[mlir][IR] Add Block::isReachable helper function (#114928)

Add a new helper function `isReachable` to `Block`. This function
traverses all successors of a block to determine if another block is
reachable from the current block.

This functionality has been reimplemented in multiple places in MLIR.
Possibly additional copies in downstream projects. Therefore, moving it
to a common place.

Author: matthias-springer

mlir/include/mlir/IR/Block.h

@@ -16,6 +16,8 @@
 #include "mlir/IR/BlockSupport.h"
 #include "mlir/IR/Visitors.h"
 
+#include "llvm/ADT/SmallPtrSet.h"
+
 namespace llvm {
 class BitVector;
 class raw_ostream;
@@ -264,6 +266,19 @@ class alignas(8) Block : public IRObjectWithUseList<BlockOperand>,
   succ_iterator succ_end() { return getSuccessors().end(); }
   SuccessorRange getSuccessors() { return SuccessorRange(this); }
 
+  /// Return "true" if there is a path from this block to the given block
+  /// (according to the successors relationship). Both blocks must be in the
+  /// same region. Paths that contain a block from `except` do not count.
+  /// This function returns "false" if `other` is in `except`.
+  ///
+  /// Note: This function performs a block graph traversal and its complexity
+  /// linear in the number of blocks in the parent region.
+  ///
+  /// Note: Reachability is a necessary but insufficient condition for
+  /// dominance. Do not use this function in places where you need to check for
+  /// dominance.
+  bool isReachable(Block *other, SmallPtrSet<Block *, 16> &&except = {});
+
   //===--------------------------------------------------------------------===//
   // Walkers
   //===--------------------------------------------------------------------===//

mlir/lib/Dialect/Bufferization/Transforms/OneShotAnalysis.cpp

@@ -273,25 +273,6 @@ static bool happensBefore(Operation *a, Operation *b,
   return false;
 }
 
-static bool isReachable(Block *from, Block *to, ArrayRef<Block *> except) {
-  DenseSet<Block *> visited;
-  SmallVector<Block *> worklist;
-  for (Block *succ : from->getSuccessors())
-    worklist.push_back(succ);
-  while (!worklist.empty()) {
-    Block *next = worklist.pop_back_val();
-    if (llvm::is_contained(except, next))
-      continue;
-    if (next == to)
-      return true;
-    if (!visited.insert(next).second)
-      continue;
-    for (Block *succ : next->getSuccessors())
-      worklist.push_back(succ);
-  }
-  return false;
-}
-
 /// Return `true` if op dominance can be used to rule out a read-after-write
 /// conflicts based on the ordering of ops. Returns `false` if op dominance
 /// cannot be used to due region-based loops.
@@ -427,8 +408,8 @@ static bool canUseOpDominanceDueToBlocks(OpOperand *uRead, OpOperand *uWrite,
   Block *writeBlock = uWrite->getOwner()->getBlock();
   for (Value def : definitions) {
     Block *defBlock = def.getParentBlock();
-    if (isReachable(readBlock, writeBlock, {defBlock}) &&
-        isReachable(writeBlock, readBlock, {defBlock}))
+    if (readBlock->isReachable(writeBlock, {defBlock}) &&
+        writeBlock->isReachable(readBlock, {defBlock}))
       return false;
   }
 

mlir/lib/Dialect/Vector/Transforms/VectorTransferOpTransforms.cpp

@@ -73,20 +73,7 @@ bool TransferOptimization::isReachable(Operation *start, Operation *dest) {
   // Simple case where the start op dominate the destination.
   if (dominators.dominates(start, dest))
     return true;
-  Block *startBlock = start->getBlock();
-  Block *destBlock = dest->getBlock();
-  SmallVector<Block *, 32> worklist(startBlock->succ_begin(),
-                                    startBlock->succ_end());
-  SmallPtrSet<Block *, 32> visited;
-  while (!worklist.empty()) {
-    Block *bb = worklist.pop_back_val();
-    if (!visited.insert(bb).second)
-      continue;
-    if (dominators.dominates(bb, destBlock))
-      return true;
-    worklist.append(bb->succ_begin(), bb->succ_end());
-  }
-  return false;
+  return start->getBlock()->isReachable(dest->getBlock());
 }
 
 /// For transfer_write to overwrite fully another transfer_write must:

mlir/lib/IR/Block.cpp

@@ -7,9 +7,12 @@
 //===----------------------------------------------------------------------===//
 
 #include "mlir/IR/Block.h"
+
 #include "mlir/IR/Builders.h"
 #include "mlir/IR/Operation.h"
 #include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
+
 using namespace mlir;
 
 //===----------------------------------------------------------------------===//
@@ -331,7 +334,7 @@ unsigned PredecessorIterator::getSuccessorIndex() const {
 }
 
 //===----------------------------------------------------------------------===//
-// SuccessorRange
+// Successors
 //===----------------------------------------------------------------------===//
 
 SuccessorRange::SuccessorRange() : SuccessorRange(nullptr, 0) {}
@@ -349,6 +352,26 @@ SuccessorRange::SuccessorRange(Operation *term) : SuccessorRange() {
     base = term->getBlockOperands().data();
 }
 
+bool Block::isReachable(Block *other, SmallPtrSet<Block *, 16> &&except) {
+  assert(getParent() == other->getParent() && "expected same region");
+  if (except.contains(other)) {
+    // Fast path: If `other` is in the `except` set, there can be no path from
+    // "this" to `other` (that does not pass through an excluded block).
+    return false;
+  }
+  SmallVector<Block *> worklist(succ_begin(), succ_end());
+  while (!worklist.empty()) {
+    Block *next = worklist.pop_back_val();
+    if (next == other)
+      return true;
+    // Note: `except` keeps track of already visited blocks.
+    if (!except.insert(next).second)
+      continue;
+    worklist.append(next->succ_begin(), next->succ_end());
+  }
+  return false;
+}
+
 //===----------------------------------------------------------------------===//
 // BlockRange
 //===----------------------------------------------------------------------===//