Re: [PATCH GCC]Improve bound information in loop niter analysis

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

On Mon, Aug 17, 2015 at 6:49 PM, Ajit Kumar Agarwal
<ajit.kumar.agarwal@xilinx.com> wrote:
> All:
>
> Does the Logic to calculate the Loop bound information through Value Range Analyis uses the post dominator and
> Dominator info. The iteration branches instead of Loop exit condition can be calculated through post dominator info.
> If the node in the Loop has two successors and post dominates the two successors then the iteration branch can be
> The same node.
>
> For All the nodes L in the Loop B
> If (L1, L2  belongs to successors of (L) && L1,L2 belongs to PosDom(Header of Loop))
> {
>   I = I union L1
> }
>
> Thus "I" will have all set of iteration branches. This will handle more cases of Loop bound information that
> Will be accurate through the exact iteration count that are known cases along with Value Range Information
> Where the condition is instead not the Loop exits but other nodes in the Loop.

I don't quite follow your words here.  Could you please give a simple
example about it?  Especially I don't know how post-dom helps the loop
bound analysis.  Seems your pseudo code is collecting some comparison
basic block of loop?

Thanks,
bin
>
> Thanks & Regards
> Ajit
>
>
> -----Original Message-----
> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-owner@gcc.gnu.org] On Behalf Of Bin.Cheng
> Sent: Monday, August 17, 2015 3:32 PM
> To: Richard Biener
> Cc: Bin Cheng; GCC Patches
> Subject: Re: [PATCH GCC]Improve bound information in loop niter analysis
>
> Thanks for all your reviews.
>
> On Fri, Aug 14, 2015 at 4:17 PM, Richard Biener <richard.guenther@gmail.com> wrote:
>> On Tue, Jul 28, 2015 at 11:36 AM, Bin Cheng <bin.cheng@arm.com> wrote:
>>> Hi,
>>> Loop niter computes inaccurate bound information for different loops.
>>> This patch is to improve it by using loop initial condition in
>>> determine_value_range.  Generally, loop niter is computed by
>>> subtracting start var from end var in loop exit condition.  Moreover,
>>> loop bound is computed using value range information of both start and end variables.
>>> Basic idea of this patch is to check if loop initial condition
>>> implies more range information for both start/end variables.  If yes,
>>> we refine range information and use that to compute loop bound.
>>> With this improvement, more accurate loop bound information is
>>> computed for test cases added by this patch.
>>
>> +      c0 = fold_convert (type, c0);
>> +      c1 = fold_convert (type, c1);
>> +
>> +      if (operand_equal_p (var, c0, 0))
>>
>> I believe if c0 is not already of type type operand-equal_p will never succeed.
> It's quite specific case targeting comparison between var and it's range bounds.  Given c0 is in form of "var + offc0", then the comparison "var + offc0 != range bounds" doesn't have any useful information.  Maybe useless type conversion can be handled here though, it might be even corner case.
>
>>
>> (side-note: we should get rid of the GMP use, that's expensive and now
>> we have wide-int available which should do the trick as well)
>>
>> +         /* Case of comparing with the bounds of the type.  */
>> +         if (TYPE_MIN_VALUE (type)
>> +             && operand_equal_p (c1, TYPE_MIN_VALUE (type), 0))
>> +           cmp = GT_EXPR;
>> +         if (TYPE_MAX_VALUE (type)
>> +             && operand_equal_p (c1, TYPE_MAX_VALUE (type), 0))
>> +           cmp = LT_EXPR;
>>
>> don't use TYPE_MIN/MAX_VALUE.  Instead use the types precision and all
>> wide_int operations (see match.pd wi::max_value use).
> Done.
>
>>
>> +  else if (!operand_equal_p (var, varc0, 0))
>> +    goto end_2;
>>
>> ick - goto.  We need sth like a auto_mpz class with a destructor.
> Label end_2 removed.
>
>>
>> struct auto_mpz
>> {
>>   auto_mpz () { mpz_init (m_val); }
>>   ~auto_mpz () { mpz_clear (m_val); }
>>   mpz& operator() { return m_val; }
>>   mpz m_val;
>> };
>>
>>> Is it OK?
>>
>> I see the code follows existing practice in niter analysis even though
>> my overall plan was to transition its copying of value-range related
>> optimizations to use VRP infrastructure.
> Yes, I think it's easy to push it to VRP infrastructure.  Actually from the name of the function, it's more vrp related.  For now, the function is called only by bound_difference, not so many as vrp queries.  We need cache facility in vrp otherwise it would be expensive.
>
>>
>> I'm still ok with improving the existing code on the basis that I
>> won't get to that for GCC 6.
>>
>> So - ok with the TYPE_MIN/MAX_VALUE change suggested above.
>>
>> Refactoring with auto_mpz welcome.
> That will be an independent patch, so I skipped it in this one.
>
> New version attached.  Bootstrap and test on x86_64.
>
> Thanks,
> bin
>>
>> Thanks,
>> RIchard.
>>
>>> Thanks,
>>> bin
>>>
>>> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>>>
>>>         * tree-ssa-loop-niter.c (refine_value_range_using_guard): New.
>>>         (determine_value_range): Call refine_value_range_using_guard for
>>>         each loop initial condition to improve value range.
>>>
>>> gcc/testsuite/ChangeLog
>>> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>>>
>>>         * gcc.dg/tree-ssa/loop-bound-1.c: New test.
>>>         * gcc.dg/tree-ssa/loop-bound-3.c: New test.
>>>         * gcc.dg/tree-ssa/loop-bound-5.c: New test.