[SCF][Transform] Add support for scf.for in LoopFuseSibling op

Adds support for fusing two scf.for loops occurring in the same block.
Implementation mirrors that of LoopFuseSibling's support for scf.forall,
including only rudimentary checks, like the target loop's operands being
dominated by the source loop.

Fixes a bug in the dominance check whereby it was checked that values in the
target loop themselves dominated the source loop rather than (the ops) where
these values originate.

Adds tests for using LoopFuseSibling on scf.for loops, including one which
fails without the fix for the dominance check.

Author: Rolf Morel

mlir/include/mlir/Dialect/SCF/TransformOps/SCFTransformOps.td

@@ -333,23 +333,25 @@ def TakeAssumedBranchOp : Op<Transform_Dialect, "scf.take_assumed_branch", [
   }];
 }
 
-def LoopFuseSibling : Op<Transform_Dialect, "loop.fuse_sibling",
+def LoopFuseSiblingOp : Op<Transform_Dialect, "loop.fuse_sibling",
   [FunctionalStyleTransformOpTrait, MemoryEffectsOpInterface,
    DeclareOpInterfaceMethods<TransformOpInterface>]> {
   let summary = "Fuse a loop into another loop, assuming the fusion is legal.";
 
   let description = [{
     Fuses the `target` loop into the `source` loop assuming they are
-    independent of each other. It is the responsibility of the user to ensure
-    that the given two loops are independent of each other, this operation will
-    not performa any legality checks and will simply fuse the two given loops.
+    independent of each other. In the fused loop, the arguments, body and
+    results of `target` are placed _before_ those of `source`.
 
-    Currently, the only fusion supported is when both `target` and `source`
-    are `scf.forall` operations. For `scf.forall` fusion, the bounds and the
-    mapping must match, otherwise a silencable failure is produced.
+    For fusion of two `scf.for` loops, the bounds and step size must match. For
+    fusion of two `scf.forall` loops, the bounds and the mapping must match.
+    Otherwise a silencable failure is produced. Attempting to fuse any other kinds
+    of loops/ops will produce a definite failure.
 
-    The input handles `target` and `source` must map to exactly one operation,
-    a definite failure is produced otherwise.
+    The `target` and `source` handles must refer to exactly one operation,
+    otherwise a definite failure is produced. It is the responsibility of the
+    user to ensure that the `target` and `source` loops are independent of each
+    other -- this op will not perform any legality checks.
 
     #### Return modes
 
@@ -362,10 +364,6 @@ def LoopFuseSibling : Op<Transform_Dialect, "loop.fuse_sibling",
   let results = (outs TransformHandleTypeInterface:$fused_loop);
   let assemblyFormat = "$target `into` $source attr-dict "
                        " `:` functional-type(operands, results)";
-
-  let builders = [
-    OpBuilder<(ins "Value":$loop, "Value":$fused_loop)>
-  ];
 }
 
 #endif // SCF_TRANSFORM_OPS

mlir/include/mlir/Dialect/SCF/Utils/Utils.h

@@ -162,6 +162,16 @@ scf::ForallOp fuseIndependentSiblingForallLoops(scf::ForallOp target,
                                                 scf::ForallOp source,
                                                 RewriterBase &rewriter);
 
+/// Given two scf.for loops, `target` and `source`, fuses `target` into
+/// `source`. Assumes that the given loops are siblings and are independent of
+/// each other.
+///
+/// This function does not perform any legality checks and simply fuses the
+/// loops. The caller is responsible for ensuring that the loops are legal to
+/// fuse.
+scf::ForOp fuseIndependentSiblingForLoops(scf::ForOp target, scf::ForOp source,
+                                          RewriterBase &rewriter);
+
 } // namespace mlir
 
 #endif // MLIR_DIALECT_SCF_UTILS_UTILS_H_

mlir/lib/Dialect/SCF/TransformOps/SCFTransformOps.cpp

@@ -384,7 +384,7 @@ void transform::TakeAssumedBranchOp::getEffects(
 }
 
 //===----------------------------------------------------------------------===//
-// LoopFuseSibling
+// LoopFuseSiblingOp
 //===----------------------------------------------------------------------===//
 
