[autovect] [patch] clean up latch blocks (+rfc)

[Zdenek Dvorak]

Hello,

> +	  FOR_EACH_SSA_DEF_OPERAND (def_p, stmt, iter, SSA_OP_ALL_DEFS)
> +	    {
> +	      tree def = DEF_FROM_PTR (def_p);
> +	      imm_use_iterator imm_iter;
> +	      use_operand_p use_p;
> +
> +	      FOR_EACH_IMM_USE_FAST (use_p, imm_iter, def)
> +		{
> +		  tree use_stmt = USE_STMT (use_p);
> +		  basic_block use_bb = bb_for_stmt (use_stmt);
> +
> +		  if (!flow_bb_inside_loop_p (loop, use_bb))

this is weird.  I don't think this can ever be true -- definitions in
the latch of the loop can only be used in the latch itself, or as an
argument of a phi node of the loop, since the loop latch cannot dominate
any other location. What you probably want to check is whether you do
not create an overlapping live range by moving the assignments.  But
this is very ugly.

I think it would be much cleaner to just persuade code sinking
not to move statements into empty loop latches (one simple way might be
to eliminate the latch forwarder blocks before sinking is run, since
sinking will only move the code to the existing blocks).

Zdenek