Re: Ping: RFA: Improve doloop_begin support

[Joern Rennecke <joern dot rennecke at embecosm dot com>]

Quoting Zdenek Dvorak <rakdver@iuuk.mff.cuni.cz>:

> +  entered_at_top = loop_preheader_edge (loop)->dest == desc->in_edge->dest;
>
> is equivalent to
>
> +  entered_at_top = loop->header == desc->in_edge->dest;
>
> But, I don't think it will do what you want.  Loops are canonicalized so that
> their latch blocks have single successors.  So, desc->in_edge->dest will be
> the latch block, not the header, for the loop entered at the top.  I think
>
> +  entered_at_top = (loop->latch == desc->in_edge->dest
> +		     && forwarder_block_p (loop->latch));
>
> is what you want.  Other than that, the patch seems ok to me,

I'm pretty sure that I checked that this did what I wanted it to do when
I wrote the code originally.  Alas, that was a few years ago; the  
canonicalization probably has come in in the meantime.

So I made up another test and tried it:

int f(int a, int b)
{
  while (--b >= 0)
    {
      a <<= 1;
      a += 42;
    }
  return a;
}

The loop appears to be entered at the top, yet both my original and your test
fail.

Looking what happens with your test in more detail, I find that

 loop->latch == desc->in_edge->dest

is true, but forwarder_block_p (loop->latch) fails:

562           if (dest->loop_father->header == dest)
(gdb)
563             return false;
(gdb) p dest
$7 = (basic_block) 0xb7be8198
(gdb) p dest->loop_father->header
$8 = (basic_block) 0xb7be8198

The comment in front of this code says:
  /* Protect loop latches, headers and preheaders.  */

So, presumably, the loop latch will remain a forwarder block precisely because
forwarder_block_p denies that it is one.

So, may I just write:
entered_at_top = (loop->latch == desc->in_edge->dest);