Re: [PATCH PR64705]Don't aggressively expand induction variable's base

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

On Tue, Feb 10, 2015 at 6:02 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Mon, Feb 9, 2015 at 11:33 AM, Bin Cheng <bin.cheng@arm.com> wrote:
>> The second time I missed patch in one day, I hate myself.
>> Here it is.
>
> I think the patch is reasonable but I would have used a default = NULL
> arg for 'stop' to make the patch smaller.  You don't constrain 'stop'
> to being an SSA name - any particular reason for that?  It would
The check is from the first version patch, in which I just passed the
whole IV's step to expand_simple_operations.  Yes, it should be
changed accordingly.

> make the comparison in expand_simple_operations simpler
> and it could be extended to be a bitmap of SSA name versions.
Yes, that's exactly what I want to do.  BTW, per for previous comment,
I don't think GCC expands IV's step in either IVOPT or SCEV, right?
As a result, it's unlikely to have an IV's step referring to multiple
ssa names.  And that's why I didn't extend it to a ssa name versions
bitmap.

>
> So - I'd like you to constrain 'stop' and check it like
>
>   if (TREE_CODE (expr) != SSA_NAME
Hmm, won't this effectively disable the expansion?

>      || expr == stop)
>     return expr;
>
> and declare
>
> -extern tree expand_simple_operations (tree);
> +extern tree expand_simple_operations (tree, tree = NULL_TREE);
I am still living in the C world...

>
> Ok with that change.
>
> Thanks,
> Richard.
>
>>> -----Original Message-----
>>> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
>>> owner@gcc.gnu.org] On Behalf Of Bin Cheng
>>> Sent: Monday, February 09, 2015 6:10 PM
>>> To: gcc-patches@gcc.gnu.org
>>> Subject: [PATCH PR64705]Don't aggressively expand induction variable's
>> base
>>>
>>> Hi,
>>> As comments in the PR, root cause is GCC aggressively expand induction
>>> variable's base.  This patch avoids that by adding new parameter to
>>> expand_simple_operations thus we can stop expansion whenever ssa var
>>> referred by IV's step is encountered.  As comments in patch, we could stop
>>> expanding at each ssa var referred by IV's step, but that's expensive and
>> not
>>> likely to happen, this patch only extracts the first ssa var and skips
>> expanding
>>> accordingly.
>>> For the new test case, currently the loop body is bloated as below after
>>> IVOPT:
>>>
>>> <bb 5>:
>>>   # ci_28 = PHI <ci_12(D)(4), ci_17(6)>
>>>   # ivtmp.13_31 = PHI <ivtmp.13_25(4), ivtmp.13_27(6)>
>>>   ci_17 = ci_28 + 1;
>>>   _1 = (void *) ivtmp.13_31;
>>>   MEM[base: _1, offset: 0B] = 0;
>>>   ivtmp.13_27 = ivtmp.13_31 + _26;
>>>   _34 = (unsigned long) _13;
>>>   _35 = (unsigned long) flags_8(D);
>>>   _36 = _34 - _35;
>>>   _37 = (unsigned long) step_14;
>>>   _38 = _36 - _37;
>>>   _39 = ivtmp.13_27 + _38;
>>>   _40 = _39 + 3;
>>>   iter_33 = (long int) _40;
>>>   if (len_16(D) >= iter_33)
>>>     goto <bb 6>;
>>>   else
>>>     goto <bb 7>;
>>>
>>> <bb 6>:
>>>   goto <bb 5>;
>>>
>>> And it can be improved by this patch as below:
>>>
>>> <bb 5>:
>>>   # steps_28 = PHI <steps_12(D)(4), steps_17(6)>
>>>   # iter_29 = PHI <iter_15(4), iter_21(6)>
>>>   steps_17 = steps_28 + 1;
>>>   _31 = (sizetype) iter_29;
>>>   MEM[base: flags_8(D), index: _31, offset: 0B] = 0;
>>>   iter_21 = step_14 + iter_29;
>>>   if (len_16(D) >= iter_21)
>>>     goto <bb 6>;
>>>   else
>>>     goto <bb 7>;
>>>
>>> <bb 6>:
>>>   goto <bb 5>;
>>>
>>>
>>> I think this is a corner case, it only changes several files' assembly
>> code
>>> slightly in spec2k6.  Among these files, only one of them is regression.
>> I
>>> looked into the regression and thought it was because of passes after
>> IVOPT.
>>> The IVOPT dump is at least not worse than the original version.
>>>
>>> Bootstrap and test on x86_64 and AArch64, so is it OK?
>>>
>>> 2015-02-09  Bin Cheng  <bin.cheng@arm.com>
>>>
>>>       PR tree-optimization/64705
>>>       * tree-ssa-loop-niter.h (expand_simple_operations): New
>>> parameter.
>>>       * tree-ssa-loop-niter.c (expand_simple_operations): New parameter.
>>>       (tree_simplify_using_condition_1, refine_bounds_using_guard)
>>>       (number_of_iterations_exit): Pass new argument to
>>>       expand_simple_operations.
>>>       * tree-ssa-loop-ivopts.c (extract_single_var_from_expr): New.
>>>       (find_bivs, find_givs_in_stmt_scev): Pass new argument to
>>>       expand_simple_operations.  Call extract_single_var_from_expr.
>>>       (difference_cannot_overflow_p): Pass new argument to
>>>       expand_simple_operations.
>>>
>>> gcc/testsuite/ChangeLog
>>> 2015-02-09  Bin Cheng  <bin.cheng@arm.com>
>>>
>>>       PR tree-optimization/64705
>>>       * gcc.dg/tree-ssa/pr64705.c: New test.
>>>
>>>
>>>
>>>