[llvm-dev] Load combine pass

Philip Reames via llvm-dev
Wed, 25 Sep 2019 00:12:33 -0000

If we do load combining at the IR level, one thing we'll need to give
some thought to is atomicity.  Combining two atomic loads into a wider
(legal) atomic load is not a reversible transformation given our current
specification.

I've been thinking about a concept I've been tentatively calling
"element wise atomicity" which would make this a reversible transform
by
representing the load as either a vector or struct load on which the
atomicity requirement was clearly expressed on the individual elements,
not the vector/struct itself.  (We have no way to spell that today.)

Not having load combining be a reversible transformation is really
unfortunate.  Consider this small example:

store i32 0, a+4
load i32, a
load i32, a+4

If you happen to visit the load pair first, you really want to be able
to split them again once you discover the store.

Note that the same theoretical problem exists in MI/SelectionDAG.  We
just do so much less memory optimization there we get away with mostly
not caring.  :)

Philip

On 9/24/2019 4:50 PM, Artur Pilipenko via llvm-dev
wrote:
> Implementing load combine/widening on the IR level sounds like a
> reasonable enhancement to me. Although, we (Azul) don't have any plans
> of doing that in the near future.
>
> Artur
>
>> On 12 Sep 2019, at 00:58, Paweł Bylica <chfast at gmail.com
>> <mailto:chfast at gmail.com>> wrote:
>>
>> Ok, thanks.
>>
>> Are there any plans to reintroduce it on the IR level? I'm not
>> confident this is strictly necessary, but in some cases not having
>> load widening ends up really bad.
>> Like in the case where vectorizer tries to do something about it:
>> https://godbolt.org/z/60RuEw
>> https://bugs.llvm.org/show_bug.cgi?id=42708
>>
>> At the current state I'm forced to use memset() to express uint64
>> load from an array of bytes.
>>
>> // P.
>>
>> On Thu, Sep 12, 2019 at 4:22 AM Artur Pilipenko <apilipenko at
azul.com
>> <mailto:apilipenko at azul.com>> wrote:
>>
>>     Hi,
>>
>>     Load widening/combine for the original pattern was implemented in
>>     DAG combiner. See:
>>
https://github.com/llvm/llvm-project/blob/23bbeb52f392d88bf0d0527392a7a11561ee09c0/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L6872
>>
>>     Later a similar transform was introduced fro stores:
>>
https://github.com/llvm/llvm-project/blob/23bbeb52f392d88bf0d0527392a7a11561ee09c0/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L6700
>>
>>     There is no load widening on the IR level as of now.
>>
>>     Artur
>>
>>>     On 11 Sep 2019, at 04:47, Paweł Bylica <chfast at gmail.com
>>>     <mailto:chfast at gmail.com>> wrote:
>>>
>>>     Hi,
>>>
>>>     Can I ask what is the status of load widening. It seems there
is
>>>     no load widening on IR at all.
>>>
>>>     // Paweł
>>>
>>>     On Wed, Oct 5, 2016 at 1:49 PM Artur Pilipenko via llvm-dev
>>>     <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>>
>>>         Philip and I talked about this is person. Given the fact
>>>         that load widening in presence of atomics is irreversible
>>>         transformation we agreed that we don't want to do this
>>>         early. For now it can be implemented as a peephole
>>>         optimization over machine IR. MIR is preferred here because
>>>         X86 backend does GEP reassociation at MIR level and it can
>>>         make information about addresses being adjacent available.
>>>
>>>         Inline cost of the original pattern is a valid concern and
>>>         we might want to revisit our decision later. But in order
to
>>>         do widening earlier we need to have a way to undo this
>>>         transformation.
>>>
>>>         I’m going to try implementing MIR optimization but not in
>>>         the immediate future.
>>>
>>>         Artur
>>>
>>>         > On 28 Sep 2016, at 19:32, Artur Pilipenko
>>>         <apilipenko at azulsystems.com
>>>         <mailto:apilipenko at azulsystems.com>> wrote:
>>>         >
>>>         > One of the arguments for doing this earlier is inline
cost
>>>         perception of the original pattern. Reading i32/i64 by
bytes
>>>         look much more expensive than it is and can prevent
inlining
>>>         of interesting function.
