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[NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient #111514

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Merged
merged 4 commits into from
Oct 15, 2024
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[NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient #111514

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389ab09
[NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient
david-arm Oct 8, 2024
6c45224
Address review comments
david-arm Oct 11, 2024
3da4e4e
Make new replaceXYZ functions private
david-arm Oct 14, 2024
d1b45da
Ensure reassociateBlocks leaves Old and New in a decent state
david-arm Oct 15, 2024
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13 changes: 6 additions & 7 deletions llvm/lib/Transforms/Vectorize/VPlan.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1004,13 +1004,12 @@ static void replaceVPBBWithIRVPBB(VPBasicBlock *VPBB, BasicBlock *IRBB) {
assert(!R.isPhi() && "Tried to move phi recipe to end of block");
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@ayalz ayalz Oct 25, 2024

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While we're here, worth adding in documentation above that VPBB, having a single predecessor, is expected to be free of phi recipes. A more informative error message may be "VPBB to be replaced by IRBB must be free of phi recipes".

Note that, in general, replacing a VPBB with an IRBB is a temporary solution, until the full skeleton is built in VPlan. Blocks would clearly be identified as IRBB's or not, from the outset.

R.moveBefore(*IRVPBB, IRVPBB->end());
}
VPBlockBase *PredVPBB = VPBB->getSinglePredecessor();
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@ayalz ayalz Oct 25, 2024

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This assert of having a single predecessor is now lost, yet still documented. The function traverses multiple predecessors, but probably works and tested for single predecessor only, as VPBB is still expected to be free of phi recipes? Better be clear, assert and test what is supported.

VPBlockUtils::disconnectBlocks(PredVPBB, VPBB);
VPBlockUtils::connectBlocks(PredVPBB, IRVPBB);
for (auto *Succ : to_vector(VPBB->getSuccessors())) {
VPBlockUtils::connectBlocks(IRVPBB, Succ);
VPBlockUtils::disconnectBlocks(VPBB, Succ);
}

VPBlockUtils::reassociateBlocks(VPBB, IRVPBB);
IRVPBB->setPredecessors(VPBB->getPredecessors());
IRVPBB->setSuccessors(VPBB->getSuccessors());

// Any successor/predecessor lists will be cleared on deletion.
delete VPBB;
}

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29 changes: 29 additions & 0 deletions llvm/lib/Transforms/Vectorize/VPlan.h
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Expand Up @@ -556,6 +556,26 @@ class VPBlockBase {
return getEnclosingBlockWithPredecessors()->getSinglePredecessor();
}

/// This function replaces one predecessor with another, useful when
/// trying to replace an old block in the CFG with a new one.
void replacePredecessor(VPBlockBase *Old, VPBlockBase *New) {
auto I = find(Predecessors, Old);
assert(I != Predecessors.end());
assert(Old->getParent() == New->getParent() &&
"replaced predecessor must have the same parent");
*I = New;
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@fhahn fhahn Oct 13, 2024

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I think this may leaves Old in an invalid state, as we remove it from the list of predecessors here, but don't remove this from the Successors of Old. Can we also remove it from there? Otherwise the callers of the API must either remove the node itself or completely delete the block

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@david-arm david-arm Oct 14, 2024

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Hmm, but if we go down that route then surely we should also care about setting the successor of New to this as well so that we leave it in a valid state? I guess if this function was private then it wouldn't be part of the interface so it wouldn't matter as much. If we do the extra work here as you suggest, then this patch becomes less about efficiency and more about retaining ordering, which is fine but then I probably ought to change the title!

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@david-arm david-arm Oct 14, 2024 •
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For example, removePredecessor does not leave the Predecessor argument in a valid state either because this will still be a successor of Predecessor . How about I make the functions private and then we can behave like removePredecessor, unless you see value in these new interfaces being public?

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@fhahn fhahn Oct 14, 2024

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Right, let me take a look tomorrow

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@david-arm david-arm Oct 15, 2024

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Sure, no problem. I just added a new commit as I realised the the public reassociateBlocks interface was leaving Old and New in an invalid state. Hopefully, this now addresses your concern!

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@fhahn fhahn Oct 15, 2024

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Thanks this was what I was about to suggest, as this ensures that reassociateBlocks leaves the CFG in a valid state.

}

/// This function replaces one successor with another, useful when
/// trying to replace an old block in the CFG with a new one.
void replaceSuccessor(VPBlockBase *Old, VPBlockBase *New) {
auto I = find(Successors, Old);
assert(I != Successors.end());
assert(Old->getParent() == New->getParent() &&
"replaced successor must have the same parent");
*I = New;
}

/// Set a given VPBlockBase \p Successor as the single successor of this
/// VPBlockBase. This VPBlockBase is not added as predecessor of \p Successor.
/// This VPBlockBase must have no successors.
Expand Down Expand Up @@ -3862,6 +3882,15 @@ class VPBlockUtils {
To->removePredecessor(From);
}

/// Reassociate all the blocks connected to \p Old so that they now point to
/// \p New.
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@ayalz ayalz Oct 25, 2024

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Suggested change
/// \p New.
/// \p New instead.

static void reassociateBlocks(VPBlockBase *Old, VPBlockBase *New) {
for (auto *Pred : to_vector(Old->getPredecessors()))
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@ayalz ayalz Oct 25, 2024

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Is this to_vector() needed, given that Old's predecessors remain intact?

Pred->replaceSuccessor(Old, New);
for (auto *Succ : to_vector(Old->getSuccessors()))
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@ayalz ayalz Oct 25, 2024

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Is this to_vector() needed, given that Old's successors remain intact?

Succ->replacePredecessor(Old, New);
}

/// Return an iterator range over \p Range which only includes \p BlockTy
/// blocks. The accesses are casted to \p BlockTy.
template <typename BlockTy, typename T>
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