[RFC][PATCH] Preferred rename register in regrename pass

[Bernd Schmidt]

Hi Robert,
> gcc/
> 	* regrename.c (create_new_chain): Initialize terminated_dead,
> 	renamed and tied_chain.
> 	(find_best_rename_reg): Pick and check register from the tied chain.
> 	(regrename_do_replace): Mark head as renamed.
> 	(scan_rtx_reg): Tie chains in move insns.  Set terminate_dead flag.
> 	* regrename.h (struct du_head): Add tied_chain, renamed and
> 	terminated_dead members.

Thanks - this looks a lot better already. You didn't say how it was 
bootstrapped and tested; please include this information for future 
submissions. For a patch like this, some data on the improvement you got 
would also be appreciated.

I'd still like to investigate the possibility of further simplification:

> +	    {
> +	      /* Find the input chain.  */
> +	      for (i = c->id - 1; id_to_chain.iterate (i, &head); i--)
> +		if (head->last && head->last->insn == insn
> +		    && head->terminated_dead)
> +		  {
> +		    gcc_assert (head->regno == REGNO (recog_data.operand[1]));
> +		    c->tied_chain = head;
> +		    head->tied_chain = c;
> +
> +		    if (dump_file)
> +		      fprintf (dump_file, "Tying chain %s (%d) with %s (%d)\n",
> +			       reg_names[c->regno], c->id,
> +			       reg_names[head->regno], head->id);
> +		    /* Once tied, we're done.  */
> +		    break;
> +		  }
> +	    }
> +	}
> +
This looks like it's a little more complicated than necessary. Couldn't 
you add a static var "terminated_this_insn" which gets initialized to 
NULL and set when a reg dies, and then you check this here rather than 
having a loop? That would also eliminate the new "terminated_dead" field.

Other than that I'm pretty happy with this.


Bernd