Loop invariant code motion - remove reliance on getForwardSlice. Add more tests.

--

PiperOrigin-RevId: 250950703

1 files changed, 151 insertions, 22 deletions

diff --git a/mlir/lib/Transforms/LoopInvariantCodeMotion.cpp b/mlir/lib/Transforms/LoopInvariantCodeMotion.cpp
index 402f7d9..3187566 100644
--- a/mlir/lib/Transforms/LoopInvariantCodeMotion.cpp
+++ b/mlir/lib/Transforms/LoopInvariantCodeMotion.cpp
@@ -35,15 +35,13 @@
 #include "mlir/Transforms/Utils.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
-#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 
 #define DEBUG_TYPE "licm"
 
-using llvm::SetVector;
-
 using namespace mlir;
 
 namespace {
@@ -57,45 +55,177 @@ struct LoopInvariantCodeMotion : public FunctionPass<LoopInvariantCodeMotion> {
 };
 } // end anonymous namespace
 
+static bool
+checkInvarianceOfNestedIfOps(Operation *op, Value *indVar,
+                             SmallPtrSetImpl<Operation *> &definedOps,
+                             SmallPtrSetImpl<Operation *> &opsToHoist);
+static bool isOpLoopInvariant(Operation &op, Value *indVar,
+                              SmallPtrSetImpl<Operation *> &definedOps,
+                              SmallPtrSetImpl<Operation *> &opsToHoist);
+
+static bool
+areAllOpsInTheBlockListInvariant(Region &blockList, Value *indVar,
+                                 SmallPtrSetImpl<Operation *> &definedOps,
+                                 SmallPtrSetImpl<Operation *> &opsToHoist);
+
+static bool isMemRefDereferencingOp(Operation &op) {
+  // TODO(asabne): Support DMA Ops.
+  if (isa<LoadOp>(op) || isa<StoreOp>(op)) {
+    return true;
+  }
+  return false;
+}
+
 FunctionPassBase *mlir::createLoopInvariantCodeMotionPass() {
   return new LoopInvariantCodeMotion();
 }
 
+// Returns true if the individual op is loop invariant.
+bool isOpLoopInvariant(Operation &op, Value *indVar,
+                       SmallPtrSetImpl<Operation *> &definedOps,
+                       SmallPtrSetImpl<Operation *> &opsToHoist) {
+  LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
+
+  if (isa<AffineIfOp>(op)) {
+    if (!checkInvarianceOfNestedIfOps(&op, indVar, definedOps, opsToHoist)) {
+      return false;
+    }
+  } else if (isa<AffineForOp>(op)) {
+    // If the body of a predicated region has a for loop, we don't hoist the
+    // 'affine.if'.
+    return false;
+  } else if (isa<DmaStartOp>(op) || isa<DmaWaitOp>(op)) {
+    // TODO(asabne): Support DMA ops.
+    return false;
+  } else if (!isa<ConstantOp>(op)) {
+    if (isMemRefDereferencingOp(op)) {
+      Value *memref = isa<LoadOp>(op) ? cast<LoadOp>(op).getMemRef()
+                                      : cast<StoreOp>(op).getMemRef();
+      for (auto *user : memref->getUsers()) {
+        // If this memref has a user that is a DMA, give up because these
+        // operations write to this memref.
+        if (isa<DmaStartOp>(op) || isa<DmaWaitOp>(op)) {
+          return false;
+        }
+        // If the memref used by the load/store is used in a store elsewhere in
+        // the loop nest, we do not hoist. Similarly, if the memref used in a
+        // load is also being stored too, we do not hoist the load.
+        if (isa<StoreOp>(user) || (isa<LoadOp>(user) && isa<StoreOp>(op))) {
+          if (&op != user) {
+            SmallVector<AffineForOp, 8> userIVs;
+            getLoopIVs(*user, &userIVs);
+            // Check that userIVs don't contain the for loop around the op.
+            if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar))) {
+              return false;
+            }
+          }
+        }
+      }
+    }
+
+    // Insert this op in the defined ops list.
+    definedOps.insert(&op);
+
+    if (op.getNumOperands() == 0 && !isa<AffineTerminatorOp>(op)) {
+      LLVM_DEBUG(llvm::dbgs() << "\nNon-constant op with 0 operands\n");
+      return false;
+    }
+    for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
+      auto *operandSrc = op.getOperand(i)->getDefiningOp();
+
+      LLVM_DEBUG(
+          op.getOperand(i)->print(llvm::dbgs() << "\nIterating on operand\n"));
+
+      // If the loop IV is the operand, this op isn't loop invariant.
+      if (indVar == op.getOperand(i)) {
+        LLVM_DEBUG(llvm::dbgs() << "\nLoop IV is the operand\n");
+        return false;
+      }
+
+      if (operandSrc != nullptr) {
+        LLVM_DEBUG(llvm::dbgs()
+                   << *operandSrc << "\nIterating on operand src\n");
+
+        // If the value was defined in the loop (outside of the
+        // if/else region), and that operation itself wasn't meant to
+        // be hoisted, then mark this operation loop dependent.
+        if (definedOps.count(operandSrc) && opsToHoist.count(operandSrc) == 0) {
+          return false;
+        }
+      }
+    }
+  }
+
+  // If no operand was loop variant, mark this op for motion.
+  opsToHoist.insert(&op);
+  return true;
+}
+
+// Checks if all ops in a region (i.e. list of blocks) are loop invariant.
+bool areAllOpsInTheBlockListInvariant(
+    Region &blockList, Value *indVar, SmallPtrSetImpl<Operation *> &definedOps,
+    SmallPtrSetImpl<Operation *> &opsToHoist) {
+
+  for (auto &b : blockList) {
+    for (auto &op : b) {
+      if (!isOpLoopInvariant(op, indVar, definedOps, opsToHoist)) {
+        return false;
+      }
+    }
+  }
+
+  return true;
+}
+
+// Returns true if the affine.if op can be hoisted.
+bool checkInvarianceOfNestedIfOps(Operation *op, Value *indVar,
+                                  SmallPtrSetImpl<Operation *> &definedOps,
+                                  SmallPtrSetImpl<Operation *> &opsToHoist) {
+  assert(isa<AffineIfOp>(op));
+  auto ifOp = cast<AffineIfOp>(op);
+
+  if (!areAllOpsInTheBlockListInvariant(ifOp.getThenBlocks(), indVar,
+                                        definedOps, opsToHoist)) {
+    return false;
+  }
+
+  if (!areAllOpsInTheBlockListInvariant(ifOp.getElseBlocks(), indVar,
+                                        definedOps, opsToHoist)) {
+    return false;
+  }
+
+  return true;
+}
+
 void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
   auto *loopBody = forOp.getBody();
+  auto *indVar = forOp.getInductionVar();
 
