[PATCH] Loop unswitching: support gswitch statements.

Richard Biener richard.guenther@gmail.com
Wed Nov 10 08:59:37 GMT 2021


On Tue, Nov 9, 2021 at 5:44 PM Martin Liška <mliska@suse.cz> wrote:
>
> On 11/9/21 14:37, Richard Biener wrote:
> > On Mon, Nov 8, 2021 at 8:45 PM Andrew MacLeod <amacleod@redhat.com> wrote:
> >>
> >> On 11/8/21 10:05 AM, Martin Liška wrote:
> >>> On 9/28/21 22:39, Andrew MacLeod wrote:
> >>>> In Theory, modifying the IL should be fine, it happens already in
> >>>> places, but its not extensively tested under those conditions yet.
> >>>
> >>> Hello Andrew.
> >>>
> >>> I've just tried using a global gimple_ranger and it crashes when loop
> >>> unswitching duplicates
> >>> some BBs.
> >>>
> >>> Please try the attached patch for:
> >>
> >> hey Martin,
> >>
> >> try using this in your tree.  Since nothing else is using a growing BB
> >> right now, I'll let you work with it and see if everything works as
> >> expected before checking it in, just in case we need more tweaking.
> >> With this,
> >>
> >> make RUNTESTFLAGS=dg.exp=loop-unswitch*.c check-gcc
> >>
> >> runs clean.
> >>
> >>
> >> basically, I tried to grow it by either a factor of 10% for the current
> >> BB size when the grow is requested, or some double the needed extra
> >> size, or 128... whichever value is "maximum"    That means it shoudnt be
> >> asking for tooo much each time, but also not a minimum amount.
> >>
> >> Im certainly open to suggestion on how much to grow it each time.
> >> Note the vector being grown is ONLY fo the SSA_NAme being asked for.. so
> >> it really an on-demand thing just for specific names, in your case,
> >> mostly just the switch index.
> >>
> >> Let me know how this works for you, and if you have any other issues.
> >
> > So I think in the end we shouldn't need the growing.  Ideally we'd do all
> > the analysis before the first transform, but for that we'd need ranger to
> > be able to "simplify" conditions based on a known true/false predicate
> > that's not yet in the IL.  Consider
> >
> >   for (;;)
> >     {
> >          if (invariant < 3) // A
> >            {
> > ...
> >            }
> >          if (invariant < 5) // B
> >            {
> > ...
> >            }
> >     }
> >
> > unswitch analysis will run into the condition 'A' and determine the loop
> > can be unswitched with the condition 'invariant < 3'.  To be able to
> > perform cost assessment and to avoid redundant unswitching we
> > want to determine that if we unswitch with 'invariant < 3' being
> > true then the condition at 'B' is true as well before actually inserting
> > the if (invariant < 3) outside of the loop.
> >
> > So I'm thinking of assigning a gimple_uid to each condition we want to
> > unswitch on and have an array indexed by the uid with meta-data on
> > the unswitch opportunity, the "related" conditions could be marked with
> > the same uid (or some other), and the folding result recorded so that
> > at transform time we can just do the appropriate replacement without
> > invoking ranger again.
>
> Calculating all this before transformation is quite ambitious based on the code
> we have now.
>
> Note one can have in a loop:
>
> if (a > 100)
>     ...
>
> switch (a)
>     case 1000:
>       ...
>     case 20:
>       ...
>     case 200:
>       ...
>
> which means the first predicate effectively makes some cases unreachable. Moreover
> one can have
>
> if (a > 100 && b < 300)
>     ...
>
> and more complex conditions.

True - I guess we should do two things.

 1) keep simplify_using_entry_checks like code for symbolic conditions
 2) add integer ranges for unswitch conditions producing them, that
     includes all unswitching of switch stmts - we might be able to use
     the ranger queries (with global ranges) to simplify stmts with the
     known ranges as noted by Andrew

I do think that pre-computing the simplifications is what we should do
to be able to make the cost modeling sane.  What we can avoid
trying is evaluating multiple unswitch possibilities to pick the "best".

I think changing the code do to the analysis first should be done
before wiring in gcond support, even adding the additional 'range'
capability will be useful without that since the current code
wont figure out a > 5 is true when we unswitch on a > 3.

> >
> > Now, but how do we arrange for the ranger analysis here?
>
> That's likely something we need support from ranger, yes.
>
> >
> > We might also somehow want to remember that on the
> > 'invariant < 3' == false copy of the loop there's still the
> > unswitching opportunity on 'invariant < 5', but not on the
> > 'invariant < 5' == true copy.
> >
> > Currently unswitching uses a custom simplify_using_entry_checks
> > which tries to do simplification only after the fact (and so costing
> > also is far from costing the true cost and ordering of the opportunities
> > to do the best first is not implemented either).
>
> I'm sending updated version of the patch where I changed:
> - simplify_using_entry_checks is put back for the floating point expressions
> - all scans utilize scan-tree-dump-times
> - some new tests were added
> - global ranger is used (I rely on the growing patch provided by Andrew)
> - better ranger API is used for gcond expression: ranger.range_of_stmt (r, stmt) && r.singleton_p (&result))
> - auto_edge_flag is used now
>
> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>
> Thoughts?
> Martin
>
> >
> > Richard.
> >
> >> Andrew
> >>


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