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llvm-project/mlir/lib/Transforms/LoopFusion.cpp
MLIR Team 27d067e164 LoopFusion improvements: 
*) Adds support for fusing into consumer loop nests with multiple loads from the same memref.
*) Adds support for reducing slice loop trip count by projecting out destination loop IVs greater than destination loop depth.
*) Removes dependence on src loop depth and simplifies cost model computation.

PiperOrigin-RevId: 229575126
2019-03-29 15:21:59 -07:00

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//===- LoopFusion.cpp - Code to perform loop fusion -----------------------===//
//
// Copyright 2019 The MLIR Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// =============================================================================
//
// This file implements loop fusion.
//
//===----------------------------------------------------------------------===//
#include "mlir/Analysis/AffineAnalysis.h"
#include "mlir/Analysis/AffineStructures.h"
#include "mlir/Analysis/LoopAnalysis.h"
#include "mlir/Analysis/Utils.h"
#include "mlir/IR/AffineExpr.h"
#include "mlir/IR/AffineMap.h"
#include "mlir/IR/Builders.h"
#include "mlir/IR/BuiltinOps.h"
#include "mlir/IR/InstVisitor.h"
#include "mlir/Pass.h"
#include "mlir/StandardOps/StandardOps.h"
#include "mlir/Transforms/LoopUtils.h"
#include "mlir/Transforms/Passes.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#define DEBUG_TYPE "loop-fusion"
using llvm::SetVector;
using namespace mlir;
namespace {
/// Loop fusion pass. This pass currently supports a greedy fusion policy,
/// which fuses loop nests with single-writer/single-reader memref dependences
/// with the goal of improving locality.
// TODO(andydavis) Support fusion of source loop nests which write to multiple
// memrefs, where each memref can have multiple users (if profitable).
// TODO(andydavis) Extend this pass to check for fusion preventing dependences,
// and add support for more general loop fusion algorithms.
struct LoopFusion : public FunctionPass {
LoopFusion() : FunctionPass(&LoopFusion::passID) {}
PassResult runOnFunction(Function *f) override;
static char passID;
};
} // end anonymous namespace
char LoopFusion::passID = 0;
FunctionPass *mlir::createLoopFusionPass() { return new LoopFusion; }
namespace {
// LoopNestStateCollector walks loop nests and collects load and store
// operations, and whether or not an IfInst was encountered in the loop nest.
class LoopNestStateCollector : public InstWalker<LoopNestStateCollector> {
public:
SmallVector<ForInst *, 4> forInsts;
SmallVector<OperationInst *, 4> loadOpInsts;
SmallVector<OperationInst *, 4> storeOpInsts;
bool hasIfInst = false;
void visitForInst(ForInst *forInst) { forInsts.push_back(forInst); }
void visitIfInst(IfInst *ifInst) { hasIfInst = true; }
void visitOperationInst(OperationInst *opInst) {
if (opInst->isa<LoadOp>())
loadOpInsts.push_back(opInst);
if (opInst->isa<StoreOp>())
storeOpInsts.push_back(opInst);
}
};
// MemRefDependenceGraph is a graph data structure where graph nodes are
// top-level instructions in a Function which contain load/store ops, and edges
// are memref dependences between the nodes.
// TODO(andydavis) Add a depth parameter to dependence graph construction.
struct MemRefDependenceGraph {
public:
// Node represents a node in the graph. A Node is either an entire loop nest
// rooted at the top level which contains loads/stores, or a top level
// load/store.
struct Node {
// The unique identifier of this node in the graph.
unsigned id;
// The top-level statment which is (or contains) loads/stores.
Instruction *inst;
// List of load operations.
SmallVector<OperationInst *, 4> loads;
// List of store op insts.
SmallVector<OperationInst *, 4> stores;
Node(unsigned id, Instruction *inst) : id(id), inst(inst) {}
// Returns the load op count for 'memref'.
unsigned getLoadOpCount(Value *memref) {
unsigned loadOpCount = 0;
for (auto *loadOpInst : loads) {
if (memref == loadOpInst->cast<LoadOp>()->getMemRef())
++loadOpCount;
}
return loadOpCount;
}
// Returns the store op count for 'memref'.
unsigned getStoreOpCount(Value *memref) {
unsigned storeOpCount = 0;
for (auto *storeOpInst : stores) {
if (memref == storeOpInst->cast<StoreOp>()->getMemRef())
++storeOpCount;
}
return storeOpCount;
}
};
// Edge represents a memref data dependece between nodes in the graph.
struct Edge {
// The id of the node at the other end of the edge.
unsigned id;
// The memref on which this edge represents a dependence.
