This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [RFC PR43721] Optimize a/b and a%b to single divmod call


On 30 October 2015 at 15:57, Richard Biener <richard.guenther@gmail.com> wrote:
> On Fri, Oct 30, 2015 at 8:39 AM, Prathamesh Kulkarni
> <prathamesh.kulkarni@linaro.org> wrote:
>> Hi,
>> I have attached revamped version of Kugan's patch
>> (https://gcc.gnu.org/ml/gcc/2013-06/msg00100.html) for PR43721.
>> Test-case: http://pastebin.com/QMfpXLD9
>> divmod pass dump: http://pastebin.com/yMY1ikCp
>> Assembly: http://pastebin.com/kk2HZpvA
>>
>> The approach I took is similar to sincos pass, which involves two parts:
>> a) divmod pass that transforms:
>> t1 = a / b;
>> t2 = a % b;
>> to the following sequence:
>> complex_tmp = DIVMOD (a, b);
>> t1 = REALPART_EXPR(a);
>> t2 = IMAGPART_EXPR(b);
>>
>> b) DIVMOD(a, b) is represented as an internal-fn and is expanded by
>> expand_DIVMOD().
>> I am not sure if I have done this correctly. I was somehow looking to
>> reuse expand_divmod() but am not able to figure out how to do that
>> (AFAIU expand_divmod() strictly returns either the quotient or
>> remainder but never both which is what I want), so ended up with
>> manually expanding to rtx.
>>
>> While going through the sincos pass in execute_cse_sincos_1, I didn't
>> understand the following:
>>  if (gimple_purge_dead_eh_edges (gimple_bb (stmt)))
>>           cfg_changed = true;
>> Um why is the call to gimple_purge_dead_eh_edges necessary here?
>
> The call replacement might no longer throw.
>
>> A silly question, what options to pass to gcc to print statistics ? I
>> added statistics_counter_event to keep track of number of calls
>> inserted/used but not sure how to print them :/
>
> -fdump-tree-XXX-stats or -fdump-statistics-stats
Thanks, I can see it now -;)
>
>> I would be grateful for suggestions for improving the patch.
>
> First of all new functions need comments (expand_DIVMOD).
>
> Second - what's the reasoning for the pass placement seen?  I think
> the transform is a lowering one, improving RTL expansion.  The
> DIVMOD representation is harmful for GIMPLE optimizers.
>
> Third - I'd have integrated this with an existing pass - we have another
> lowering / RTL expansion kind pass which is pass_optimize_widening_mul.
> Please piggy-back on it.
>
> You seem to do the transform unconditionally even if the target has
> instructions for division and modulus but no instruction for divmod
> in which case you'll end up emitting a library call in the end instead
> of a div and mod instruction.  I think the transform should be gated on
> missing div/mod instructions or the availability of a divmod one.
>
> +         if (is_gimple_assign (stmt)
> +             && TREE_CODE_CLASS (gimple_assign_rhs_code (stmt)) == tcc_binary)
> +           {
> +             if (gimple_assign_rhs_code (stmt) == TRUNC_DIV_EXPR)
> +               cfg_changed |= execute_divmod_1 (stmt);
>
> you can directly check gimple_assign_rhs_code.  I'd check for TRUNC_MOD_EXPR
> which seems to be less common and thus should cut down compile-time.
>
> +  /* Case 2: When both are in top_bb */
> +  else if (op1_def_in_bb && op2_def_in_bb)
> +    {
> +      /* FIXME: better way to compare iterators?.  */
> +      gimple_stmt_iterator gsi = get_later_stmt (top_bb,
> def_stmt_op1, def_stmt_op2);
>
> You can't use get_later_stmt, it's a vectorizer speciality.  To do
> this test efficiently
> you need stmt UIDs computed.
>
> I miss a testcase (or two).
I have tried to address your comments in the attached patch.
Could you please review it for me ?

I have a few questions:

a) Is the check for availability of divmod correct ?

b) Assume TRUNC_DIV_EXPR stmt is in bb0 and TRUNC_DIV_MOD in bb1.
I choose to transform if bb0 dominates bb1 or bb1 dominates bb0 (or bb0 == bb1).
However I wonder if we should also check that if bb0 dominates bb1
then bb1 should
post-dominate bb0 ?
For instance the transform gets applied to test-case in pr43721-2.c.
bb containing rem = x % y doesn't post-dominate bb containing quot = x / y;
I wonder if we want to perform the transform for this case since control flow
may not always reach from quot = x / y to rem = x % y ?

c) A silly queston, I wonder if vec<gimple *> stmts  in convert_to_divmod() will
have at most 2 entries (assuming TRUNC_MOD_EXPR stmt is also added in
the vector) ?
I assume that cse will take place before widening_mul pass executes
(since pass_fre/pass_fre executes earlier), and there will be no
duplicate gimple stmts ?
So vec<gimple *> stmts would contain at most 2 entries - gimple stmt
with subcode  TRUNC_MOD_EXPR
and gimple stmt with subcode TRUNC_DIV_EXPR having same operands.
There won't be another gimple stmt with subcode TRUNC_DIV_EXPR with
same operands ?

d) I am not sure if I have correctly done expansion of divmod to rtx
in expand_DIVMOD() (I fear I may be missing
many checks that are in expmed.c:expand_divmod).

Thanks,
Prathamesh
>
> Richard.
>
>
>> Thank you,
>> Prathamesh

Attachment: ChangeLog.txt
Description: Text document

Attachment: patch-v2.diff
Description: Text document


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]