Re: [PATCH] PR86844: Fix for store merging

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Sep 10, 2018 at 04:05:26PM +0200, Andreas Krebbel wrote:
> On 20.08.2018 16:30, Jeff Law wrote:
> > On 08/18/2018 03:20 AM, Eric Botcazou wrote:
> >>> Eric, didn't your patches explicitely handle this case of a non-constant
> >>> inbetween?
> >>
> >> Only if there is no overlap at all, otherwise you cannot do things simply.
> >>
> >>> Can you have a look / review here?
> >>
> >> Jakub is probably more qualified to give a definitive opinion, as he wrote 
> >> check_no_overlap and the bug is orthogonal to my patches since it is present 
> >> in 8.x; in any case, all transformations are supposed to be covered by the 
> >> testsuite.
> > FYI. Jakub is on PTO through the end of this week and will probably be
> > buried when he returns.
> 
> Jakub, could you please have a look whether that's the right fix?
> 
> https://gcc.gnu.org/ml/gcc-patches/2018-08/msg00474.html

It is a fix, but not optimal.
We have essentially:
     MEM[(int *)p_28] = 0;
     MEM[(char *)p_28 + 3B] = 1;
     MEM[(char *)p_28 + 1B] = 2;
     MEM[(char *)p_28 + 2B] = MEM[(char *)p_28 + 6B];
It is useful to merge the first 3 stores into:
     MEM[(int *)p_28] = 0x01000200; // or 0x00020001; depending on endianity
     MEM[(char *)p_28 + 2B] = MEM[(char *)p_28 + 6B];
rather than punt, and just ignore (i.e. make sure it isn't merged with
anything else) the non-INTEGER_CST store).  If you don't mind, I'll take this
PR over and handle it tomorrow.

Slightly tweaked testcase:
__attribute__((noipa)) void
foo (int *p)
{
  *p = 0;
  *((char *)p + 3) = 1;
  *((char *)p + 1) = 2;
  *((char *)p + 2) = *((char *)p + 6);
}

int
main ()
{
  int a[2] = { -1, 0 };
  if (sizeof (int) != 4)
    return 0;
  ((char *)a)[6] = 3;
  foo (a);
  if (((char *)a)[0] != 0 || ((char *)a)[1] != 2
      || ((char *)a)[2] != 3 || ((char *)a)[3] != 1)
    __builtin_abort ();
}

	Jakub