 /// Check if `target` and `source` are siblings, in the context that `target`
@@ -408,7 +408,7 @@ static DiagnosedSilenceableFailure isOpSibling(Operation *target,
   // Check if fusion will violate dominance.
   DominanceInfo domInfo(source);
   if (target->isBeforeInBlock(source)) {
-    // Since, `target` is before `source`, all users of results of `target`
+    // Since `target` is before `source`, all users of results of `target`
     // need to be dominated by `source`.
     for (Operation *user : target->getUsers()) {
       if (!domInfo.properlyDominates(source, user, /*enclosingOpOk=*/false)) {
@@ -424,9 +424,8 @@ static DiagnosedSilenceableFailure isOpSibling(Operation *target,
     // Check if operands of `target` are dominated by `source`.
     for (Value operand : target->getOperands()) {
       Operation *operandOp = operand.getDefiningOp();
-      // If operand does not have a defining operation, it is a block arguement,
-      // which will always dominate `source`, since `target` and `source` are in
-      // the same block and the operand dominated `source` before.
+      // Operands without defining operations are block arguments. When `target`
+      // and `source` occur in the same block, these operands dominate `source`.
       if (!operandOp)
         continue;
 
@@ -441,8 +440,12 @@ static DiagnosedSilenceableFailure isOpSibling(Operation *target,
     bool failed = false;
     OpOperand *failedValue = nullptr;
     visitUsedValuesDefinedAbove(target->getRegions(), [&](OpOperand *operand) {
-      if (!domInfo.properlyDominates(operand->getOwner(), source,
-                                     /*enclosingOpOk=*/false)) {
+      Operation *operandOp = operand->get().getDefiningOp();
+      if (!operandOp && !domInfo.properlyDominates(operandOp, source,
+                                                   /*enclosingOpOk=*/false)) {
+        // `operand` is not a block argument of an enclosing block or otherwise
+        // `operand`'s defining op is outside `target` but does not dominate
+        // `source`.
         failed = true;
         failedValue = operand;
       }
@@ -476,21 +479,40 @@ static bool isForallWithIdenticalConfiguration(Operation *target,
          targetOp.getMapping() == sourceOp.getMapping();
 }
 
-/// Fuse `target` into `source` assuming they are siblings and indepndent.
-/// TODO: Add fusion for more operations. Currently, we handle only scf.forall.
+static bool isForWithIdenticalConfiguration(Operation *target,
+                                            Operation *source) {
+  auto targetOp = dyn_cast<scf::ForOp>(target);
+  auto sourceOp = dyn_cast<scf::ForOp>(source);
+  if (!targetOp || !sourceOp)
+    return false;
+
+  return targetOp.getLowerBound() == sourceOp.getLowerBound() &&
+         targetOp.getUpperBound() == sourceOp.getUpperBound() &&
+         targetOp.getStep() == sourceOp.getStep();
+}
+
+/// Fuse `target` into `source` assuming they are siblings and independent.
+/// TODO: Support fusion for operations besides scf.for and scf.forall.
 static Operation *fuseSiblings(Operation *target, Operation *source,
                                RewriterBase &rewriter) {
-  auto targetOp = dyn_cast<scf::ForallOp>(target);
-  auto sourceOp = dyn_cast<scf::ForallOp>(source);
-  if (!targetOp || !sourceOp)
-    return nullptr;
-  return fuseIndependentSiblingForallLoops(targetOp, sourceOp, rewriter);
+  auto targetForOp = dyn_cast<scf::ForOp>(target);
+  auto sourceForOp = dyn_cast<scf::ForOp>(source);
+  if (targetForOp && sourceForOp)
+    return fuseIndependentSiblingForLoops(targetForOp, sourceForOp, rewriter);
+
+  auto targetForallOp = dyn_cast<scf::ForallOp>(target);
+  auto sourceForallOp = dyn_cast<scf::ForallOp>(source);
+  if (targetForallOp && sourceForallOp)
+    return fuseIndependentSiblingForallLoops(targetForallOp, sourceForallOp,
+                                             rewriter);
+
+  return nullptr;
 }
 
