Re: [PATCH PR90078]Capping comp_cost computation in ivopts

[Richard Biener <richard.guenther@gmail.com>]

On Sun, May 5, 2019 at 8:03 AM bin.cheng <bin.cheng@linux.alibaba.com> wrote:
>
> > ------------------------------------------------------------------
> > Sender:Jakub Jelinek <jakub@redhat.com>
> > Sent At:2019 Apr. 17 (Wed.) 19:27
> > Recipient:Bin.Cheng <amker.cheng@gmail.com>
> > Cc:bin.cheng <bin.cheng@linux.alibaba.com>; GCC Patches <gcc-patches@gcc.gnu.org>
> > Subject:Re: [PATCH PR90078]Capping comp_cost computation in ivopts
> >
> >
> > On Wed, Apr 17, 2019 at 07:14:05PM +0800, Bin.Cheng wrote:
> > > > As
> > > > #define INFTY 10000000
> > > > what is the reason to keep the previous condition as well?
> > > > I mean, if cost1.cost == INFTY or cost2.cost == INFTY,
> > > > cost1.cost + cost2.cost >= INFTY too.
> > > > Unless costs can go negative.
> > > It's a bit complicated, but in general, costs can go negative.
> >
> > Ok, no objections from me then (but as I don't know anything about it,
> > not an ack either; you are ivopts maintainer, so you don't need one).
>
> Hi,
> The previous patch was reverted on GCC-9 because of PR90240.  PR90240 is now
> fixed by another patch.  This is the updated patch for PR90078.  It promotes type
> of ivopts cost from int to int64_t, as well as change behavior of infinite_cost overflow
> from saturation to assert.
> Please note, implicit conversions are kept in cost computation as before without
> introducing any narrowing.
>
> Bootstrap/test on x86_64 along with PR90240 patch.  Is it OK?

Do not include system headers in .c files, instead those need to be
(and are already)
included via system.h.

 /* The infinite cost.  */
-#define INFTY 10000000
+#define INFTY 1000000000L

do we actually need this?  What happens on a ilp32 host?  That is, I believe
you can drop the 'L' (it fits into an int anyways)

@@ -256,6 +259,7 @@ operator- (comp_cost cost1, comp_cost cost2)
     return infinite_cost;

   gcc_assert (!cost2.infinite_cost_p ());
+  gcc_assert (cost1.cost - cost2.cost < infinite_cost.cost);

   cost1.cost -= cost2.cost;
   cost1.complexity -= cost2.complexity;

probably a pre-existing issue, but we do not seem to handle underflow
here in general, nor check that underflow doesn't get us below -INFTY.

I guess we really don't want negative costs?  That doesn't seem to be
documented and I was also wondering why the cost isn't unsigned...

@@ -638,7 +646,7 @@ struct iv_ca
   comp_cost cand_use_cost;

   /* Total cost of candidates.  */
-  unsigned cand_cost;
+  int64_t cand_cost;

   /* Number of times each invariant variable is used.  */
   unsigned *n_inv_var_uses;

shows this "issue".  Can't we use uint64_t throughout the patch?

Otherwise this looks OK.

Thanks,
Richard.

> Thanks,
> bin
> 2019-05-05  Bin Cheng  <bin.cheng@linux.alibaba.com>
>
>         PR tree-optimization/90078
>         * tree-ssa-loop-ivopts.c (inttypes.h): Include new header file.
>         (INFTY): Increase the value for infinite cost.
>         (struct comp_cost): Promote type of members to int64_t.
>         (infinite_cost): Don't set complexity in initialization.
>         (comp_cost::operator +,-,+=,-+,/=,*=): Assert when cost computation
>         overflows to infinite_cost.
>         (adjust_setup_cost): Promote type of parameter and cost computation
>         to int64_t.
>         (struct ainc_cost_data, struct iv_ca): Promote type of member to
>         int64_t.
>         (get_scaled_computation_cost_at, determine_iv_cost): Promote type of
>         cost computation to int64_t.
>         (determine_group_iv_costs, iv_ca_dump, find_optimal_iv_set): Use
>         int64_t's format specifier in dump.
>
> 2018-05-05  Bin Cheng  <bin.cheng@linux.alibaba.com>
>
>         PR tree-optimization/90078
>         * g++.dg/tree-ssa/pr90078.C: New test.