Re: [PATCH GCC][V2]A simple implementation of loop interchange

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Thu, Nov 30, 2017 at 1:01 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Nov 28, 2017 at 4:26 PM, Bin Cheng <Bin.Cheng@arm.com> wrote:
>> Hi,
>> This is updated patch with review comments resolved.  Some explanation embedded below.
>>
>>> +
>>> +  iloop->nb_iterations = nb_iterations;
>>>
>>> use std::swap?  Also I think if you can keep nb_iterations you
>>> can certainly keep the upper bounds.  You're probably
>>> afraid of the ->stmt references in the nb_iter_bound entries?
>>>
>>> Anyway, either scrap everything or try to keep everything.
>> Yeah, not only the stmts, but also the control_iv information because the SCEV
>> information may be corrupted during code transformation.
>> Now I discarded all the information.
>
> Note that given you interchange the loops but not the CFG or the loop structures
> you might want to swap loop->num and flags like ->force_vectorize.  That is,
> essentially change the ->header/latch association (and other CFG related stuff
> like recorded exits).
>
> It might also be we want to / need to disable interchange for, say,
> ->force_vectorize
> inner loops or ->unroll != 0?  Or we need to clear them, maybe
> optionally diagnosing
> that fact.
>
> At least we need to think about what it means to preserve loop
> structure (semantically,
> loop->num should maintain association to the same source-level loop
> throughout the
> compilation) for transforms like interchange.
>
>>>
>>> +  for (i = 0; oloop.reductions.iterate (i, &re); ++i)
>>> +    {
>>> +      if (re->type != DOUBLE_RTYPE)
>>> +       gcc_unreachable ();
>>> +
>>> +      use_operand_p use_p;
>>> +      imm_use_iterator iterator;
>>> +      FOR_EACH_IMM_USE_FAST (use_p, iterator, re->var)
>>> +       mark_or_remove_dbg_stmt (USE_STMT (use_p), re->var);
>>> +      FOR_EACH_IMM_USE_FAST (use_p, iterator, re->next)
>>> +       mark_or_remove_dbg_stmt (USE_STMT (use_p), re->next);
>>> +      if (TREE_CODE (re->init) == SSA_NAME)
>>> +       {
>>> +         FOR_EACH_IMM_USE_FAST (use_p, iterator, re->init)
>>> +           mark_or_remove_dbg_stmt (USE_STMT (use_p), re->init);
>>> +       }
>>>
>>> can you add a comment what you are doing here?
>>>
>>> Note that other loop opts simply scrap all debug stmts ...
>> As mentioned above, updated patch doesn't try hard to maintain debug use info any more.
>>
>>>
>>> +static void
>>> +compute_access_stride (struct loop *loop_nest, struct loop *loop,
>>> +                      data_reference_p dr)
>>> +{
>>> ...
>>> +  tree ref = DR_REF (dr);
>>> +  tree scev_base = build_fold_addr_expr (ref);
>>> +  tree access_size = TYPE_SIZE_UNIT (TREE_TYPE (ref));
>>> +  tree niters = build_int_cst (sizetype, AVG_LOOP_NITER);
>>> +  access_size = fold_build2 (MULT_EXPR, sizetype, niters, access_size);
>>> +
>>> +  do {
>>> +    tree scev_fn = analyze_scalar_evolution (loop, scev_base);
>>> +    if (chrec_contains_undetermined (scev_fn)
>>> +       || chrec_contains_symbols_defined_in_loop (scev_fn, loop->num))
>>> +      break;
>>> ...
>>> +    strides->safe_push (scev_step);
>>> +  } while (loop != loop_nest && (loop = loop_outer (loop)) != NULL);
>>> +
>>>
>>> I _think_ you want to do
>>>
>>>    scev_fn = analyze_scalar_evolution (loop, scev_base); // assuming
>>> DR_STMT (dr) is in loop
>>>    scev_fn = instantiate_parameters (nest, scev_fn);
>>>    if (chrec_contains_undetermined (scev_fn))
>>>      return; // false?
>>>
>>> and analyze the result which should be of the form
>>>
>>>   { { { init, +, step1 }_1, +, step2 }_2, + , step3 }_3 ...
>>>
>>> if canonical.  I think estimate_val_by_simplify_replace isn't needed
>>> if you do that
>>> (it also looks odd to replace all vairables in step by niter...).
>> I replied on this in previous message, instantiate_parameters doesn't always
>> give canonical form result as expected.  The loop here could be seen as a
>> local instantiate process, right?
>
> Kind of.  I'll see if I can reproduce the difference with any of your
> intercahnge
> testcases - any hint which one to look at?
For added tests, I think there will be no difference between the two.
I noticed the difference for
pointer cases like:
for (i...)
  for (j...)
    for (k...)
       p[i*n*n + j*n+ k] =...

>
>> Also estimate_val_by_simplify_replace is needed for pointers, where strides
>> are computed from niters of loops which could be non compilation time constant.
