[patch, ia64] Fix unaligned accesses on IA64 from dwarf2out.c

[Steve Ellcey <sje at catbert dot cup dot hp dot com>]

Some recent changes in dwarf2out.c have caused GCC to generate unaligned
data traps on IA64 Linux.  In looking a this I tracked it down to
md5_read_ctx in libiberty, which is called by md5_finish_ctx, which in
turn is called by various routines in dwarf2out.c.

md5_read_ctx has a comment that the buffer must be 32 bit aligned but
there is nothing in dwarf2out.c to enforce this alignment to the
checksum buffer passed in to md5_finish_ctx.  I looked at calls to
md5_finish_ctx in fold-const.c to see why these didn't seem to have a
problem and noticed that the buffers used were always declared after the
md5_ctx structure and I think that 'accidentaly' got everything aligned
correctly.  If I do the same thing on the three buffer declarations in
dwarf2out.c the misaligned messages go away and things work fine.

Obviously this isn't a perfect fix, it is relying on how GCC is laying
out local variables which isn't gauranteed, but it fixes the problem and
I thought I would see if I could get approval for this simple fix or if
people think we need a more complete fix.  If we need a better fix, what
should I do?  It doesn't look like we use the alignment attribute in the
GCC code anywhere.  I could change the type of the checksum buffer from an
array of unsigned char to something that has a stronger alignment
requirement, but that would probably require a number of casts be added
where the checksum variable is used.

So, can I get someone to approve this simple, if imperfect, patch?
Tested on IA64 Linux.

Steve Ellcey
sje@cup.hp.com


2011-08-05  Steve Ellcey  <sje@cup.hp.com>

	* dwarf2out.c (generate_type_signature): Change decl order to force
	alignment.
	(compute_section_prefix): 
	(optimize_macinfo_range): Ditto.


Index: dwarf2out.c
===================================================================
--- dwarf2out.c	(revision 177422)
+++ dwarf2out.c	(working copy)
@@ -6369,8 +6369,8 @@ generate_type_signature (dw_die_ref die,
 {
   int mark;
   const char *name;
-  unsigned char checksum[16];
   struct md5_ctx ctx;
+  unsigned char checksum[16];
   dw_die_ref decl;
 
   name = get_AT_string (die, DW_AT_name);
@@ -6602,8 +6602,8 @@ compute_section_prefix (dw_die_ref unit_
   char *name = XALLOCAVEC (char, strlen (base) + 64);
   char *p;
   int i, mark;
-  unsigned char checksum[16];
   struct md5_ctx ctx;
+  unsigned char checksum[16];
 
   /* Compute the checksum of the DIE, then append part of it as hex digits to
      the name filename of the unit.  */
@@ -20662,8 +20662,8 @@ optimize_macinfo_range (unsigned int idx
 {
   macinfo_entry *first, *second, *cur, *inc;
   char linebuf[sizeof (HOST_WIDE_INT) * 3 + 1];
-  unsigned char checksum[16];
   struct md5_ctx ctx;
+  unsigned char checksum[16];
   char *grp_name, *tail;
   const char *base;
   unsigned int i, count, encoded_filename_len, linebuf_len;