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[MLIR] Add logging to eraseUnreachableBlocks (NFC) #153968

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Merged
merged 1 commit into from
Aug 18, 2025
+29 −4
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[MLIR] Add logging to eraseUnreachableBlocks (NFC) #153968

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33 changes: 29 additions & 4 deletions mlir/lib/Transforms/Utils/RegionUtils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -23,12 +23,15 @@
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/DebugLog.h"

#include <deque>
#include <iterator>

using namespace mlir;

#define DEBUG_TYPE "region-utils"

void mlir::replaceAllUsesInRegionWith(Value orig, Value replacement,
Region &region) {
for (auto &use : llvm::make_early_inc_range(orig.getUses())) {
Expand Down Expand Up @@ -182,19 +185,34 @@ SmallVector<Value> mlir::makeRegionIsolatedFromAbove(
// TODO: We could likely merge this with the DCE algorithm below.
LogicalResult mlir::eraseUnreachableBlocks(RewriterBase &rewriter,
MutableArrayRef<Region> regions) {
LDBG() << "Starting eraseUnreachableBlocks with " << regions.size()
<< " regions";

// Set of blocks found to be reachable within a given region.
llvm::df_iterator_default_set<Block *, 16> reachable;
// If any blocks were found to be dead.
bool erasedDeadBlocks = false;
int erasedDeadBlocks = 0;
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@jpienaar jpienaar Aug 18, 2025

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If this weren't a util, a statistic would have been useful for additional reporting.

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@joker-eph joker-eph Aug 18, 2025 •
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Instead of the API being

Returns success if any blocks were erased, failure otherwise.

We could return the number of block erased, WDYT? (I'll do it in a follow-up)

I also don't like using a "failure" when the API does not find something to do: it didn't fail, it succeeded with 0 replacements!


SmallVector<Region *, 1> worklist;
worklist.reserve(regions.size());
for (Region &region : regions)
worklist.push_back(&region);

LDBG(2) << "Initial worklist size: " << worklist.size();

while (!worklist.empty()) {
Region *region = worklist.pop_back_val();
if (region->empty())
if (region->empty()) {
LDBG(2) << "Skipping empty region";
continue;
}

LDBG(2) << "Processing region with " << region->getBlocks().size()
<< " blocks";
if (region->getParentOp())
LDBG(2) << " -> for operation: "
<< OpWithFlags(region->getParentOp(),
OpPrintingFlags().skipRegions());

// If this is a single block region, just collect the nested regions.
if (region->hasOneBlock()) {
Expand All @@ -209,13 +227,17 @@ LogicalResult mlir::eraseUnreachableBlocks(RewriterBase &rewriter,
for (Block *block : depth_first_ext(&region->front(), reachable))
(void)block /* Mark all reachable blocks */;

LDBG(2) << "Found " << reachable.size() << " reachable blocks out of "
<< region->getBlocks().size() << " total blocks";

// Collect all of the dead blocks and push the live regions onto the
// worklist.
for (Block &block : llvm::make_early_inc_range(*region)) {
if (!reachable.count(&block)) {
LDBG() << "Erasing unreachable block: " << &block;
block.dropAllDefinedValueUses();
rewriter.eraseBlock(&block);
erasedDeadBlocks = true;
++erasedDeadBlocks;
continue;
}

Expand All @@ -226,7 +248,10 @@ LogicalResult mlir::eraseUnreachableBlocks(RewriterBase &rewriter,
}
}

return success(erasedDeadBlocks);
LDBG() << "Finished eraseUnreachableBlocks, erased " << erasedDeadBlocks
<< " dead blocks";

return success(erasedDeadBlocks > 0);
}

//===----------------------------------------------------------------------===//
Expand Down