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[mlir][IR] Set insertion point when erasing an operation #146955

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[mlir][IR] Set insertion point when erasing an operation #146955

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[mlir][IR][WIP] Set insertion point when erasing an operation
matthias-springer Jul 3, 2025
c2c125f
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matthias-springer Jul 4, 2025
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38 changes: 38 additions & 0 deletions mlir/lib/IR/PatternMatch.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -152,12 +152,45 @@ void RewriterBase::replaceOp(Operation *op, Operation *newOp) {
eraseOp(op);
}

/// Returns the given block iterator if it lies within the block `b`.
/// Otherwise, otherwise finds the ancestor of the given block iterator that
/// lies within `b`. Returns and "empty" iterator if the latter fails.
///
/// Note: This is a variant of Block::findAncestorOpInBlock that operates on
/// block iterators instead of ops.
static std::pair<Block *, Block::iterator>
findAncestorIteratorInBlock(Block *b, Block *itBlock, Block::iterator it) {
// Case 1: The iterator lies within the block.
if (itBlock == b)
return std::make_pair(itBlock, it);

// Otherwise: Find ancestor iterator. Bail if we run out of parent ops.
Operation *parentOp = itBlock->getParentOp();
if (!parentOp)
return std::make_pair(static_cast<Block *>(nullptr), Block::iterator());
Operation *op = b->findAncestorOpInBlock(*parentOp);
if (!op)
return std::make_pair(static_cast<Block *>(nullptr), Block::iterator());
return std::make_pair(op->getBlock(), op->getIterator());
}

/// This method erases an operation that is known to have no uses. The uses of
/// the given operation *must* be known to be dead.
void RewriterBase::eraseOp(Operation *op) {
assert(op->use_empty() && "expected 'op' to have no uses");
auto *rewriteListener = dyn_cast_if_present<Listener>(listener);

// If the current insertion point is before/within the erased operation, we
// need to adjust the insertion point to be after the operation.
if (getInsertionBlock()) {
Block *insertionBlock;
Block::iterator insertionPoint;
std::tie(insertionBlock, insertionPoint) = findAncestorIteratorInBlock(
op->getBlock(), getInsertionBlock(), getInsertionPoint());
if (insertionBlock && insertionPoint == op->getIterator())
setInsertionPointAfter(op);
}
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What do you think about simplifying this to just:

  if (getInsertionBlock())
    if (getInsertionPoint() == op->getIterator())
      setInsertionPointAfter(op);

I agree with you that we don't want to make this too expensive. I think the above should cover the cases discussed in #146908 and is a rather very small cost.

I personally do not have expectations of the insertion point still working if the block is being deleted that one is inserting, but I think few users are thinking "My insertion point is before this op and the op is used as an anchor inside the builder to insert new ops, therefore I now need to set the insertion point again after I deleted the op". Even less so in patterns where the framework sets the insertion point for you!


// Fast path: If no listener is attached, the op can be dropped in one go.
if (!rewriteListener) {
op->erase();
Expand Down Expand Up @@ -322,6 +355,11 @@ void RewriterBase::inlineBlockBefore(Block *source, Block *dest,
moveOpBefore(&source->front(), dest, before);
}

// If the current insertion point is within the source block, adjust the
// insertion point to the destination block.
if (getInsertionBlock() == source)
setInsertionPoint(dest, getInsertionPoint());
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Does this deserve an API documentation update as well?


// Erase the source block.
assert(source->empty() && "expected 'source' to be empty");
eraseBlock(source);
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