Re: [RFC][PR82479] missing popcount builtin detection

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

On Mon, Mar 5, 2018 at 3:24 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Thu, Feb 8, 2018 at 1:41 AM, Kugan Vivekanandarajah
> <kugan.vivekanandarajah@linaro.org> wrote:
>> Hi Richard,
>>
>> On 1 February 2018 at 23:21, Richard Biener <richard.guenther@gmail.com> wrote:
>>> On Thu, Feb 1, 2018 at 5:07 AM, Kugan Vivekanandarajah
>>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>> Hi Richard,
>>>>
>>>> On 31 January 2018 at 21:39, Richard Biener <richard.guenther@gmail.com> wrote:
>>>>> On Wed, Jan 31, 2018 at 11:28 AM, Kugan Vivekanandarajah
>>>>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>>>> Hi Richard,
>>>>>>
>>>>>> Thanks for the review.
>>>>>> On 25 January 2018 at 20:04, Richard Biener <richard.guenther@gmail.com> wrote:
>>>>>>> On Wed, Jan 24, 2018 at 10:56 PM, Kugan Vivekanandarajah
>>>>>>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>>>>>> Hi All,
>>>>>>>>
>>>>>>>> Here is a patch for popcount builtin detection similar to LLVM. I
>>>>>>>> would like to queue this for review for next stage 1.
>>>>>>>>
>>>>>>>> 1. This is done part of loop-distribution and effective for -O3 and above.
>>>>>>>> 2. This does not distribute loop to detect popcount (like
>>>>>>>> memcpy/memmove). I dont think that happens in practice. Please correct
>>>>>>>> me if I am wrong.
>>>>>>>
>>>>>>> But then it has no business inside loop distribution but instead is
>>>>>>> doing final value
>>>>>>> replacement, right?  You are pattern-matching the whole loop after all.  I think
>>>>>>> final value replacement would already do the correct thing if you
>>>>>>> teached number of
>>>>>>> iteration analysis that niter for
>>>>>>>
>>>>>>>   <bb 3> [local count: 955630224]:
>>>>>>>   # b_11 = PHI <b_5(5), b_8(6)>
>>>>>>>   _1 = b_11 + -1;
>>>>>>>   b_8 = _1 & b_11;
>>>>>>>   if (b_8 != 0)
>>>>>>>     goto <bb 6>; [89.00%]
>>>>>>>   else
>>>>>>>     goto <bb 8>; [11.00%]
>>>>>>>
>>>>>>>   <bb 6> [local count: 850510900]:
>>>>>>>   goto <bb 3>; [100.00%]
>>>>>>
>>>>>> I am looking into this approach. What should be the scalar evolution
>>>>>> for b_8 (i.e. b & (b -1) in a loop) should be? This is not clear to me
>>>>>> and can this be represented with the scev?
>>>>>
>>>>> No, it's not affine and thus cannot be represented.  You only need the
>>>>> scalar evolution of the counting IV which is already handled and
>>>>> the number of iteration analysis needs to handle the above IV - this
>>>>> is the missing part.
>>>> Thanks for the clarification. I am now matching this loop pattern in
>>>> number_of_iterations_exit when number_of_iterations_exit_assumptions
>>>> fails. If the pattern matches, I am inserting the _builtin_popcount in
>>>> the loop preheater and setting the loop niter with this. This will be
>>>> used by the final value replacement. Is this what you wanted?
>>>
>>> No, you shouldn't insert a popcount stmt but instead the niter
>>> GENERIC tree should be a CALL_EXPR to popcount with the
>>> appropriate argument.
>>
>> Thats what I tried earlier but ran into some ICEs. I wasn't sure if
>> niter in tree_niter_desc can take such.
>>
>> Attached patch now does this. Also had to add support for CALL_EXPR in
>> few places to handle niter with CALL_EXPR. Does this look OK?
>
> Overall this looks ok - the patch includes changes in places that I don't think
> need changes such as chrec_convert_1 or extract_ops_from_tree.
> The expression_expensive_p change should be more specific than making
> all calls inexpensive as well.
>
> The verify_ssa change looks bogus, you do
>
> +  dest = gimple_phi_result (count_phi);
> +  tree var = make_ssa_name (TREE_TYPE (dest), NULL);
> +  tree fn = builtin_decl_implicit (BUILT_IN_POPCOUNT);
> +
> +  var = build_call_expr (fn, 1, src);
> +  *niter = fold_build2 (MINUS_EXPR, TREE_TYPE (dest), var,
> +                       build_int_cst (TREE_TYPE (dest), 1));
>
> why do you allocate a new SSA name here?  It seems unused
> as you overwrive 'var' with the CALL_EXPR immediately.
>
> I didn't review the pattern matching thoroughly nor the exact place you
> call it.  But
>
> +      if (check_popcount_pattern (loop, &count))
> +       {
> +         niter->assumptions = boolean_false_node;
> +         niter->control.base = NULL_TREE;
> +         niter->control.step = NULL_TREE;
> +         niter->control.no_overflow = false;
> +         niter->niter = count;
> +         niter->assumptions = boolean_true_node;
> +         niter->may_be_zero = boolean_false_node;
> +         niter->max = -1;
> +         niter->bound = NULL_TREE;
> +         niter->cmp = ERROR_MARK;
> +         return true;
> +       }
>
> simply setting may_be_zero to false looks fishy.  Try
> with -fno-tree-loop-ch.  Also max should not be negative,
> it should be the number of bits in the IV type?
>
> A related testcase could be that we can completely peel
> a loop like the following which iterates at most 8 times:
>
> int a[8];
> void foo (unsigned char ctrl)
> {
>   int c = 0;
>   while (ctrl)
>     {
>        ctrl = ctrl & (ctrl - 1);
>        a[c++] = ctrl;
>     }
> }
>
> This is now stage1 material so please update and re-post.  Maybe Bin has
> further suggestions as well.
Sorry for being late on this.  Actually I thought about popcount in
distribution before.  More like the first patch, but handled as
another distribution pattern rather than a special case.  It's a bit
strange to compute and store the info in niters.  It's also not
straight forward when/where the transformation is finally done.
I haven't looked into the details so not sure how appropriate it will
be as a distribution pattern (current ones are only about data
references).  So I am okay with this version if it's more appropriate.

