[SLP] Add support for throttling.

[dtemirbulatov]

Summary

Here is support for SLP throttling, when cost is high to vectorize the whole tree we can reduce the number of proposed vectorizable elements and partially vectorize the tree. https://www.youtube.com/watch?v=xxtA2XPmIug&t=5s

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3443–3446

I mean, that the check in the previous if must be Entry->State == TreeEntry::ProposedToGather, not != TreeEntry::Vectorize

3462–3465

Why?

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3462–3465

For example, in the first loops, we could change from Entry1 TreeEntry::ProposedToGather to TreeEntry::NeedToGather status, but we later could encounter another use of this Entry1 and from another Entry2()let's say) with TreeEntry::Vectorize status and we could tell difference with just canceled item and not considered to vectorize Entry. thus ExternalUses would not be properly populated.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3462–3465

The first loop does not change the state of the tree entries.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3462–3465

I mean if we merge them into one loop.

[dtemirbulatov]

oh, sorry I misspelled:
For example, in the first loops, we could change from Entry1 TreeEntry::ProposedToGather to TreeEntry::NeedToGather status, but we later could encounter another use of this Entry1 and from another Entry2()let's say) with TreeEntry::Vectorize status and we could NOT tell difference with just canceled item and not considered to vectorize Entry. thus ExternalUses would not be properly populated.

[dtemirbulatov]

Rebased, Ping

[RKSimon]

clang/tools/clang-tidy

2 ↗

(On Diff #284313)

Remove this

llvm/tools/mlir

2 ↗

(On Diff #284313)

remove this

[dtemirbulatov]

Fixed.

[RKSimon]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

7402

This looks like a NFC clang-format change now - either pre-commit or discard from the patch?

llvm/test/Transforms/SLPVectorizer/X86/load-merge.ll

59

rebase - this was committed at rG90f721404ff8

[dtemirbulatov]

Rebased, Fixed.

[dtemirbulatov]

oh, I missed to fully remove from diff at 7269, Fixed

[RKSimon]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4402–4414

Maybe pull this NDEBUG change out into its own patch?

6656

This still looks wrong - isn't the UserCost only used locally in the CompensateUseCost path?

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4402–4414

yes, it could go as NFC.

6656

No, there is another instance of UserCost at 6476, We have to compare the cost to SLPCostThreshold inside findSubTree() and subtract UserCost.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

6656

I mean the instance of usage.

[dtemirbulatov]

Rebased after rGb1600d8b8971

[RKSimon]

@ABataev @anton-afanasyev Any more comments on this?

llvm/test/Transforms/SLPVectorizer/X86/slp-throttle.ll

2–3

Is it worth adding a second RUN with -slp-throttle=false ?

[xbolva00]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

640

Please mention paper name:

“Throttling Automatic Vectorization: When Less Is More”

https://www.cl.cam.ac.uk/~tmj32/papers/docs/porpodas15-pact.pdf

Slides are good, but paper is paper :)

[dtemirbulatov]

Corrected paper citation, added -slp-throttle=false to llvm/test/Transforms/SLPVectorizer/X86/slp-throttle.ll, rebased.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3450

What if the user does not have corresponding tree entry, i.e. it is initially scalar? What if the Scalar itself is going to remain scalar?

4294–4301

Just:

for (Value *V : Entry->Scalars) {
  auto *Inst = cast<Instruction>(V);
  if (llvm::any_of(Inst->users(), [this](User *Op){ return Tree->ScalarToTreeEntry.count(Op) > 0; }))
      return InsertCost + getEntryCost(Entry);
}

Also, check code formatting

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3450

What if the Scalar itself is going to remain scalar?

At this point, the decision to cut the tree was made and the Scalar could be only with intend to vectorize. Note about that 3295 we are ignoring any tree entries without State not equals TreeEntry::Vectorize.

What if the user does not have corresponding tree entry, i.e. it is initially scalar?

ah, yes. I have to check that !UserTE at 3305 and just continue if it is true.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4294–4301

hmm, I think this is not a correct suggestion, there might be several tree entries with TreeEntry::ProposedToGather status and we have to calculate Insert cost for the whole tree here.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4294–4301

Yeah, maybe. But you van do something similar, like

InsertCost += ...
break;

instead of setting flag and do a check after the loop.

[dtemirbulatov]

Fixed remarks, rebased.

