See http://gcc.gnu.org/ml/gcc/2003-08/msg00959.html

Compiling glibc-2.3.2 with a gcc-3.4 snapshot, I'm seeing
a whole bunch of warnings like this:

../string/bits/string2.h:80: warning: `packed' attribute ignored
../string/bits/string2.h:81: warning: `packed' attribute ignored
...

Looks like it was bad code that was silently ignored by previous compilers.
Ulrich fixed it as follows:

===================================================================
RCS file: /cvs/glibc/libc/string/bits/string2.h,v
retrieving revision 1.68
retrieving revision 1.69
diff -u -r1.68 -r1.69
--- libc/string/bits/string2.h	2002/05/25 06:10:18	1.68
+++ libc/string/bits/string2.h	2003/08/20 00:07:37	1.69
@@ -1,5 +1,5 @@
 /* Machine-independant string function optimizations.
-   Copyright (C) 1997,1998,1999,2000,2001,2002 Free Software Foundation, Inc.
+   Copyright (C) 1997-2002, 2003 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
 
@@ -76,7 +76,7 @@
    use unaligned memory accesses.  */
 # define __STRING2_COPY_TYPE(N) \
   typedef struct { unsigned char __arr[N]; }				      \
-    __STRING2_COPY_ARR##N __attribute__ ((packed))
+    __attribute__ ((__packed__)) __STRING2_COPY_ARR##N
 __STRING2_COPY_TYPE (2);
 __STRING2_COPY_TYPE (3);
 __STRING2_COPY_TYPE (4);