Thanks,
bin
>
> Thanks,
> Richard.
>
>> Thanks,
>> Kugan
>>
>>
>> gcc/ChangeLog:
>>
>> 2018-02-08  Kugan Vivekanandarajah  <kuganv@linaro.org>
>>
>>     * gimple-expr.c (extract_ops_from_tree): Handle CALL_EXPR.
>>     * tree-chrec.c (chrec_convert_1): Likewise.
>>     * tree-scalar-evolution.c (expression_expensive_p): Likewise.
>>     * tree-ssa-loop-ivopts.c (contains_abnormal_ssa_name_p): Likewise.
>>     * tree-ssa-loop-niter.c (check_popcount_pattern): New.
>>     (number_of_iterations_exit): Record niter for popcount patern.
>>     * tree-ssa.c (verify_ssa): Check stmt to be non NULL.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 2018-02-08  Kugan Vivekanandarajah  <kuganv@linaro.org>
>>
>>     * gcc.dg/tree-ssa/popcount.c: New test.
>>
>>
>>>
>>>> In general, there is also a condition gating the loop like
>>>>
>>>> if (b_4 != 0)
>>>>   goto loop;
>>>> else
>>>>   end:
>>>>
>>>> This of course will not be removed by final value replacement. Since
>>>> popcount (0) is defined, this is redundant and could be removed but
>>>> not removed.
>>>
>>> Yeah, that's probably sth for another pass though.  I suppose the
>>> end: case just uses zero in which case you'll have a PHI and you
>>> can optimize
>>>
>>>   if (b != 0)
>>>     return popcount (b);
>>>   return 0;
>>>
>>> in phiopt.
>>>
>>> Richard.
>>>
>>>> Thanks,
>>>> Kugan
>>>>
>>>>>
>>>>> Richard.
>>>>>
>>>>>> Thanks,
>>>>>> Kugan
>>>>>>>
>>>>>>> is related to popcount (b_5).
>>>>>>>
>>>>>>> Richard.
>>>>>>>
>>>>>>>> Bootstrapped and regression tested on aarch64-linux-gnu with no new regressions.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Kugan
>>>>>>>>
>>>>>>>> gcc/ChangeLog:
>>>>>>>>
>>>>>>>> 2018-01-25  Kugan Vivekanandarajah  <kuganv@linaro.org>
>>>>>>>>
>>>>>>>>     PR middle-end/82479
>>>>>>>>     * tree-loop-distribution.c (handle_popcount): New.
>>>>>>>>     (pass_loop_distribution::execute): Use handle_popcount.
>>>>>>>>
>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>
>>>>>>>> 2018-01-25  Kugan Vivekanandarajah  <kuganv@linaro.org>
>>>>>>>>
>>>>>>>>     PR middle-end/82479
>>>>>>>>     * gcc.dg/tree-ssa/popcount.c: New test.