Re: [PATCH PR90078]Capping comp_cost computation in ivopts

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

On Mon, May 6, 2019 at 12:24 PM Bin.Cheng <amker.cheng@gmail.com> wrote:
>
> On Mon, May 6, 2019 at 6:11 PM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > 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)
> Yeah, now I think if int64_t is necessary or not.  With the scaling
> bound and assertions.
> >
> > @@ -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?
> Oh, it's actually explained in previous message,
> https://gcc.gnu.org/ml/gcc-patches/2019-04/msg00697.html
> In short, yes the cost can be negative.  The negative cost should be
> small in absolute value, and we don't check -INFTY.  With the scaling
> bound change, I don't think -INFTY is possible IIUC.

Ah, I see.  The patch is OK then with 1000000000L in the #define
changed to just 1000000000

Richard.

> Thanks,
> bin
> >
> > 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.