[Bug target/22447] gcc produces code that generate unaligned exceptions

["falk at debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From falk at debian dot org  2005-07-12 22:27 -------
(In reply to comment #0)


> Instead of generating the code to store a byte using unaligned load/store
> instruction it uses aligned LDL/STL. The assembler code looks correct using -O0
> optimization flag and incorrect with -O1 and -O2. I tried to write smaler test
> case with the same pattern, but compiler compiles it correctly, so I am
> attaching the original preprocessed source. The function to look at:
> init_one_value.

typedef char *gptr;
typedef char my_bool;
[...]
static void init_one_value(const struct my_option *option, gptr *variable,
      longlong value)
{
  switch ((option->var_type & 127)) {
  case 2:
    *((my_bool*) variable)= (my_bool) value;
    break;
[...]

At entry of the function, variable must be a null pointer, or point
to an object of type gptr = char*. Because of the dereferencing, gcc can
assume the former, and therefore *variable can be deduced to be 8-byte
aligned. (BTW, the other cases violate aliasing rules, since
they're not covered by the "lvalue of type char" exception.)

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22447