byte in the old chunk.
      If newmem is NULL, then the old chunk will still be used though,
      so we need to invert that change here.  */
   if (newmem == NULL)
     *magic_p ^= 0xFF;
+  DIAG_POP_NEEDS_COMMENT;
 
   __libc_lock_unlock (main_arena.mutex);