Re: [PATCH] tcg: `reachable_code_pass()` remove empty else-branch

[Anton Johansson]

On 3/1/23 23:37, Taylor Simpson wrote:
> > diff --git a/tcg/tcg.c b/tcg/tcg.c
> > index a4a3da6804..531bc74231 100644
> > --- a/tcg/tcg.c
> > +++ b/tcg/tcg.c
> > @@ -2664,21 +2664,40 @@ static void reachable_code_pass(TCGContext *s)
> >                   dead = false;
> >                   remove = false;
> >
> > -                /*
> > -                 * Optimization can fold conditional branches to unconditional.
> > -                 * If we find a label with one reference which is preceded by
> > -                 * an unconditional branch to it, remove both.  This needed to
> > -                 * wait until the dead code in between them was removed.
> > -                 */
> > -                if (label->refs == 1) {
> > -                    TCGOp *op_prev = QTAILQ_PREV(op, link);
> Can't we just insert a while loop here to move op_prev back across labels?
>
>      while (op_next->opc == INDEX_op_set_label) {
>          op_prev = QTAILQ_PREV(op, op_prev);
>      }
>
> > -                    if (op_prev->opc == INDEX_op_br &&
> > -                        label == arg_label(op_prev->args[0])) {
Ah I see, you're thinking something like

    dead = false;
    remove = false;

    if (label->refs == 1) {
        TCGOp *op_prev = NULL;
        do {
            op_prev = QTAILQ_PREV(op_prev, link);
        } while (op_prev->opc == INDEX_op_set_label);

        if (op_prev->opc == INDEX_op_br &&
            label == arg_label(op_prev->args[0])) {
            tcg_op_remove(s, op_prev);
            remove = true;
        }
    }

to handle

    br
    set_label
    ...
    set_label

I do like being able to combine both branches, and we're no longer 
relying on
the next iteration of the loop to clean up the final set_label. Since we 
won't
ever encounter more than one intermediate set_label this might be overkill,
but either way I think it's an improvement.

Thanks for the feedback, I'll resubmit with this change! :)

--
Anton Johansson,
rev.ng Labs Srl.