 DiagnosedSilenceableFailure
-transform::LoopFuseSibling::apply(transform::TransformRewriter &rewriter,
-                                  transform::TransformResults &results,
-                                  transform::TransformState &state) {
+transform::LoopFuseSiblingOp::apply(transform::TransformRewriter &rewriter,
+                                    transform::TransformResults &results,
+                                    transform::TransformState &state) {
   auto targetOps = state.getPayloadOps(getTarget());
   auto sourceOps = state.getPayloadOps(getSource());
 
@@ -511,7 +533,8 @@ transform::LoopFuseSibling::apply(transform::TransformRewriter &rewriter,
     return diag;
 
   // Check if the target can be fused into source.
-  if (!isForallWithIdenticalConfiguration(target, source)) {
+  if (!isForallWithIdenticalConfiguration(target, source) &&
+      !isForWithIdenticalConfiguration(target, source)) {
     return emitSilenceableFailure(target->getLoc())
            << "operations cannot be fused";
   }

mlir/lib/Dialect/SCF/Utils/Utils.cpp

@@ -910,61 +910,96 @@ scf::ForallOp mlir::fuseIndependentSiblingForallLoops(scf::ForallOp target,
   unsigned numTargetOuts = target.getNumResults();
   unsigned numSourceOuts = source.getNumResults();
 
-  OperandRange targetOuts = target.getOutputs();
-  OperandRange sourceOuts = source.getOutputs();
-
   // Create fused shared_outs.
   SmallVector<Value> fusedOuts;
-  fusedOuts.reserve(numTargetOuts + numSourceOuts);
-  fusedOuts.append(targetOuts.begin(), targetOuts.end());
-  fusedOuts.append(sourceOuts.begin(), sourceOuts.end());
+  llvm::append_range(fusedOuts, target.getOutputs());
+  llvm::append_range(fusedOuts, source.getOutputs());
 
-  // Create a new scf::forall op after the source loop.
+  // Create a new scf.forall op after the source loop.
   rewriter.setInsertionPointAfter(source);
   scf::ForallOp fusedLoop = rewriter.create<scf::ForallOp>(
       source.getLoc(), source.getMixedLowerBound(), source.getMixedUpperBound(),
       source.getMixedStep(), fusedOuts, source.getMapping());
 
   // Map control operands.
-  IRMapping fusedMapping;
-  fusedMapping.map(target.getInductionVars(), fusedLoop.getInductionVars());
-  fusedMapping.map(source.getInductionVars(), fusedLoop.getInductionVars());
+  IRMapping mapping;
+  mapping.map(target.getInductionVars(), fusedLoop.getInductionVars());
+  mapping.map(source.getInductionVars(), fusedLoop.getInductionVars());
 
   // Map shared outs.
-  fusedMapping.map(target.getRegionIterArgs(),
-                   fusedLoop.getRegionIterArgs().slice(0, numTargetOuts));
-  fusedMapping.map(
-      source.getRegionIterArgs(),
-      fusedLoop.getRegionIterArgs().slice(numTargetOuts, numSourceOuts));
+  mapping.map(target.getRegionIterArgs(),
+              fusedLoop.getRegionIterArgs().take_front(numTargetOuts));
+  mapping.map(source.getRegionIterArgs(),
+              fusedLoop.getRegionIterArgs().take_back(numSourceOuts));
 
   // Append everything except the terminator into the fused operation.
   rewriter.setInsertionPointToStart(fusedLoop.getBody());
   for (Operation &op : target.getBody()->without_terminator())
-    rewriter.clone(op, fusedMapping);
+    rewriter.clone(op, mapping);
   for (Operation &op : source.getBody()->without_terminator())
-    rewriter.clone(op, fusedMapping);
+    rewriter.clone(op, mapping);
 