Value *memref;
};
// Map from node id to Node.
DenseMap<unsigned, Node> nodes;
// Map from node id to list of input edges.
DenseMap<unsigned, SmallVector<Edge, 2>> inEdges;
// Map from node id to list of output edges.
DenseMap<unsigned, SmallVector<Edge, 2>> outEdges;
MemRefDependenceGraph() {}
// Initializes the dependence graph based on operations in 'f'.
// Returns true on success, false otherwise.
bool init(Function *f);
// Returns the graph node for 'id'.
Node *getNode(unsigned id) {
auto it = nodes.find(id);
assert(it != nodes.end());
return &it->second;
}
// Returns true iff there is an edge from node 'srcId' to node 'dstId' for
// 'memref'. Returns false otherwise.
bool hasEdge(unsigned srcId, unsigned dstId, Value *memref) {
if (outEdges.count(srcId) == 0 || inEdges.count(dstId) == 0) {
return false;
}
bool hasOutEdge = llvm::any_of(outEdges[srcId], [=](Edge &edge) {
return edge.id == dstId && edge.memref == memref;
});
bool hasInEdge = llvm::any_of(inEdges[dstId], [=](Edge &edge) {
return edge.id == srcId && edge.memref == memref;
});
return hasOutEdge && hasInEdge;
}
// Adds an edge from node 'srcId' to node 'dstId' for 'memref'.
void addEdge(unsigned srcId, unsigned dstId, Value *memref) {
if (!hasEdge(srcId, dstId, memref)) {
outEdges[srcId].push_back({dstId, memref});
inEdges[dstId].push_back({srcId, memref});
}
}
// Removes an edge from node 'srcId' to node 'dstId' for 'memref'.
void removeEdge(unsigned srcId, unsigned dstId, Value *memref) {
assert(inEdges.count(dstId) > 0);
assert(outEdges.count(srcId) > 0);
// Remove 'srcId' from 'inEdges[dstId]'.
for (auto it = inEdges[dstId].begin(); it != inEdges[dstId].end(); ++it) {
if ((*it).id == srcId && (*it).memref == memref) {
inEdges[dstId].erase(it);
break;
}
}
// Remove 'dstId' from 'outEdges[srcId]'.
for (auto it = outEdges[srcId].begin(); it != outEdges[srcId].end(); ++it) {
if ((*it).id == dstId && (*it).memref == memref) {
outEdges[srcId].erase(it);
break;
}
}
}
// Returns the input edge count for node 'id' and 'memref'.
unsigned getInEdgeCount(unsigned id, Value *memref) {
unsigned inEdgeCount = 0;
if (inEdges.count(id) > 0)
for (auto &inEdge : inEdges[id])
if (inEdge.memref == memref)
++inEdgeCount;
return inEdgeCount;
}
// Returns the output edge count for node 'id' and 'memref'.
unsigned getOutEdgeCount(unsigned id, Value *memref) {
unsigned outEdgeCount = 0;
if (outEdges.count(id) > 0)
for (auto &outEdge : outEdges[id])
if (outEdge.memref == memref)
++outEdgeCount;
return outEdgeCount;
}
// Returns the min node id of all output edges from node 'id'.
unsigned getMinOutEdgeNodeId(unsigned id) {
unsigned minId = std::numeric_limits<unsigned>::max();
if (outEdges.count(id) > 0)
for (auto &outEdge : outEdges[id])
minId = std::min(minId, outEdge.id);
return minId;
}
// Updates edge mappings from node 'srcId' to node 'dstId' and removes
// state associated with node 'srcId'.
void updateEdgesAndRemoveSrcNode(unsigned srcId, unsigned dstId) {
// For each edge in 'inEdges[srcId]': add new edge remaping to 'dstId'.
if (inEdges.count(srcId) > 0) {
SmallVector<Edge, 2> oldInEdges = inEdges[srcId];
for (auto &inEdge : oldInEdges) {
// Remove edge from 'inEdge.id' to 'srcId'.
removeEdge(inEdge.id, srcId, inEdge.memref);
// Add edge from 'inEdge.id' to 'dstId'.
addEdge(inEdge.id, dstId, inEdge.memref);
}
}
// For each edge in 'outEdges[srcId]': add new edge remaping to 'dstId'.
if (outEdges.count(srcId) > 0) {
SmallVector<Edge, 2> oldOutEdges = outEdges[srcId];
for (auto &outEdge : oldOutEdges) {
// Remove edge from 'srcId' to 'outEdge.id'.
removeEdge(srcId, outEdge.id, outEdge.memref);
// Add edge from 'dstId' to 'outEdge.id' (if 'outEdge.id' != 'dstId').
