[Bug tree-optimization/54031] [4.8 Regression] Revision 189607 miscompiles Linux kernel

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54031

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-07-19 13:36:34 UTC ---
I have spotted an error in the patch, can you try

Index: gcc/builtins.c
===================================================================
--- gcc/builtins.c      (revision 189656)
+++ gcc/builtins.c      (working copy)
@@ -346,12 +346,10 @@ get_object_alignment_2 (tree exp, unsign

       known_alignment
        = get_pointer_alignment_1 (addr, &ptr_align, &ptr_bitpos);
-      bitpos += ptr_bitpos;
       align = MAX (ptr_align, align);

-      if (TREE_CODE (exp) == MEM_REF
-         || TREE_CODE (exp) == TARGET_MEM_REF)
-       bitpos += mem_ref_offset (exp).low * BITS_PER_UNIT;
+      /* The alignment of the pointer operand in a TARGET_MEM_REF
+        has to take the variable offset parts into account.  */
       if (TREE_CODE (exp) == TARGET_MEM_REF)
        {
          if (TMR_INDEX (exp))
@@ -369,9 +367,19 @@ get_object_alignment_2 (tree exp, unsign
       /* When EXP is an actual memory reference then we can use
         TYPE_ALIGN of a pointer indirection to derive alignment.
         Do so only if get_pointer_alignment_1 did not reveal absolute
-        alignment knowledge.  */
-      if (!addr_p && !known_alignment)
-       align = MAX (TYPE_ALIGN (TREE_TYPE (exp)), align);
+        alignment knowledge and if using that alignment would
+        improve the situation.  */
+      if (!addr_p && !known_alignment
+         && TYPE_ALIGN (TREE_TYPE (exp)) > align)
+       align = TYPE_ALIGN (TREE_TYPE (exp));
+      else
+       {
+         /* Else adjust bitpos accordingly.  */
+         bitpos += ptr_bitpos;
+         if (TREE_CODE (exp) == MEM_REF
+             || TREE_CODE (exp) == TARGET_MEM_REF)
+           bitpos += mem_ref_offset (exp).low * BITS_PER_UNIT;
+       }
     }
   else if (TREE_CODE (exp) == STRING_CST)
     {