Re: cprop fix for PR78626

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Thu, Dec 08, 2016 at 01:21:04PM +0100, Bernd Schmidt wrote:
> This is another case where an optimization turns a trap_if 
> unconditional. We have to defer changing the CFG, since the rest of 
> cprop seems to blow up when we modify things while scanning.
> 
> Bootstrapped and tested on x86_64-linux, and reportedly fixes the 
> problem on ppc, ok?

This fixes PR78626, but not yet PR78727.

> @@ -1825,11 +1828,26 @@ one_cprop_pass (void)
>  		       insn into a NOTE, or deleted the insn.  */
>  		if (! NOTE_P (insn) && ! insn->deleted ())
>  		  mark_oprs_set (insn);
> +
> +		if (first_uncond_trap == NULL
> +		    && GET_CODE (PATTERN (insn)) == TRAP_IF
> +		    && XEXP (PATTERN (insn), 0) == const1_rtx)
> +		  first_uncond_trap = insn;
>  	      }
> +	  if (first_uncond_trap != NULL && first_uncond_trap != BB_END (bb))
> +	    uncond_traps.safe_push (first_uncond_trap);
>  	}

The problem for PR78727 is that we also need to do this for insns that
already are the last insn in the block:

> +      while (!uncond_traps.is_empty ())
> +	{
> +	  rtx_insn *insn = uncond_traps.pop ();
> +	  basic_block to_split = BLOCK_FOR_INSN (insn);
> +	  remove_edge (split_block (to_split, insn));
> +	  emit_barrier_after_bb (to_split);
> +	}

We need that barrier, and we also need the successor edges removed
(which split_block+remove_edge does).

(PR78727 works fine with just that BB_END test deleted).


Segher