if (outEdge.id != dstId)
addEdge(dstId, outEdge.id, outEdge.memref);
}
}
// Remove 'srcId' from graph state.
inEdges.erase(srcId);
outEdges.erase(srcId);
nodes.erase(srcId);
}
// Adds ops in 'loads' and 'stores' to node at 'id'.
void addToNode(unsigned id, const SmallVectorImpl<OperationInst *> &loads,
const SmallVectorImpl<OperationInst *> &stores) {
Node *node = getNode(id);
for (auto *loadOpInst : loads)
node->loads.push_back(loadOpInst);
for (auto *storeOpInst : stores)
node->stores.push_back(storeOpInst);
}
void print(raw_ostream &os) const {
os << "\nMemRefDependenceGraph\n";
os << "\nNodes:\n";
for (auto &idAndNode : nodes) {
os << "Node: " << idAndNode.first << "\n";
auto it = inEdges.find(idAndNode.first);
if (it != inEdges.end()) {
for (const auto &e : it->second)
os << " InEdge: " << e.id << " " << e.memref << "\n";
}
it = outEdges.find(idAndNode.first);
if (it != outEdges.end()) {
for (const auto &e : it->second)
os << " OutEdge: " << e.id << " " << e.memref << "\n";
}
}
}
void dump() const { print(llvm::errs()); }
};
// Intializes the data dependence graph by walking instructions in 'f'.
// Assigns each node in the graph a node id based on program order in 'f'.
// TODO(andydavis) Add support for taking a Block arg to construct the
// dependence graph at a different depth.
bool MemRefDependenceGraph::init(Function *f) {
unsigned id = 0;
DenseMap<Value *, SetVector<unsigned>> memrefAccesses;
// TODO: support multi-block functions.
if (f->getBlocks().size() != 1)
return false;
for (auto &inst : f->front()) {
if (auto *forInst = dyn_cast<ForInst>(&inst)) {
// Create graph node 'id' to represent top-level 'forInst' and record
// all loads and store accesses it contains.
LoopNestStateCollector collector;
collector.walkForInst(forInst);
// Return false if IfInsts are found (not currently supported).
if (collector.hasIfInst)
return false;
Node node(id++, &inst);
for (auto *opInst : collector.loadOpInsts) {
node.loads.push_back(opInst);
auto *memref = opInst->cast<LoadOp>()->getMemRef();
memrefAccesses[memref].insert(node.id);
}
for (auto *opInst : collector.storeOpInsts) {
node.stores.push_back(opInst);
auto *memref = opInst->cast<StoreOp>()->getMemRef();
memrefAccesses[memref].insert(node.id);
}
nodes.insert({node.id, node});
}
if (auto *opInst = dyn_cast<OperationInst>(&inst)) {
if (auto loadOp = opInst->dyn_cast<LoadOp>()) {
// Create graph node for top-level load op.
Node node(id++, &inst);
node.loads.push_back(opInst);
auto *memref = opInst->cast<LoadOp>()->getMemRef();
memrefAccesses[memref].insert(node.id);
nodes.insert({node.id, node});
}
if (auto storeOp = opInst->dyn_cast<StoreOp>()) {
// Create graph node for top-level store op.
Node node(id++, &inst);
node.stores.push_back(opInst);
auto *memref = opInst->cast<StoreOp>()->getMemRef();
memrefAccesses[memref].insert(node.id);
nodes.insert({node.id, node});
}
}
// Return false if IfInsts are found (not currently supported).
if (isa<IfInst>(&inst))
return false;
}
// Walk memref access lists and add graph edges between dependent nodes.
for (auto &memrefAndList : memrefAccesses) {
unsigned n = memrefAndList.second.size();
for (unsigned i = 0; i < n; ++i) {
unsigned srcId = memrefAndList.second[i];
bool srcHasStore =
getNode(srcId)->getStoreOpCount(memrefAndList.first) > 0;
for (unsigned j = i + 1; j < n; ++j) {
unsigned dstId = memrefAndList.second[j];
bool dstHasStore =
getNode(dstId)->getStoreOpCount(memrefAndList.first) > 0;
if (srcHasStore || dstHasStore)
addEdge(srcId, dstId, memrefAndList.first);
}
}
}
return true;
}
namespace {
// LoopNestStats aggregates various per-loop statistics (eg. loop trip count
// and operation count) for a loop nest up until the innermost loop body.
struct LoopNestStats {
// Map from ForInst to immediate child ForInsts in its loop body.
DenseMap<ForInst *, SmallVector<ForInst *, 2>> loopMap;
// Map from ForInst to count of operations in its loop body.