>> But yes, it's an odd fixup after I failed to do anything better.
>
> But you are for example computing _1 - _2 to zero, right?  Because both _1
> and _2 are not constant and thus you replace it with the same (symbolical)
> constant 'niter'.
>
> I think that asks for garbage-in-garbage-out ...
>
> Which testcase is this important for so I can have a look?
So far this is only for the above pointer case.  Actually I don't
think it's that important, and thought about skip it.
So we don't have to do estimate_val_by_simplify_replace.

Thanks,
bin
>
>>>
>>> I think keeping the chrec in the above form is also more suitable for what
>>> the caller does so the outermost loop is simply
>>>
>>>   loop = loop_nest;
>>>   loop-over-all-dr-chrecs
>>>     if (flow_loop_nested_p (loop, CHREC_LOOP (chrec)))
>>>       loop = CHREC_LOOP (chrec);
>>>
>>> given the outermost loop is the outer evolution.  If you sort the
>>> stride vecs from inner
>>> to outer you don't need prune_access_strides_not_in_loop.
>> Hmm, So stripping outer loops prefer inner to outer sort of strides, but cost computation
>> during interchange prefers outer to inner sort because loop_nest in tree-data-ref is sorted
>> in this way.  Seems a single prune_* function is better than fiddling with cost computation.
>
> Not sure how to interpret your answer...  I'll see to have a more
> detailed suggestion
> after playing with the code a bit.
>
>>>
>>> +/* Count and return the number of loops in LOOP_NEST.  */
>>> +
>>> +unsigned int
>>> +num_loops_in_loop_nest (struct loop *loop_nest)
>>> +{
>>> +  unsigned num_loops;
>>> +  for (num_loops = 0; loop_nest; num_loops++, loop_nest = loop_nest->inner)
>>> +    ;
>>> +  return num_loops;
>>>
>>> loop_depth (innermost) - loop_depth (nest)?
>> Done.
>>
>>>
>>> +static bool
>>> +should_interchange_loops (unsigned i_idx, unsigned o_idx,
>>> +                         vec<data_reference_p> datarefs,
>>> +                         bool innermost_loops_p, bool dump_info_p = true)
>>> +{
>>>
>>> isn't all we need associating the above CHREC to sort after the CHREC_RIGHTs
>>> and figure a permutation sequence to arrive there?  That is for the local
>>> decision you compute here it is CHREC_RIGHT [i_idx] > CHREC_RIGHT [o_idx]
>>> when we should interchange?
>>>
>>> That subloop_stride_p and tracking invariant DRs looks a bit odd.  For loops
>>> where a DR is invariant you simply do not have an evolution in that loop.
>>> You seem to simply add strides in the inner and outer loops for each DR,
>>> can you explain how that works?  Also I guess strides bigger than the
>>> various cache-line size parameters should be treated 'equal'?  That is,
>>> if we don't get any DR to a step that results in L1 cache hits because
>>> two DRs share a cache line the interchange is pointless?
>> So given below loop:
>>
>>   for (int i = 0; i < M; i++)
>>     for (int j = 0; j < M; j++)
>>       for (int k = 0; k < M; k++)
>>         a[k][0][i] = b[k][0][i]
>>
>> We check if memory reference is invariant wrto a loop only if it has zero strides within
>> current loop nest.  In this example, there is no invariant given address changes in the
>> innermost loop.
>
> But they simply wouldn't take part in the sorting?  That is, invariant
> refs in a loop
> shouldn't prevent it becoming more inner or more outer, no?
>
>> For strides bigger than cache-line size, it's also possible the interchange is wanted, as
>> in below example:
>>
>>   for (int i = 0; i < M; i++)          //loop 1
>>     for (int j = 0; j < M; j++)        //loop 2
>>       for (int k = 0; k < M; k++)    //loop 3
>>         a[j][i][k] = b[j][i][k]
>>
>> Strides for loop 1/2 are very like to be big, but after interchange, we will have stream
>> access of both arrays.
>>
>> More advanced heuristics may be possible, but so far the estimation is quite good by
>> checking all interchanges I looked into.
>>
>>>
>>> +/* Prune DATAREFS by removing any data reference not inside of LOOP.  */
>>> +
>>> +static inline void
>>> +prune_datarefs_not_in_loop (struct loop *loop, vec<data_reference_p> datarefs)
>>> +{
>>> +  struct data_reference *dr;
>>> +
>>> +  for (unsigned i = 0; datarefs.iterate (i, &dr);)
>>> +    if (flow_bb_inside_loop_p (loop, gimple_bb (DR_STMT (dr))))
>>> +      i++;
>>> +    else
>>> +      {
>>> +       datarefs.ordered_remove (i);
>>>
>>> that's expensive.  It's better to keep moving DRs we want to keep
>>> when walking the array.  That is, add a j you increment only when
>>> we keep a DR, moving *i to *j.
>> Done.
