Re: [PATCH] rtl-optimization/110587 - speedup find_hard_regno_for_1

[Richard Biener via Gcc-patches]

On Mon, 31 Jul 2023, Jeff Law wrote:

> 
> 
> On 7/31/23 04:54, Richard Biener via Gcc-patches wrote:
> > On Tue, 25 Jul 2023, Richard Biener wrote:
> > 
> >> The following applies a micro-optimization to find_hard_regno_for_1,
> >> re-ordering the check so we can easily jump-thread by using an else.
> >> This reduces the time spent in this function by 15% for the testcase
> >> in the PR.
> >>
> >> Bootstrap & regtest running on x86_64-unknown-linux-gnu, OK if that
> >> passes?
> > 
> > Ping.
> > 
> >> Thanks,
> >> Richard.
> >>
> >>  PR rtl-optimization/110587
> >>  * lra-assigns.cc (find_hard_regno_for_1): Re-order checks.
> >> ---
> >>   gcc/lra-assigns.cc | 9 +++++----
> >>   1 file changed, 5 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/gcc/lra-assigns.cc b/gcc/lra-assigns.cc
> >> index b8582dcafff..d2ebcfd5056 100644
> >> --- a/gcc/lra-assigns.cc
> >> +++ b/gcc/lra-assigns.cc
> >> @@ -522,14 +522,15 @@ find_hard_regno_for_1 (int regno, int *cost, int
> >> @@ try_only_hard_regno,
> >>           r2 != NULL;
> >>           r2 = r2->start_next)
> >>        {
> >> -        if (r2->regno >= lra_constraint_new_regno_start
> >> +        if (live_pseudos_reg_renumber[r2->regno] < 0
> >> +            && r2->regno >= lra_constraint_new_regno_start
> >>       && lra_reg_info[r2->regno].preferred_hard_regno1 >= 0
> >> -            && live_pseudos_reg_renumber[r2->regno] < 0
> >>       && rclass_intersect_p[regno_allocno_class_array[r2->regno]])
> >>     sparseset_set_bit (conflict_reload_and_inheritance_pseudos,
> >>                               r2->regno);
> >> -        if (live_pseudos_reg_renumber[r2->regno] >= 0
> >> -            && rclass_intersect_p[regno_allocno_class_array[r2->regno]])
> >> +        else if (live_pseudos_reg_renumber[r2->regno] >= 0
> >> +                 && rclass_intersect_p
> >> +                      [regno_allocno_class_array[r2->regno]])
> >>     sparseset_set_bit (live_range_hard_reg_pseudos, r2->regno);
> My biggest concern here would be r2->regno < 0  in the new code which could
> cause an OOB array reference in the first condition of the test.
> 
> Isn't that the point if the original ordering?  Test that r2->regno is
> reasonable before using it as an array index?

Note the original code is

              if (r2->regno >= lra_constraint_new_regno_start
...
              if (live_pseudos_reg_renumber[r2->regno] >= 0
...

so we are going to access live_pseudos_reg_renumber[r2->regno]
independent on the r2->regno >= lra_constraint_new_regno_start check,
so I don't think that's the point of the original ordering.  Note
I preserved the ordering with respect to other array accesses,
the speedup seen is because we now have the


   if (live_pseudos_reg_renumber[r2->regno] < 0
       ...
   else if (live_pseudos_reg_renumber[r2->regno] >= 0
            ...

structure directly exposed which helps the compiler.

I think the check on r2->regno is to decide whether to alter
conflict_reload_and_inheritance_pseudos or
live_range_hard_reg_pseudos (so it's also somewhat natural to check
that first).

Thanks,
Richard.