DenseMap<ForInst *, uint64_t> opCountMap;
// Map from ForInst to its constant trip count.
DenseMap<ForInst *, uint64_t> tripCountMap;
};
// LoopNestStatsCollector walks a single loop nest and gathers per-loop
// trip count and operation count statistics and records them in 'stats'.
class LoopNestStatsCollector : public InstWalker<LoopNestStatsCollector> {
public:
LoopNestStats *stats;
bool hasLoopWithNonConstTripCount = false;
LoopNestStatsCollector(LoopNestStats *stats) : stats(stats) {}
void visitForInst(ForInst *forInst) {
auto *parentInst = forInst->getParentInst();
if (parentInst != nullptr) {
assert(isa<ForInst>(parentInst) && "Expected parent ForInst");
// Add mapping to 'forInst' from its parent ForInst.
stats->loopMap[cast<ForInst>(parentInst)].push_back(forInst);
}
// Record the number of op instructions in the body of 'forInst'.
unsigned count = 0;
stats->opCountMap[forInst] = 0;
for (auto &inst : *forInst->getBody()) {
if (isa<OperationInst>(&inst))
++count;
}
stats->opCountMap[forInst] = count;
// Record trip count for 'forInst'. Set flag if trip count is not constant.
Optional<uint64_t> maybeConstTripCount = getConstantTripCount(*forInst);
if (!maybeConstTripCount.hasValue()) {
hasLoopWithNonConstTripCount = true;
return;
}
stats->tripCountMap[forInst] = maybeConstTripCount.getValue();
}
};
// Computes the total cost of the loop nest rooted at 'forInst'.
// Currently, the total cost is computed by counting the total operation
// instance count (i.e. total number of operations in the loop bodyloop
// operation count * loop trip count) for the entire loop nest.
// If 'tripCountOverrideMap' is non-null, overrides the trip count for loops
// specified in the map when computing the total op instance count.
// NOTE: this is used to compute the cost of computation slices, which are
// sliced along the iteration dimension, and thus reduce the trip count.
// If 'computeCostMap' is non-null, the total op count for forInsts specified
// in the map is increased (not overridden) by adding the op count from the
// map to the existing op count for the for loop. This is done before
// multiplying by the loop's trip count, and is used to model the cost of
// inserting a sliced loop nest of known cost into the loop's body.
// NOTE: this is used to compute the cost of fusing a slice of some loop nest
// within another loop.
static uint64_t getComputeCost(
ForInst *forInst, LoopNestStats *stats,
llvm::SmallDenseMap<ForInst *, uint64_t, 8> *tripCountOverrideMap,
DenseMap<ForInst *, uint64_t> *computeCostMap) {
// 'opCount' is the total number operations in one iteration of 'forInst' body
uint64_t opCount = stats->opCountMap[forInst];
if (stats->loopMap.count(forInst) > 0) {
for (auto *childForInst : stats->loopMap[forInst]) {
opCount += getComputeCost(childForInst, stats, tripCountOverrideMap,
computeCostMap);
}
}
// Add in additional op instances from slice (if specified in map).
if (computeCostMap != nullptr) {
auto it = computeCostMap->find(forInst);
if (it != computeCostMap->end()) {
opCount += it->second;
}
}
// Override trip count (if specified in map).
uint64_t tripCount = stats->tripCountMap[forInst];
if (tripCountOverrideMap != nullptr) {
auto it = tripCountOverrideMap->find(forInst);
if (it != tripCountOverrideMap->end()) {
tripCount = it->second;
}
}
// Returns the total number of dynamic instances of operations in loop body.
return tripCount * opCount;
}
} // end anonymous namespace
static Optional<uint64_t> getConstDifference(AffineMap lbMap, AffineMap ubMap) {
assert(lbMap.getNumResults() == 1);
assert(ubMap.getNumResults() == 1);
assert(lbMap.getNumDims() == ubMap.getNumDims());
assert(lbMap.getNumSymbols() == ubMap.getNumSymbols());
// TODO(andydavis) Merge this code with 'mlir::getTripCountExpr'.
// ub_expr - lb_expr
AffineExpr lbExpr(lbMap.getResult(0));
AffineExpr ubExpr(ubMap.getResult(0));
auto loopSpanExpr = simplifyAffineExpr(ubExpr - lbExpr, lbMap.getNumDims(),
lbMap.getNumSymbols());
auto cExpr = loopSpanExpr.dyn_cast<AffineConstantExpr>();
if (!cExpr)
return None;
return cExpr.getValue();
}
// Builds a map 'tripCountMap' from ForInst to constant trip count for loop
// nest surrounding 'srcAccess' utilizing slice loop bounds in 'sliceState'.
