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

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

On Thu, Nov 30, 2017 at 3:13 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> 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.

Ok, I'd like to "dumb" the pass down to the level we can solve the
bwave case (which I realize is already one of the more complicated ones).

Just because it's already late for GCC 8.

Richard.

> 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.