Re: [PATCH][auto-inc-dec.c] Account for cost of move operation in FORM_PRE_ADD and FORM_POST_ADD cases

[Bernd Schmidt <bschmidt@redhat.com>]

On 10/26/2015 11:40 AM, Kyrill Tkachov wrote:
> In the FORM_POST_ADD case the pass transforms:
>             *a
>             ...
>             b <- a + c
>
> into
>
>             b <- a
>             ...
>             *(b += c) post
>
>
> However, the code in attempt_change that compares the costs of the
> before and after sequences
> has an oversight. When calculating the cost of the new sequence it
> doesn't take into account the cost of the
> b <- a move. This patch fixes the calculation by calling seq_cost on the
> result of the emit_move_insn call
> we do to emit that move.

But isn't that balanced by the fact that it doesn't seem to take into 
account the gain of removing the inc_insn either? So I think this can't 
be right.

> +      new_mov_cost = seq_cost (mov_insn, speed);
> +    }
> +
> +  new_cost = new_mem_cost + new_mov_cost;

Here I'd just replace the first line with
   new_cost += seq_cost (...)
and lose the extra variable.

I seem to recall Richard had a rewrite of all the autoinc code. I wonder 
what happened to that?


Bernd