// Returns true on success, false otherwise (if a non-constant trip count
// was encountered).
// TODO(andydavis) Make this work with non-unit step loops.
static bool buildSliceTripCountMap(
OperationInst *srcOpInst, ComputationSliceState *sliceState,
llvm::SmallDenseMap<ForInst *, uint64_t, 8> *tripCountMap) {
SmallVector<ForInst *, 4> srcLoopIVs;
getLoopIVs(*srcOpInst, &srcLoopIVs);
unsigned numSrcLoopIVs = srcLoopIVs.size();
// Populate map from ForInst -> trip count
for (unsigned i = 0; i < numSrcLoopIVs; ++i) {
AffineMap lbMap = sliceState->lbs[i];
AffineMap ubMap = sliceState->ubs[i];
if (lbMap == AffineMap::Null() || ubMap == AffineMap::Null()) {
// The iteration of src loop IV 'i' was not sliced. Use full loop bounds.
if (srcLoopIVs[i]->hasConstantLowerBound() &&
srcLoopIVs[i]->hasConstantUpperBound()) {
(*tripCountMap)[srcLoopIVs[i]] =
srcLoopIVs[i]->getConstantUpperBound() -
srcLoopIVs[i]->getConstantLowerBound();
continue;
}
return false;
}
Optional<uint64_t> tripCount = getConstDifference(lbMap, ubMap);
if (!tripCount.hasValue())
return false;
(*tripCountMap)[srcLoopIVs[i]] = tripCount.getValue();
}
return true;
}
// Removes load operations from 'srcLoads' which operate on 'memref', and
// adds them to 'dstLoads'.
static void
moveLoadsAccessingMemrefTo(Value *memref,
SmallVectorImpl<OperationInst *> *srcLoads,
SmallVectorImpl<OperationInst *> *dstLoads) {
dstLoads->clear();
SmallVector<OperationInst *, 4> srcLoadsToKeep;
for (auto *load : *srcLoads) {
if (load->cast<LoadOp>()->getMemRef() == memref)
dstLoads->push_back(load);
else
srcLoadsToKeep.push_back(load);
}
srcLoads->swap(srcLoadsToKeep);
}
// Returns the innermost common loop depth for the set of operations in 'ops'.
static unsigned getInnermostCommonLoopDepth(ArrayRef<OperationInst *> ops) {
unsigned numOps = ops.size();
assert(numOps > 0);
std::vector<SmallVector<ForInst *, 4>> loops(numOps);
unsigned loopDepthLimit = std::numeric_limits<unsigned>::max();
for (unsigned i = 0; i < numOps; ++i) {
getLoopIVs(*ops[i], &loops[i]);
loopDepthLimit =
std::min(loopDepthLimit, static_cast<unsigned>(loops[i].size()));
}
unsigned loopDepth = 0;
for (unsigned d = 0; d < loopDepthLimit; ++d) {
unsigned i;
for (i = 1; i < numOps; ++i) {
if (loops[i - 1][d] != loops[i][d]) {
break;
}
}
if (i != numOps)
break;
++loopDepth;
}
return loopDepth;
}
// Returns true if 'map' is a single result constant or single result
// dim expr where its corresponding loop IV in 'operands' has zero constant
// lower bound.
static bool hasZeroMinValue(AffineMap map, ArrayRef<Value *> operands) {
if (map.isSingleConstant() && map.getSingleConstantResult() == 0)
return true;
if (map.getNumResults() != 1 || !map.getResult(0).isa<AffineDimExpr>())
return false;
// Get operand position of single dim expr result.
unsigned pos = map.getResult(0).cast<AffineDimExpr>().getPosition();
// Check if loop IV at 'pos' has zero constant lower bound.
auto *operand = operands[pos];
assert(isa<ForInst>(operand));
auto *forInst = cast<ForInst>(operand);
return forInst->hasConstantLowerBound() &&
forInst->getConstantLowerBound() == 0;
}
// Returns the slice bound union of 'sliceStateA' and 'sliceStateB' in
// 'sliceStateB'.
// TODO(andydavis) This function assumes that lower bounds for 'sliceStateA'
// and 'sliceStateB' are aligned.
// Specifically, when taking the union of overlapping intervals, it assumes
// that both intervals start at zero. Support needs to be added to take into
// account interval start offset when computing the union.
