Re: [PATCH GCC]Improve loop bound info by simplifying conversions in iv base

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Aug 20, 2015 at 10:22 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Fri, Aug 14, 2015 at 4:28 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Tue, Jul 28, 2015 at 11:38 AM, Bin Cheng <bin.cheng@arm.com> wrote:
>>> Hi,
>>> For now, SCEV may compute iv base in the form of "(signed T)((unsigned
>>> T)base + step))".  This complicates other optimizations/analysis depending
>>> on SCEV because it's hard to dive into type conversions.  For many cases,
>>> such type conversions can be simplified with additional range information
>>> implied by loop initial conditions.  This patch does such simplification.
>>> With simplified iv base, loop niter analysis can compute more accurate bound
>>> information since sensible value range can be derived for "base+step".  For
>>> example, accurate loop bound&may_be_zero information is computed for cases
>>> added by this patch.
>>> The code is actually borrowed from loop_exits_before_overflow.  Moreover,
>>> with simplified iv base, the second case handled in that function now
>>> becomes the first case.  I didn't remove that part of code because it may(?)
>>> still be visited in scev analysis itself and simple_iv isn't an interface
>>> for that.
>>>
>>> Is it OK?
>>
>> It looks quite special given it only handles a very specific pattern.  Did you
>> do any larger collecting of statistics on how many times this triggers,
>> esp. how many times simplify_using_initial_conditions succeeds and
>> how many times not?  This function is somewhat expensive.
> Yes, this is corner case targeting induction variables of small signed
> types, just like added test cases.  We need to convert it to unsigned,
> do the stepping, and convert back.  I collected statistics for gcc
> bootstrap and spec2k6.  The function is called about 400-500 times in
> both case.  About 45% of calls succeeded in bootstrap, while only ~3%
> succeeded in spec2k6.
>
> I will prepare a new version patch if you think it's worthwhile in
> terms of compilation cost and benefit.

Yes.

Richard.

> Thanks,
> bin
>>
>> +      || !operand_equal_p (iv->step,
>> +                          fold_convert (type,
>> +                                        TREE_OPERAND (e, 1)), 0))
>>
>> operand_equal_p can handle sign-differences in integer constants,
>> no need to fold_convert here.  Also if you know that you are comparing
>> integer constants please use tree_int_cst_equal_p.
>>
>> +      extreme = lower_bound_in_type (type, type);
>>
>> that's a strange function to call here (with two same types).  Looks like
>> just wide_int_to_tree (type, wi::max/min_value (type)).
>>
>> +  extreme = fold_build2 (MINUS_EXPR, type, extreme, iv->step);
>>
>> so as iv->step is an INTEGER_CST please do this whole thing using
>> wide_ints and only build trees here:
>>
>> +  e = fold_build2 (code, boolean_type_node, base, extreme);
>>
>> Thanks,
>> Richard.
>>
>>> Thanks,
>>> bin
>>>
>>> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>>>
>>>         * tree-ssa-loop-niter.c (tree_simplify_using_condition): Export
>>>         the interface.
>>>         * tree-ssa-loop-niter.h (tree_simplify_using_condition): Declare.
>>>         * tree-scalar-evolution.c (simple_iv): Simplify type conversions
>>>         in iv base using loop initial conditions.
>>>
>>> gcc/testsuite/ChangeLog
>>> 2015-07-28  Bin Cheng  <bin.cheng@arm.com>
>>>
>>>         * gcc.dg/tree-ssa/loop-bound-2.c: New test.
>>>         * gcc.dg/tree-ssa/loop-bound-4.c: New test.
>>>         * gcc.dg/tree-ssa/loop-bound-6.c: New test.