[llvm-dev] Regarding Dependence distance dump

[Venkataramanan Kumar via llvm-dev]

Hi Sebastian ,

On Thu, 20 Sep 2018 at 03:57, Sebastian Pop <sebpop at gmail.com> wrote:

>
>
> On Wed, Sep 19, 2018 at 4:58 AM Venkataramanan Kumar <
> venkataramanan.kumar.llvm at gmail.com> wrote:
>
>> Hi,
>>
>> I tired to see when this behavior changed in LLVM.
>> It seems to start from.
>> --snip--
>> commit 95e5d37d5868ebde2302bc302c1e0af407c5646d
>> Author: Sebastian Pop <sebpop at gmail.com>
>> Date:   Tue Mar 6 21:55:59 2018 +0000
>>
>>     DA: remove uses of GEP, only ask SCEV
>> --snip--
>>
>> https://reviews.llvm.org/D35430
>> Review URL says "GEP" based approach was wrong for multidimensional array
>> references. The LLVM-DA now uses SCEV representation.  But why we are not
>> getting the distance with SCEV?  Current DA will not work for
>> multidimensional arrays?
>>
>
> It should work with delinearization of multi-dim arrays.
>
I thought "delinearization"  is enabled by default on trunk now.

>
>
>>
>> I need to get dependence distance for a transformation that I am working
>> on.  How do I get distance for such cases, esp multidimensional arrays?
>>
>> Thanks and regards,
>> Venkat.
>>
>> On Tue, 18 Sep 2018 at 19:00, Venkataramanan Kumar <
>> venkataramanan.kumar.llvm at gmail.com> wrote:
>>
>>> Hi,
>>>
>>> For the below test case, I need the dependence distance between the
>>> array references  "a[j][i]"  and "a[j-1][i-2]".
>>>
>>> --Snip--
>>> int a[5][5];
>>> int b[10];
>>>
>>> void test()
>>> {
>>>         for (int j=1;j<5;j++)
>>>                 for (int i=2;i<5;i++)
>>>                 {
>>>                         a[j][i] = 3.0;
>>>                         b[i]= a[j-1][i-2] ;
>>>                 }
>>> }
>>> --Snip-
>>>
>>> Compile steps
>>> clang -O1 -emit-llvm -S test.c
>>> opt  -analyze -da dist.ll
>>>
>>>
> you are missing the -delinearize flag to opt.
>
Tried this flag and I am getting
opt -analyze -delinearize -da  dist.ll
--Snip--
Delinearization on function test:
Inst:  %arrayidx6 = getelementptr inbounds [5 x [5 x i32]], [5 x [5 x
i32]]* @a, i64 0, i64 %indvars.iv30, i64 %indvars.iv
In Loop with Header: for.body4
AccessFunction: 0
failed to delinearize

Inst:  %arrayidx6 = getelementptr inbounds [5 x [5 x i32]], [5 x [5 x
i32]]* @a, i64 0, i64 %indvars.iv30, i64 %indvars.iv
In Loop with Header: for.cond1.preheader
AccessFunction: 0
failed to delinearize
.....
--Snip--

regards.
Venkat.

>
>
>> I tried dumping the dependence information with LLVM trunk (2018_09_17),
>>> it shows:
>>>
>>> Printing analysis 'Dependence Analysis' for function 'test':
>>> da analyze - none!
>>> da analyze - flow [< <>]! <== dumps direction vector
>>> da analyze - none!
>>> da analyze - none!
>>> da analyze - none!
>>> da analyze - consistent output [S 0]!
>>>
>>> LLVM Release 6.0.1 dumps the expected distance.
>>> Printing analysis 'Dependence Analysis' for function 'test':
>>> da analyze - none!
>>> da analyze - consistent flow [1 2]!
>>> da analyze - none!
>>> da analyze - none!
>>> da analyze - none!
>>> da analyze - consistent output [S 0]!
>>>
>>> In trunk, it is now showing directions instead of distances.
>>> How to get the dependence distance?
>>>
>>> regards,
>>> Venkat.
>>>
>>
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