// TODO(andydavis) Move this function to an analysis library.
static bool getSliceBoundUnion(const ComputationSliceState &sliceStateA,
ComputationSliceState *sliceStateB) {
assert(sliceStateA.lbs.size() == sliceStateB->lbs.size());
assert(sliceStateA.ubs.size() == sliceStateB->ubs.size());
for (unsigned i = 0, e = sliceStateA.lbs.size(); i < e; ++i) {
AffineMap lbMapA = sliceStateA.lbs[i];
AffineMap ubMapA = sliceStateA.ubs[i];
if (lbMapA == AffineMap::Null()) {
assert(ubMapA == AffineMap::Null());
continue;
}
assert(ubMapA != AffineMap::Null());
// Validate that constant lower bounds are aligned at zero.
if (!hasZeroMinValue(lbMapA, sliceStateA.lbOperands[i]))
return false;
AffineMap lbMapB = sliceStateB->lbs[i];
AffineMap ubMapB = sliceStateB->ubs[i];
if (lbMapB == AffineMap::Null()) {
assert(ubMapB == AffineMap::Null());
// Union 'sliceStateB' does not have a bound for 'i' so copy from A.
sliceStateB->lbs[i] = lbMapA;
sliceStateB->ubs[i] = ubMapA;
continue;
}
// Validate that constant lower bounds are aligned at zero.
if (!hasZeroMinValue(lbMapB, sliceStateB->lbOperands[i]))
return false;
// Add bound with the largest trip count to union.
Optional<uint64_t> tripCountA = getConstDifference(lbMapA, ubMapA);
Optional<uint64_t> tripCountB = getConstDifference(lbMapB, ubMapB);
if (!tripCountA.hasValue() || !tripCountB.hasValue())
return false;
// TODO(andydavis) Change this code to take the min across all lower bounds
// and max across all upper bounds for each dimension. This code can for
// cases where a unique min or max could not be statically determined.
if (tripCountA.getValue() > tripCountB.getValue()) {
sliceStateB->lbs[i] = lbMapA;
sliceStateB->ubs[i] = ubMapA;
}
}
return true;
}
// Checks the profitability of fusing a backwards slice of the loop nest
// surrounding 'srcOpInst' into the loop nest surrounding 'dstOpInsts'.
// Returns true if it profitable to fuse the candidate loop nests. Returns
// false otherwise.
// The profitability model executes the following steps:
// *) Computes the backward computation slice at 'srcOpInst'. This
// computation slice of the loop nest surrounding 'srcOpInst' is
// represented by modified src loop bounds in 'sliceState', which are
// functions of loop IVs in the loop nest surrounding 'srcOpInst'.
// *) Computes the cost of unfused src/dst loop nests (currently the cost of a
// loop nest is the total number of dynamic operation instances in the loop
// nest).
// *) Computes the cost of fusing a slice of the src loop nest into the dst
// loop nest at various values of dst loop depth, attempting to fuse
// the largest compution slice at the maximal dst loop depth (closest to the
// load) to minimize reuse distance and potentially enable subsequent
// load/store forwarding.
// NOTE: If the dst loop nest includes multiple loads in 'dstOpInsts' for
// the same memref as is written by 'srcOpInst', then the union of slice
// loop bounds is used to compute the slice and associated slice cost.
// NOTE: 'dstLoopDepth' refers the loop depth within the destination loop
// nest, at which the src computation slice is inserted/fused.
// NOTE: We attempt to maximize the dst loop depth, but there are cases
// where a particular setting for 'dstLoopNest' might fuse an unsliced
// loop (within the src computation slice) at a depth which results in
// execessive recomputation (see unit tests for examples).
// *) Compares the total cost of the unfused loop nests to the min cost fused
// loop nest computed in the previous step, and returns true if the latter
// is lower.
static bool isFusionProfitable(OperationInst *srcOpInst,
ArrayRef<OperationInst *> dstOpInsts,
ComputationSliceState *sliceState,
unsigned *dstLoopDepth) {
// Compute cost of sliced and unsliced src loop nest.
SmallVector<ForInst *, 4> srcLoopIVs;
getLoopIVs(*srcOpInst, &srcLoopIVs);
unsigned numSrcLoopIVs = srcLoopIVs.size();
// Walk src loop nest and collect stats.
LoopNestStats srcLoopNestStats;
LoopNestStatsCollector srcStatsCollector(&srcLoopNestStats);
srcStatsCollector.walk(srcLoopIVs[0]);
// Currently only constant trip count loop nests are supported.
if (srcStatsCollector.hasLoopWithNonConstTripCount)
return false;
// Compute cost of dst loop nest.
