[Bug target/44903] [4.6 Regression] FAIL: gcc.dg/pr35258.c execution test

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2010-07-11 10:47 -------
(In reply to comment #3)
> Subject: Re:  [4.6 Regression] FAIL: gcc.dg/pr35258.c
>         execution test
> 
> On Sat, 10 Jul 2010, rguenth at gcc dot gnu dot org wrote:
> 
> > I get for all memory accesses an alignment of 8 at expansion time which looks
> > correct (on i?86).  Please debug this a bit, set_mem_attributes_minus_bitpos
> > looks conservative enough.
> 
> The rtl in question is the following:
> 
> (insn 8 6 11 /test/gnu/gcc/gcc/gcc/testsuite/gcc.dg/pr35258.c:16 (set (reg:SI
> 28 %r28 [orig:94 D.1980 ] [94])
>          (mem/c:SI (plus:SI (reg/f:SI 1 %r1 [95])
>                          (const_int 1 [0x1])) [0 MEM[(char * {ref-all})&str +
> 1B]+0 S4 A8])) 37 {*pa.md:2102} (nil))
> 
> An alignment of 8 is not sufficient for a 4 byte (SImode) load on targets
> that define STRICT_ALIGNMENT.  We need an alignment of 32.
>
> I believe the i?86 hardware allows unaligned addresses, so you wouldn't
> see the problem.

Hm.  So the MEM_REF path goes the same way as the INDIRECT_REF path for

typedef int t __attribute__((aligned(1),may_alias));
int foo(t *p)
{
  return *p;
}
int main()
{
  char c[5] = {};
  if (foo(&c[1]) != 0)
    abort ();
  return 0;
}

for example on the 4.5 branch.  I see no provision to handle not properly
aligned pointer dereferences in expansion.  So I believe this is a latent
issue - but I am quite lost here in the myriads of RTL expansion (which
isn't exactly a piece of GCC I am familiar with).

In fact with Erics fix for PRxyz (all 32bit sparc tests fail) we now claim
an alignment of 32 for the integer load.  (CCing Eric - we should factor
in the alignemnt of the pointer type as minimum here).

But back to the above testcase.  On the 4.5 branch I get on i?86:

(insn 6 5 7 3 t.c:4 (set (reg:SI 58 [ D.1952 ])
        (mem:SI (reg/f:SI 60) [0 S4 A8])) -1 (nil))

(good), but with a cross to ia64-hp-hpux11.23 (I happened to have that around)

(insn 7 6 8 3 t.c:4 (set (reg/f:DI 341)
        (unspec:DI [
                (reg:SI 342)
            ] 24)) -1 (nil))

(insn 8 7 9 3 t.c:4 (set (reg:SI 339 [ D.2007 ])
        (mem:SI (reg/f:DI 341) [0 S4 A32])) -1 (nil))

thus an alignment of 32!?  A nice way of "fixing" ;)

I am curious if the above testcase works for you on the 4.5 branch (or
for any version).


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44903