On Sat, Mar 28, 2020 at 4:19 AM xiezhiheng <[email protected]> wrote:
>
> Hi,
> I find there exists some restricts in function fwprop preventing it to
> forward propagate addresses into loops.
> /* Go through all the uses. df_uses_create will create new ones at the
> end, and we'll go through them as well.
>
> Do not forward propagate addresses into loops until after unrolling.
> CSE did so because it was able to fix its own mess, but we are not. */
> for (i = 0; i < DF_USES_TABLE_SIZE (); i++)
> {
> if (!propagations_left)
> break;
>
> df_ref use = DF_USES_GET (i);
> if (use)
> {
> if (DF_REF_TYPE (use) == DF_REF_REG_USE
> || DF_REF_BB (use)->loop_father == NULL
> <<<<<<<<<<
> /* The outer most loop is not really a loop. */
> || loop_outer (DF_REF_BB (use)->loop_father) == NULL)
> forward_propagate_into (use, fwprop_addr_p);
>
> else if (fwprop_addr_p)
> forward_propagate_into (use, false);
> }
> }
>
> And I have two questions.
> 1) What are the reasons or background for not forward propagating addresses
> into loops ?
> 2) Can we still forward propagate addresses if the def and use are in the
> same loop ?
> I mean something like:
The condiition indeed looks odd, the canonical way would be to check
!flow_loop_nested_p (DF_REF_BB (def)->loop_father, DF_REF_BB
(use)->loop_father)
which would allow propagating addresses defined in loops outside as well.
And loop_father should never be NULL I think.
> diff -Nurp a/gcc/fwprop.c b/gcc/fwprop.c
> --- a/gcc/fwprop.c 2020-03-27 03:17:50.704000000 -0400
> +++ b/gcc/fwprop.c 2020-03-27 04:58:35.148000000 -0400
> @@ -1573,10 +1573,12 @@ fwprop (bool fwprop_addr_p)
> df_ref use = DF_USES_GET (i);
> if (use)
> {
> + df_ref def = get_def_for_use (use);
> if (DF_REF_TYPE (use) == DF_REF_REG_USE
> || DF_REF_BB (use)->loop_father == NULL
> /* The outer most loop is not really a loop. */
> - || loop_outer (DF_REF_BB (use)->loop_father) == NULL)
> + || loop_outer (DF_REF_BB (use)->loop_father) == NULL
> + || (def && DF_REF_BB (def)->loop_father == DF_REF_BB
> (use)->loop_father))
> forward_propagate_into (use, fwprop_addr_p);
>
> else if (fwprop_addr_p)
>
> I would be grateful if anyone could help.
>
> Best regards
