[llvm-dev] A bug in DependenceAnalysis?

[Philip Pfaffe via llvm-dev]

Hi Stan,

can you share your example.bc? Can you reproduce your issue with llvm 4.0
or, better even, trunk?

Cheers,
Philip

2017-06-21 18:58 GMT+02:00 Stanislav Manilov <stanislav.manilov at gmail.com>:

> Hi Philip,
>
> Thanks for checking!
>
> I'm running my own Foo pass that registers DependenceAnalysisWrapperPass
> as a prerequisite and then I run it like so:
>
> opt -load libfoo.so -foo example.bc
>
> This is LLVM 3.9.
>
> Cheers,
>  - Stan
>
> On Wed, Jun 21, 2017 at 5:40 PM, Philip Pfaffe <philip.pfaffe at gmail.com>
> wrote:
>
>> Hi Stan,
>>
>> in both cases I get a consistent anti result. Can you show us the command
>> lines you're using? Which version of llvm is this?
>>
>> Best,
>> Philip
>>
>> 2017-06-21 17:56 GMT+02:00 Stanislav Manilov via llvm-dev <
>> llvm-dev at lists.llvm.org>:
>>
>>> Hello llvm-dev,
>>>
>>> I'm running a pass that uses the result of llvm::DependenceAnalysisWrapperPass
>>> to compute the dependencies between all instructions of a loop. I have the
>>> following two examples of code I wish to analyse:
>>>
>>> example A:
>>>
>>> ```
>>> void move_one(int *A, unsigned n) {
>>>   for (unsigned i = 0; i < n-1; ++i) {
>>>     A[i] = A[i + 1];
>>>   }
>>> }
>>> ```
>>> and example B:
>>> ```
>>> void move_one_alt(int *A, unsigned n) {
>>>   int *B = A + 1;
>>>   for (unsigned i = 0; i < n-1; ++i) {
>>>     A[i] = B[i];
>>>   }
>>> }
>>> ```
>>>
>>> I would expect that I get the same result for both A and B, namely a
>>> loop carried anti (WAR) dependence from the generated load instruction to
>>> the generated store instruction. This should be the case, because on
>>> iteration i+1 the loop is writing to the element that has been read in the
>>> previous iteration - iteration i.
>>>
>>> However, in example A I get a loop carried flow (RAW) dependence from
>>> the store instruction to the load instruction, while in example B I don't
>>> get any dependence at all.
>>>
>>> Am I missing something, or is the result wrong?
>>>
>>> Thanks,
>>>  - Stan
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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