------- Comment #5 from dave at hiauly1 dot hia dot nrc dot ca 2010-07-11
15:17 -------
Subject: Re: [4.6 Regression] FAIL: gcc.dg/pr35258.c
execution test
On Sun, 11 Jul 2010, rguenth at gcc dot gnu dot org wrote:
>
>
> ------- 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).
Yes, I don't believe that there ever was a general provision to handle
improperly aligned pointer dereferences in expansion. However, I think
memcpy was special.
> 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).
The test always passed before. I've attached the .expand file generated using
the 4.5 branch (32-bit) for comparison.
Dave
------- Comment #6 from dave at hiauly1 dot hia dot nrc dot ca 2010-07-11
15:17 -------
Created an attachment (id=21179)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21179&action=view)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44903
