Re: [RFA 2/n] Don't lift loads above register using jumps in postreload-gcse.c

[Steven Bosscher]

On Wed, Sep 5, 2012 at 1:42 PM, Matthew Gretton-Dann wrote:
> Whilst this fix works for this particular case I am not sure it is the
> best fix for the general issue, and so if others have a better idea how
> to fix this I would be very happy.

postreload-gcse.c is broken in "interesting" ways. Look at this gem for example:

static bool
reg_changed_after_insn_p (rtx x, int cuid)
{
  unsigned int regno, end_regno;

  regno = REGNO (x);
  end_regno = END_HARD_REGNO (x);
  do
    if (reg_avail_info[regno] > cuid)
      return true;
  while (++regno < end_regno);
  return false;
}

So the more conservative the fix, the better :-)

The patch looks correct to me. But perhaps the pass should just punt
on blocks not ending in a simple jump in
bb_has_well_behaved_predecessors?

Ciao!
Steven