+  SmallPtrSet<Operation *, 8> definedOps;
   // This is the place where hoisted instructions would reside.
   FuncBuilder b(forOp.getOperation());
 
-  // This vector is used to place loop invariant operations.
+  SmallPtrSet<Operation *, 8> opsToHoist;
   SmallVector<Operation *, 8> opsToMove;
 
-  SetVector<Operation *> loopDefinedOps;
-  // Generate forward slice which contains ops that fall under the transitive
-  // definition closure following the loop induction variable.
-  getForwardSlice(forOp, &loopDefinedOps);
-
-  LLVM_DEBUG(for (auto i
-                  : loopDefinedOps) {
-    i->print(llvm::dbgs() << "\nLoop-dependent op\n");
-  });
-
   for (auto &op : *loopBody) {
-    // If the operation is loop invariant, insert it into opsToMove.
-    if (!isa<AffineForOp>(op) && !isa<AffineTerminatorOp>(op) &&
-        loopDefinedOps.count(&op) != 1) {
-      LLVM_DEBUG(op.print(llvm::dbgs() << "\nLICM'ing op\n"));
-      opsToMove.push_back(&op);
+    // We don't hoist for loops.
+    if (!isa<AffineForOp>(op)) {
+      if (!isa<AffineTerminatorOp>(op)) {
+        if (isOpLoopInvariant(op, indVar, definedOps, opsToHoist)) {
+          opsToMove.push_back(&op);
+        }
+      }
     }
   }
 
-  // For all instructions that we found to be invariant, place them sequentially
+  // For all instructions that we found to be invariant, place sequentially
   // right before the for loop.
   for (auto *op : opsToMove) {
     op->moveBefore(forOp);
   }
 
-  LLVM_DEBUG(forOp.getOperation()->print(llvm::dbgs() << "\nModified loop\n"));
+  LLVM_DEBUG(forOp.getOperation()->print(llvm::dbgs() << "Modified loop\n"));
 
   // If the for loop body has a single operation (the terminator), erase it.
   if (forOp.getBody()->getOperations().size() == 1) {
@@ -105,7 +235,6 @@ void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
 }
 
 void LoopInvariantCodeMotion::runOnFunction() {
-
   // Walk through all loops in a function in innermost-loop-first order.  This
   // way, we first LICM from the inner loop, and place the ops in
   // the outer loop, which in turn can be further LICM'ed.