>>>         >
>>>         > Inhibiting other optimizations concern can be
addressed by
>>>         careful selection of the pattern we’d like to match. I
limit
>>>         the transformation to the case when all the individual have
>>>         no uses other than forming a wider load. In this case it’s
>>>         less likely to loose information during this
transformation.
>>>         >
>>>         > I didn’t think of atomicity aspect though.
>>>         >
>>>         > Artur
>>>         >
>>>         >> On 28 Sep 2016, at 18:50, Philip Reames
>>>         <listmail at philipreames.com
>>>         <mailto:listmail at philipreames.com>> wrote:
>>>         >>
>>>         >> There's a bit of additional context worth
adding here...
>>>         >>
>>>         >> Up until very recently, we had a form of widening
>>>         implemented in GVN.  We decided to remove this in
>>>         https://reviews.llvm.org/D24096
>>>         <https://reviews.llvm.org/D24096> precisely because
its
>>>         placement in the pass pipeline was inhibiting other
>>>         optimizations. There's also a major problem with doing
>>>         widening at the IR level which is that widening a pair of
>>>         atomic loads into a single wider atomic load can not be
>>>         undone. This creates a major pass ordering problem of its
own.
>>>         >>
>>>         >> At this point, my general view is that widening
>>>         transformations of any kind should be done very late.
>>>         Ideally, this is something the backend would do, but doing
>>>         it as a CGP like fixup pass over the IR is also reasonable.
>>>         >>
>>>         >> With that in mind, I feel both the current
placement of
>>>         LoadCombine (within the inliner iteration) and the proposed
>>>         InstCombine rule are undesirable.
>>>         >>
>>>         >> Philip
>>>         >>
>>>         >>
>>>         >> On 09/28/2016 08:22 AM, Artur Pilipenko wrote:
>>>         >>> Hi,
>>>         >>>
>>>         >>> I'm trying to optimize a pattern like this
into a single
>>>         i16 load:
>>>         >>> %1 = bitcast i16* %pData to i8*
>>>         >>> %2 = load i8, i8* %1, align 1
>>>         >>> %3 = zext i8 %2 to i16
>>>         >>> %4 = shl nuw i16 %3, 8
>>>         >>> %5 = getelementptr inbounds i8, i8* %1, i16 1
>>>         >>> %6 = load i8, i8* %5, align 1
>>>         >>> %7 = zext i8 %6 to i16
>>>         >>> %8 = shl nuw nsw i16 %7, 0
>>>         >>> %9 = or i16 %8, %4
>>>         >>>
>>>         >>> I came across load combine pass which is
motivated by
>>>         virtualliy the same pattern. Load combine optimizes the
>>>         pattern by combining adjacent loads into one load and lets
>>>         the rest of the optimizer cleanup the rest. From what I see
>>>         on the initial review for load combine
>>>         (https://reviews.llvm.org/D3580) it was not enabled by
>>>         default because it caused some performance regressions.
It's
>>>         not very surprising, I see how this type of widening can
>>>         obscure some facts for the rest of the optimizer.
>>>         >>>
>>>         >>> I can't find any backstory for this pass,
why was it
>>>         chosen to optimize the pattern in question in this way?
What
>>>         is the current status of this pass?
>>>         >>>
>>>         >>> I have an alternative implementation for it
locally. I
>>>         implemented an instcombine rule similar to recognise
>>>         bswap/bitreverse idiom. It relies on collectBitParts
>>>         (Local.cpp) to determine the origin of the bits in a given
>>>         or value. If all the bits are happen to be loaded from
>>>         adjacent locations it replaces the or with a single load or
>>>         a load plus bswap.
>>>         >>>
>>>         >>> If the alternative approach sounds reasonable
I'll post
>>>         my patches for review.
>>>         >>>
>>>         >>> Artur
>>>         >>
>>>         >
>>>
>>>         _______________________________________________
>>>         LLVM Developers mailing list
>>>         llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>>         http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>
>
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