[dtemirbulatov]

Removed unnecessary check for "UserTE" at 3305.

[dtemirbulatov]

Rebased. Ping.

[xbolva00]

Good enough for initial implementation?

[dtemirbulatov]

In D57779#2258046, @xbolva00 wrote:

Good enough for initial implementation?

yes, For me, it looks like ready.

[ABataev]

In D57779#2266618, @dtemirbulatov wrote:

In D57779#2258046, @xbolva00 wrote:

Good enough for initial implementation?

yes, For me, it looks like ready.

Will be able to review it next week, after returning from vacation.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3451–3452

Could you compare it with a similar code in BoUpSLP::buildTree? Looks like you still missed some cases for user ignoring.

[dtemirbulatov]

Rebased. Moved InternalTreeUses population out of (UseScalar != U || !InTreeUserNeedToExtract(Scalar, UserInst, TLI)) limitation at line 2661 in BoUpSLP::buildTree(), since we have to consider every interal user for partial vectorization, while calculating cost.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3451–3452

I think those ignoring cases are related to the fact that we are doing full vectorization at BoUpSLP::buildTree and we can avoid extracting for in-tree users. And here we have to extract to each user of once proposed to vectorized value.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3451–3452

And here we have to extract to each user of once proposed to vectorized value. I mean for the partial vectorization.

[dtemirbulatov]

Ping

[dtemirbulatov]

Rebased. PING

[dtemirbulatov]

Rebased. Ping.

[dtemirbulatov]

Rebased. Ping^2

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

3455–3457

Either just cast without if or dyn_cast

4378

Not sure that this is the best criterion. I think you also need to include the distance from the head of the tree to the entry, because some big costs can be compensated by the vectorizable nodes in the tree.
What I would do here is just some kind of level ordering search (BFS) starting from the deepest level.

4385

I think you can also exclude entries with the number of operands <= 1.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4378

Hmm, implemented, but I don't see any benefit from that, plus we have to do BFS search. And we are going to throw away any non-vectorizable nodes at 4295.

4385

But why? The only thing that matters here is the cost.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4378

It may trigger for targets like silvermont or in future for vectorized functions.

4385

Because the main idea is to drop gathers and drop one gather in favor of another one will not be profitable for sure. But it may improve compile time and the list of candidates, The only case you need to check for is the latest masked gather case, it may be profitable to convert it to gathers for some targets.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4378

I measured the BFS approach vs this implementation. And with BFS, it is ~10% less efficient on SPEC2006 INT and ~20% less on compilable SPEC2006 FP. By efficiency, I mean the total number of reduced trees while the whole compilation.

4385

I think I can check here if scutter/gather is supported via TargetrInfo and if it is not then move all nodes with TreeEntry::ScatterVectorize to TreeEntry::Gather.

[anton-afanasyev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4385

I believe it's wrong decision to check scatter/gather target support for the reason mentioned here https://reviews.llvm.org/D92701#2435573. Why could not we just rely on costs (node cost and total one)?

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4385

I agree with @anton-afanasyev here. I am not sure what @ABataev wants here? If I exclude (operands <= 1) then we would lose have of all tests in SLP affected by throttling.

[ABataev]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4378

Could you post it anyway to check if it may be improved?

4385

I did not say anything about checking if scatter is supported here. I just said that we can improve the criterion here by checking that the entry node has at least 2 operands (because if it has just one operand, most probably we can skip it) and we just need to check the nodes with only 1 operand if it is gather scatter node, because it may be better to represent it as simple gather.

[dtemirbulatov]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

4378

ok, I might miss something. Thanks.

[dtemirbulatov]

Here is the BFS version of the change. Rebased.

[dtemirbulatov]

And I counted the total number of nodes vectorized with throttling, instead of just the number of successful tree reductions. So, the total number is higher ~25% for INT and FP CPU2006(AVX2 and AVX512F) with Cost sort compare to Distance.

[dtemirbulatov]

Discussed with @ABataev further improvements offline and he suggested removing the throttle limiter ("slp-throttling-budget"), at least for basic blocks without calls. I am looking for new functionality.

[dtemirbulatov]

Removed "slp-throttling-budget" limiter for trees without calls
Moved the main tree reduction loop to getTreeCost() function
deleted ProposedToGather node attribute out of EntryState