[Bug tree-optimization/104543] [12 Regression] wrong code at -O3 on x86_64-linux-gnu

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104543

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matz at gcc dot gnu.org

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Hmm, no.  The issue seems to be that the outer loop is head-controlled which
means unrolling duplicates the exit check in the middle of the fused loop.
I think we miss a check for an empty header of the outer loop, the
perfect nest check is too simplistic here.  We have

  b = 0;
  do
    {
       if (b > 2)
         break;
       c = 0;
       do
         {
           a[c] ^= 1;
           ++c
         }
       while (c <= 2);
       ++b;
    }
  while (1);

and fusing ends up with something like

  b = 0;
  do
    {
       c = 0;
       do
         {
           a[c] ^= 1;
           if (b > 2)
             break;
           a[c] ^= 1;
         }
       while (c <= 2);
       b+=2;
    }
  while (1);

in particular the comment in bb_prevents_fusion_p saying

  /* BB is duplicated by outer unrolling and then all N-1 first copies
     move into the body of the fused inner loop.  If BB exits the outer loop
     the last copy still does so, and the first N-1 copies are cancelled
     by loop unrolling, so also after fusion it's the exit block.

looks wrong.  Yes, the first is cancelled but the remaining is not the
exit block.