Re: RFA: Make compare_and_jump_seq check that it has a jump insn

[Paolo Bonzini <bonzini at gnu dot org>]

On 06/24/2010 11:50 AM, Nick Clifton wrote:
> Hi Jeff,
>
> > OK.
>
> Thanks.
>
> > Presumably it was a port error that was causing do_compare_rtx_and_jump
> > to return a CODE_LABEL --
>
> Right.
>
> > it certainly seems quite odd to get that as
> > the final insn from a compare & jump sequence.

It can be a jump over another compare-and-jump.  do_compare_rtx_and_jump 
can return that even if the backend does not do it, Most of the time in 
the case of loop-unswitching the condition is taken straight from 
another compare-and-jump, but canon_condition may for example change LE 
to LT, then you can get into this code which creates a jump-over-jump:

  if ((! if_true_label
       || ! can_compare_p (code, mode, ccp_jump))
      && (! FLOAT_MODE_P (mode) || ...))
    {
      enum rtx_code rcode;
      if (FLOAT_MODE_P (mode))
        rcode = reverse_condition_maybe_unordered (code);
      else
        rcode = reverse_condition (code);

      /* Canonicalize to UNORDERED for the libcall.  */
      if (can_compare_p (rcode, mode, ccp_jump)
          || ... )
        {
          tem = if_true_label;
          if_true_label = if_false_label;
          if_false_label = tem;
          code = rcode;
          prob = inv (prob);
        }
    }

  ...

  if (! if_true_label)
    dummy_label = if_true_label = gen_label_rtx ();

  ...

  if (if_false_label)
    emit_jump (if_false_label);
  if (dummy_label)
    emit_label (dummy_label);

Since compare_and_jump_seq passes NULL if_false_label, if the jump is 
reversed the last emitted instruction will be a label.  I think it is 
rarely seen only because a) not many loops are unswitched at RTL level, 
b) loop unswitching is only run at -O3.

> The backend was converting:
>
> (set (cc) (compare) (foo) (bar))
> (set (pc) (if_then_else) (compare (cc) (const_int 0))
> (label)
> (pc))
>
> into:
>
> (set (cc) (reversed compare) (foo) (bar))
> (set (pc) (if_then_else (compare (cc) (const_int 0))
> (over-label)
> (pc))
> (set (pc) (label))
> (over-label)

That's exactly what do_compare_rtx_and_jump could do (and will often do 
for x86_64 floating-point jumps, but those are unswitched on the tree 
level).

Paolo