>>
>>>
>>> +       if (dr->aux)
>>> +         {
>>> +           DR_ACCESS_STRIDE (dr)->release ();
>>> +           free (dr->aux);
>>> +         }
>>> +       free_data_ref (dr);
>>> +      }
>>> +}
>>>
>>> +
>>> +  start_loop = prune_non_rectangle_loop_nest (innermost_loop, start_loop);
>>> +
>>>
>>> Hmm.  If you instantiate the SCEV for the niters for each loop in the nest
>>> wrt the nest you can figure if it has any evolution in sth else than the
>>> loop (evolution_function_is_univariate_p).   That is, this is not a problem
>>> until you arrive at analyzing DR strides, right?  At which point you
>>> can check for the appropriate form?
>> Hmm, not really.  The niter relation may not appear in SCEV of reference addr.
>> For example, below loop:
>>
>>   for (int i = 0; i < M; i++)          //loop 1
>>     for (int j = 0; j < M; j++)        //loop 2
>>       for (int k = 0; k < i; k++)    //loop 3
>>         a[k][0][i] = b[k][0][i]
>>
>> There is no information in data reference about i/j loops.
>> Anyway, I refactored the code and put this check in proper_loop_form_for_interchange.
>> Simpler I think.
>>
>>>
>>> +  if (find_data_references_in_loop (start_loop, datarefs) == chrec_dont_know
>>> +      /* Check if there is no data reference.  */
>>> +      || datarefs->length () == 0
>>> +      /* Check if there are too many of data references.  */
>>> +      || ((int) datarefs->length ()
>>> +         > PARAM_VALUE (PARAM_LOOP_MAX_DATAREFS_FOR_DATADEPS))
>>> +      /* Check if there is any data reference in loop latch.  We can't handle
>>> +        loops which loop header and data references have different execution
>>> +        times.  */
>>> +      || dataref_niters_diff_to_loop_header (*datarefs)
>>>
>>> this suggests to do your own find_data_references_in_loop so you can early
>>> out.
>> I refactored the code a bit.  Now this check is in proper_loop_form_for_interchange,
>> but I do customized my own data references finder.  It's needed to strip outer loops
>> once a difficult reference is found.
>>
>>>
>>> Overall the flow through the pass is a bit hard to follow given there are
>>> IMHO too many functions.
>> Yeah, I removed quite number of small functions and refactor the code a lot.  Hope this
>> version is more straightforward.
>>>
>>> +unsigned int
>>> +pass_linterchange::execute (function *fun)
>>> +{
>>> +  if (number_of_loops (fun) <= 2)
>>> +    return 0;
>>> +
>>> +  bool changed_p = false;;
>>> +  struct loop *loop;
>>> +  vec<loop_p> loop_nest;
>>> +  vec<data_reference_p> datarefs;
>>> +  vec<ddr_p> ddrs;
>>> +
>>> +  FOR_EACH_LOOP (loop, LI_ONLY_INNERMOST)
>>> +    if (prepare_perfect_loop_nest (loop, &loop_nest, &datarefs, &ddrs))
>>> +      {
>>> +       tree_loop_interchange loop_interchange (loop_nest, datarefs, ddrs);
>>> +       changed_p |= loop_interchange.interchange ();
>>> +      }
>>>
>>> you leak datarefs/ddrs?
>> It was release in destructor, but I refactored it anyway.  I will push the code to branch
>> gcc.gnu.org/svn/gcc/branches/gimple-linterchange.
>>
>> Thanks again for the comment of you two.
>
> Digging into the code now...
>
> Richard.
>
>> Thanks,
>> bin
>> 2017-11-27  Bin Cheng  <bin.cheng@arm.com>
>>
>>         * Makefile.in (gimple-loop-interchange.o): New object file.
>>         * common.opt (floop-interchange): Reuse the option from graphite.
>>         * doc/invoke.texi (-floop-interchange): Ditto.  New document.
>>         * gimple-loop-interchange.cc: New file.
>>         * params.def (PARAM_LOOP_INTERCHANGE_MAX_NUM_STMTS): New parameter.
>>         (PARAM_LOOP_INTERCHANGE_STRIDE_RATIO): New parameter.
>>         * passes.def (pass_linterchange): New pass.
>>         * timevar.def (TV_LINTERCHANGE): New time var.
>>         * tree-pass.h (make_pass_linterchange): New declaration.
>>         * tree-ssa-loop-ivcanon.c (create_canonical_iv): Change to external
>>         interchange.  Record IV before/after increment in new parameters.
>>         * tree-ssa-loop-ivopts.h (create_canonical_iv): New declaration.
>>
>> gcc/testsuite
>> 2017-11-27  Bin Cheng  <bin.cheng@arm.com>
>>
>>         * gcc.dg/tree-ssa/loop-interchange-1.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-2.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-3.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-4.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-5.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-6.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-7.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-8.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-9.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-10.c: New test.
>>         * gcc.dg/tree-ssa/loop-interchange-11.c: New test.