[Mlir-commits] [mlir] [mlir][scf] Add reductions support to `scf.parallel` fusion (PR #75955)

[Renato Golin]

================
@@ -131,29 +131,83 @@ static bool isFusionLegal(ParallelOp firstPloop, ParallelOp secondPloop,
 }
 
 /// Prepends operations of firstPloop's body into secondPloop's body.
-static void fuseIfLegal(ParallelOp firstPloop, ParallelOp secondPloop,
-                        OpBuilder b,
+/// Updates secondPloop with new loop.
+static void fuseIfLegal(ParallelOp firstPloop, ParallelOp &secondPloop,
+                        OpBuilder builder,
                         llvm::function_ref<bool(Value, Value)> mayAlias) {
+  Block *block1 = firstPloop.getBody();
+  Block *block2 = secondPloop.getBody();
   IRMapping firstToSecondPloopIndices;
-  firstToSecondPloopIndices.map(firstPloop.getBody()->getArguments(),
-                                secondPloop.getBody()->getArguments());
+  firstToSecondPloopIndices.map(block1->getArguments(), block2->getArguments());
 
   if (!isFusionLegal(firstPloop, secondPloop, firstToSecondPloopIndices,
                      mayAlias))
     return;
 
-  b.setInsertionPointToStart(secondPloop.getBody());
-  for (auto &op : firstPloop.getBody()->without_terminator())
-    b.clone(op, firstToSecondPloopIndices);
+  DominanceInfo dom;
+  for (Operation *user : firstPloop->getUsers())
+    if (!dom.properlyDominates(secondPloop, user, /*enclosingOpOk*/ false))
+      return;
+
+  ValueRange inits1 = firstPloop.getInitVals();
+  ValueRange inits2 = secondPloop.getInitVals();
+
+  SmallVector<Value> newInitVars(inits1.begin(), inits1.end());
+  newInitVars.append(inits2.begin(), inits2.end());
+
+  IRRewriter b(builder);
+  b.setInsertionPoint(secondPloop);
+  auto newSecondPloop = b.create<ParallelOp>(
+      secondPloop.getLoc(), secondPloop.getLowerBound(),
+      secondPloop.getUpperBound(), secondPloop.getStep(), newInitVars);
+
+  Block *newBlock = newSecondPloop.getBody();
+  auto term1 = cast<ReduceOp>(block1->getTerminator());
+  auto term2 = cast<ReduceOp>(block2->getTerminator());
+
+  b.inlineBlockBefore(block2, newBlock, newBlock->begin(),
+                      newBlock->getArguments());
+  b.inlineBlockBefore(block1, newBlock, newBlock->begin(),
+                      newBlock->getArguments());
+
+  ValueRange results = newSecondPloop.getResults();
+  if (!results.empty()) {
+    b.setInsertionPointToEnd(newBlock);
+
+    ValueRange reduceArgs1 = term1.getOperands();
+    ValueRange reduceArgs2 = term2.getOperands();
+    SmallVector<Value> newReduceArgs(reduceArgs1.begin(), reduceArgs1.end());
+    newReduceArgs.append(reduceArgs2.begin(), reduceArgs2.end());
+
+    auto newReduceOp = b.create<scf::ReduceOp>(term2.getLoc(), newReduceArgs);
+
+    for (auto &&[i, reg] : llvm::enumerate(llvm::concat<Region>(
+             term1.getReductions(), term2.getReductions()))) {
+      Block &oldRedBlock = reg.front();
+      Block &newRedBlock = newReduceOp.getReductions()[i].front();
+      b.inlineBlockBefore(&oldRedBlock, &newRedBlock, newRedBlock.begin(),
+                          newRedBlock.getArguments());
+    }
+
+    firstPloop.replaceAllUsesWith(results.take_front(inits1.size()));
+    secondPloop.replaceAllUsesWith(results.take_back(inits2.size()));
+  }
+  term1->erase();
+  term2->erase();
   firstPloop.erase();
+  secondPloop.erase();
+  secondPloop = newSecondPloop;
 }
 
 void mlir::scf::naivelyFuseParallelOps(
     Region &region, llvm::function_ref<bool(Value, Value)> mayAlias) {
   OpBuilder b(region);
   // Consider every single block and attempt to fuse adjacent loops.
+  SmallVector<SmallVector<ParallelOp>, 1> ploopChains;
   for (auto &block : region) {
-    SmallVector<SmallVector<ParallelOp, 8>, 1> ploopChains{{}};
+    ploopChains.clear();
----------------
rengolin wrote:

This change is unnecessary. 

The `SmallVector` empty constructor is basically a 2-word copy, with both `clear()` and `push_back()` already happening on c-tor/d-tor.

https://github.com/llvm/llvm-project/blob/aba40fb34a27f1d36b4b541bf04d414dca7d1c4c/llvm/include/llvm/ADT/SmallVector.h#L63

https://github.com/llvm/llvm-project/pull/75955