SmallVector<ForInst *, 4> dstLoopIVs;
getLoopIVs(*dstOpInsts[0], &dstLoopIVs);
LoopNestStats dstLoopNestStats;
LoopNestStatsCollector dstStatsCollector(&dstLoopNestStats);
dstStatsCollector.walk(dstLoopIVs[0]);
// Currently only constant trip count loop nests are supported.
if (dstStatsCollector.hasLoopWithNonConstTripCount)
return false;
// Compute the innermost common loop for ops in 'dstOpInst'.
unsigned maxDstLoopDepth = getInnermostCommonLoopDepth(dstOpInsts);
if (maxDstLoopDepth == 0)
return false;
// Search for min cost value for 'dstLoopDepth'. At each value of
// 'dstLoopDepth' from 'maxDstLoopDepth' to '1', compute computation slice
// bounds between 'srcOpInst' and each op in 'dstOpinsts' (taking the union
// of these bounds). Next the union slice bounds are used to calculate
// the cost of the slice and the cost of the slice inserted into the dst
// loop nest at 'dstLoopDepth'.
unsigned minFusedLoopNestComputeCost = std::numeric_limits<unsigned>::max();
unsigned bestDstLoopDepth;
SmallVector<ComputationSliceState, 4> sliceStates;
sliceStates.resize(maxDstLoopDepth);
llvm::SmallDenseMap<ForInst *, uint64_t, 8> sliceTripCountMap;
DenseMap<ForInst *, uint64_t> computeCostMap;
for (unsigned i = maxDstLoopDepth; i >= 1; --i) {
MemRefAccess srcAccess(srcOpInst);
// Handle the common case of one dst load without a copy.
if (!mlir::getBackwardComputationSliceState(
srcAccess, MemRefAccess(dstOpInsts[0]), i, &sliceStates[i - 1]))
return false;
// Compute the union of slice bound of all ops in 'dstOpInsts'.
for (int j = 1, e = dstOpInsts.size(); j < e; ++j) {
MemRefAccess dstAccess(dstOpInsts[j]);
ComputationSliceState tmpSliceState;
if (!mlir::getBackwardComputationSliceState(srcAccess, dstAccess, i,
&tmpSliceState))
return false;
// Compute slice boun dunion of 'tmpSliceState' and 'sliceStates[i - 1]'.
getSliceBoundUnion(tmpSliceState, &sliceStates[i - 1]);
}
// Build trip count map for computation slice.
sliceTripCountMap.clear();
if (!buildSliceTripCountMap(srcOpInst, &sliceStates[i - 1],
&sliceTripCountMap))
return false;
// Compute op instance count for the src loop nest with iteration slicing.
uint64_t sliceComputeCost =
getComputeCost(srcLoopIVs[0], &srcLoopNestStats, &sliceTripCountMap,
/*computeCostMap=*/nullptr);
// Compute cost of fusion for these values of 'i' and 'j'.
computeCostMap.clear();
computeCostMap[dstLoopIVs[i - 1]] = sliceComputeCost;
uint64_t fusedLoopNestComputeCost =
getComputeCost(dstLoopIVs[0], &dstLoopNestStats,
/*tripCountOverrideMap=*/nullptr, &computeCostMap);
if (fusedLoopNestComputeCost < minFusedLoopNestComputeCost) {
minFusedLoopNestComputeCost = fusedLoopNestComputeCost;
bestDstLoopDepth = i;
}
}
// Compute op instance count for the src loop nest without iteration slicing.
uint64_t srcLoopNestCost = getComputeCost(srcLoopIVs[0], &srcLoopNestStats,
/*tripCountOverrideMap=*/nullptr,
/*computeCostMap=*/nullptr);
// Compute op instance count for the src loop nest.
uint64_t dstLoopNestCost = getComputeCost(dstLoopIVs[0], &dstLoopNestStats,
/*tripCountOverrideMap=*/nullptr,
/*computeCostMap=*/nullptr);
LLVM_DEBUG(llvm::dbgs() << "LoopFusion statistics "
<< " bestDstLoopDepth: " << bestDstLoopDepth
<< " srcLoopNestCost: " << srcLoopNestCost
<< " dstLoopNestCost: " << dstLoopNestCost
<< " minFusedLoopNestComputeCost: "
<< minFusedLoopNestComputeCost << "\n");
// Do not fuse if fused loop would increase the total cost of the computation.
// TODO(andydavis) Use locality/reduction in slice memref size/opportunity
// for load/store forwarding in cost model.
if (minFusedLoopNestComputeCost > srcLoopNestCost + dstLoopNestCost)
return false;
// Update return parameter 'sliceState' with 'bestSliceState'.