   // Fuse the old terminator in_parallel ops into the new one.
   scf::InParallelOp targetTerm = target.getTerminator();
   scf::InParallelOp sourceTerm = source.getTerminator();
   scf::InParallelOp fusedTerm = fusedLoop.getTerminator();
-
   rewriter.setInsertionPointToStart(fusedTerm.getBody());
   for (Operation &op : targetTerm.getYieldingOps())
-    rewriter.clone(op, fusedMapping);
+    rewriter.clone(op, mapping);
   for (Operation &op : sourceTerm.getYieldingOps())
-    rewriter.clone(op, fusedMapping);
-
-  // Replace all uses of the old loops with the fused loop.
-  rewriter.replaceAllUsesWith(target.getResults(),
-                              fusedLoop.getResults().slice(0, numTargetOuts));
-  rewriter.replaceAllUsesWith(
-      source.getResults(),
-      fusedLoop.getResults().slice(numTargetOuts, numSourceOuts));
-
-  // Erase the old loops.
-  rewriter.eraseOp(target);
-  rewriter.eraseOp(source);
+    rewriter.clone(op, mapping);
+
+  // Replace old loops by substituting their uses by results of the fused loop.
+  rewriter.replaceOp(target, fusedLoop.getResults().take_front(numTargetOuts));
+  rewriter.replaceOp(source, fusedLoop.getResults().take_back(numSourceOuts));
+
+  return fusedLoop;
+}
+
+scf::ForOp mlir::fuseIndependentSiblingForLoops(scf::ForOp target,
+                                                scf::ForOp source,
+                                                RewriterBase &rewriter) {
+  unsigned numTargetOuts = target.getNumResults();
+  unsigned numSourceOuts = source.getNumResults();
+
+  // Create fused init_args, with target's init_args before source's init_args.
+  SmallVector<Value> fusedInitArgs;
+  llvm::append_range(fusedInitArgs, target.getInitArgs());
+  llvm::append_range(fusedInitArgs, source.getInitArgs());
+
+  // Create a new scf.for op after the source loop.
+  rewriter.setInsertionPointAfter(source);
+  scf::ForOp fusedLoop = rewriter.create<scf::ForOp>(
+      source.getLoc(), source.getLowerBound(), source.getUpperBound(),
+      source.getStep(), fusedInitArgs);
+
+  // Map original induction variables and operands to those of the fused loop.
+  IRMapping mapping;
+  mapping.map(target.getInductionVar(), fusedLoop.getInductionVar());
+  mapping.map(target.getRegionIterArgs(),
+              fusedLoop.getRegionIterArgs().take_front(numTargetOuts));
+  mapping.map(source.getInductionVar(), fusedLoop.getInductionVar());
+  mapping.map(source.getRegionIterArgs(),
+              fusedLoop.getRegionIterArgs().take_back(numSourceOuts));
+
+  // Merge target's body into the new (fused) for loop and then source's body.
+  rewriter.setInsertionPointToStart(fusedLoop.getBody());
+  for (Operation &op : target.getBody()->without_terminator())
+    rewriter.clone(op, mapping);
+  for (Operation &op : source.getBody()->without_terminator())
+    rewriter.clone(op, mapping);
+
+  // Build fused yield results by appropriately mapping original yield operands.
+  SmallVector<Value> yieldResults;
+  for (Value operand : target.getBody()->getTerminator()->getOperands())
+    yieldResults.push_back(mapping.lookupOrDefault(operand));
+  for (Value operand : source.getBody()->getTerminator()->getOperands())
+    yieldResults.push_back(mapping.lookupOrDefault(operand));
+  rewriter.create<scf::YieldOp>(source.getLoc(), yieldResults);
+
+  // Replace old loops by substituting their uses by results of the fused loop.
+  rewriter.replaceOp(target, fusedLoop.getResults().take_front(numTargetOuts));
+  rewriter.replaceOp(source, fusedLoop.getResults().take_back(numSourceOuts));
 
   return fusedLoop;
 }