llvm dev - Jul 2020 - How to get information about data dependencies?

Michael Kruse via llvm-dev <llvm-dev at lists.llvm.org> writes:
> LLVM has multiple dependence analyses, each with its up- and downsides:
>
>  * llvm::DependenceAnalysis
>  * llvm::LoopAccessAnalysis
Can someone explain the differences between these?  As far as I can tell
they essentially do the same thing (though perhaps one is more
precise?).  LAA seems to be used by vectorization (what else?) while DA
seems to be used by loop transformations (what else?).

I am not a loop opt guy so while I'm familiar with the basic ideas, the
details are somewhat lost on me.  Is there a reason to have two passes
or should they be combined and maintained as one pass?  If I have need
of dependence analysis it's not clear which I should use and the
comments are not much help.

                   -David

AFAIK they are independent developments. LoopAccessAnalysis was
extracted out of the LoopVectorizer in 2015, and first developed in
2013
(https://github.com/llvm/llvm-project/commit/d517976758c8674bdcd4c74457f7a83f20e432c5)

DependenceAnalysis was a from-scratch implementation from 2012
(https://github.com/llvm/llvm-project/commit/59b61b9e2c549956b1094417a72c3943c20c9234)

I do not know why LoopVectorize would not make use of
DependenceAnalysis in 2013.

Michael


Am Mo., 6. Juli 2020 um 09:51 Uhr schrieb David Greene <david.greene at
hpe.com>:
>
> Michael Kruse via llvm-dev <llvm-dev at lists.llvm.org> writes:
>
> > LLVM has multiple dependence analyses, each with its up- and
downsides:
> >
> >  * llvm::DependenceAnalysis
> >  * llvm::LoopAccessAnalysis
>
> Can someone explain the differences between these?  As far as I can tell
> they essentially do the same thing (though perhaps one is more
> precise?).  LAA seems to be used by vectorization (what else?) while DA
> seems to be used by loop transformations (what else?).
>
> I am not a loop opt guy so while I'm familiar with the basic ideas, the
> details are somewhat lost on me.  Is there a reason to have two passes
> or should they be combined and maintained as one pass?  If I have need
> of dependence analysis it's not clear which I should use and the
> comments are not much help.
>
>                    -David

Michael Kruse via llvm-dev <llvm-dev at lists.llvm.org> writes:
> AFAIK they are independent developments. LoopAccessAnalysis was
> extracted out of the LoopVectorizer in 2015, and first developed in
> 2013
(https://github.com/llvm/llvm-project/commit/d517976758c8674bdcd4c74457f7a83f20e432c5)
>
> DependenceAnalysis was a from-scratch implementation from 2012
>
(https://github.com/llvm/llvm-project/commit/59b61b9e2c549956b1094417a72c3943c20c9234)
>
> I do not know why LoopVectorize would not make use of
> DependenceAnalysis in 2013.
Is anyone looking at unifying these?  It's super confusing as things
stand.

                      -David
> Am Mo., 6. Juli 2020 um 09:51 Uhr schrieb David Greene <david.greene at
hpe.com>:
>>
>> Michael Kruse via llvm-dev <llvm-dev at lists.llvm.org> writes:
>>
>> > LLVM has multiple dependence analyses, each with its up- and
downsides:
>> >
>> >  * llvm::DependenceAnalysis
>> >  * llvm::LoopAccessAnalysis
>>
>> Can someone explain the differences between these?  As far as I can
tell
>> they essentially do the same thing (though perhaps one is more
>> precise?).  LAA seems to be used by vectorization (what else?) while DA
>> seems to be used by loop transformations (what else?).
>>
>> I am not a loop opt guy so while I'm familiar with the basic ideas,
the
>> details are somewhat lost on me.  Is there a reason to have two passes
>> or should they be combined and maintained as one pass?  If I have need
>> of dependence analysis it's not clear which I should use and the
>> comments are not much help.
>>
>>                    -David
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