> -Original Message-
> From: Richard Biener [mailto:richard.guent...@gmail.com]
> Sent: Monday, March 30, 2020 4:39 PM
> To: xiezhiheng
> Cc: gcc@gcc.gnu.org
> Subject: Re: [QUESTION] About RTL optimization at forward propagation
>
> On Sat, Mar 28, 2020 at 4:19 AM xiezhiheng
> 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.
Thanks for replying!
I think your change will also propagate addresses from one loop to another.
If def's loop is less frequent than use's, maybe it'll get worse performance.
I suggest we can go with the following condition
DF_REF_BB(def)->loop_father == DF_REF_BB(use)->loop_father
|| flow_loop_nested_p (DF_REF_BB(use)->loop_father,
DF_REF_BB(def)->loop_father)
Also I find that function should_replace_address returns true
when it has a positive gain.
/* OLD is a memory address. Return whether it is good to use NEW instead,
for a memory access in the given MODE. */
static bool
should_replace_address (rtx old_rtx, rtx new_rtx, machine_mode mode,
addr_space_t as, bool speed)
{
int gain;
if (rtx_equal_p (old_rtx, new_rtx)
|| !memory_address_addr_space_p (mode, new_rtx, as))
return false;
/* Copy propagation is always ok. */
if (REG_P (old_rtx) && REG_P (new_rtx))
return true;
/* Prefer the new address if it is less expensive. */
gain = (address_cost (old_rtx, mode, as, speed)
- address_cost (new_rtx, mode, as, speed));
/* If the addresses have equivalent cost, prefer the new address
if it has the highest `set_src_cost'. That has the potential of
eliminating the most insns without additional costs, and it
is the same that cse.c used to do. */
if (gain == 0)
gain = (set_src_cost (new_rtx, VOIDmode, speed)
- set_src_cost (old_rtx, VOIDmode, speed));
return (gain > 0); <<<<<<<<
}
Actually we can choose the new address if they have the same cost,
because we may get some other benefits.
Compare
a) m = n + 10
MEM[m + 20] = 0
and
b) m = n + 10
MEM[n + 30] = 0
Although we have no gain in memory operation (they have the same
` address_cost' and ` set_src_cost'), we may have a chance to eliminate
the dead code ` m = n + 10' in (b).
Best regards!
>
> > diff -Nurp a/gcc/fwprop.c b/gcc/fwprop.c
> > --- a/gcc/fwprop.c 2020-03-27 03:17:50.70400 -0400
> > +++ b/gcc/fwprop.c 2020-03-27 04:58:35.14800 -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