ComputationSliceState *bestSliceState = &sliceStates[bestDstLoopDepth - 1];
sliceState->lbs = bestSliceState->lbs;
sliceState->ubs = bestSliceState->ubs;
sliceState->lbOperands = bestSliceState->lbOperands;
sliceState->ubOperands = bestSliceState->ubOperands;
// Set dstLoopDepth based on best values from search.
*dstLoopDepth = bestDstLoopDepth;
// Canonicalize slice bound affine maps.
for (unsigned i = 0; i < numSrcLoopIVs; ++i) {
if (sliceState->lbs[i] != AffineMap::Null()) {
canonicalizeMapAndOperands(&sliceState->lbs[i],
&sliceState->lbOperands[i]);
}
if (sliceState->ubs[i] != AffineMap::Null()) {
canonicalizeMapAndOperands(&sliceState->ubs[i],
&sliceState->ubOperands[i]);
}
}
return true;
}
// GreedyFusion greedily fuses loop nests which have a producer/consumer
// relationship on a memref, with the goal of improving locality. Currently,
// this the producer/consumer relationship is required to be unique in the
// Function (there are TODOs to relax this constraint in the future).
//
// The steps of the algorithm are as follows:
//
// *) A worklist is initialized with node ids from the dependence graph.
// *) For each node id in the worklist:
// *) Pop a ForInst of the worklist. This 'dstForInst' will be a candidate
// destination ForInst into which fusion will be attempted.
// *) Add each LoadOp currently in 'dstForInst' into list 'dstLoadOps'.
// *) For each LoadOp in 'dstLoadOps' do:
// *) Lookup dependent loop nests at earlier positions in the Function
// which have a single store op to the same memref.
// *) Check if dependences would be violated by the fusion. For example,
// the src loop nest may load from memrefs which are different than
// the producer-consumer memref between src and dest loop nests.
// *) Get a computation slice of 'srcLoopNest', which adjusts its loop
// bounds to be functions of 'dstLoopNest' IVs and symbols.
// *) Fuse the 'srcLoopNest' computation slice into the 'dstLoopNest',
// just before the dst load op user.
// *) Add the newly fused load/store operation instructions to the state,
// and also add newly fuse load ops to 'dstLoopOps' to be considered
// as fusion dst load ops in another iteration.
// *) Remove old src loop nest and its associated state.
//
// Given a graph where top-level instructions are vertices in the set 'V' and
// edges in the set 'E' are dependences between vertices, this algorithm
// takes O(V) time for initialization, and has runtime O(V + E).
//
// This greedy algorithm is not 'maximal' due to the current restriction of
// fusing along single producer consumer edges, but there is a TODO to fix this.
//
// TODO(andydavis) Experiment with other fusion policies.
// TODO(andydavis) Add support for fusing for input reuse (perhaps by
// constructing a graph with edges which represent loads from the same memref
// in two different loop nestst.
struct GreedyFusion {
public:
MemRefDependenceGraph *mdg;
SmallVector<unsigned, 4> worklist;
GreedyFusion(MemRefDependenceGraph *mdg) : mdg(mdg) {
// Initialize worklist with nodes from 'mdg'.
worklist.resize(mdg->nodes.size());
std::iota(worklist.begin(), worklist.end(), 0);
}
void run() {
while (!worklist.empty()) {
unsigned dstId = worklist.back();
worklist.pop_back();
// Skip if this node was removed (fused into another node).
if (mdg->nodes.count(dstId) == 0)
continue;
// Get 'dstNode' into which to attempt fusion.
auto *dstNode = mdg->getNode(dstId);
// Skip if 'dstNode' is not a loop nest.
if (!isa<ForInst>(dstNode->inst))
continue;
SmallVector<OperationInst *, 4> loads = dstNode->loads;
SmallVector<OperationInst *, 4> dstLoadOpInsts;
while (!loads.empty()) {
// Get memref of load on top of the stack.
auto *memref = loads.back()->cast<LoadOp>()->getMemRef();
// Move all loads in 'loads' accessing 'memref' to 'dstLoadOpInsts'.
moveLoadsAccessingMemrefTo(memref, &loads, &dstLoadOpInsts);
// Skip if no input edges along which to fuse.
if (mdg->inEdges.count(dstId) == 0)
continue;
// Iterate through in edges for 'dstId'.
for (auto &srcEdge : mdg->inEdges[dstId]) {
// Skip 'srcEdge' if not for 'memref'.
if (srcEdge.memref != memref)
continue;
auto *srcNode = mdg->getNode(srcEdge.id);
// Skip if 'srcNode' is not a loop nest.
if (!isa<ForInst>(srcNode->inst))
continue;
// Skip if 'srcNode' has more than one store to 'memref'.
if (srcNode->getStoreOpCount(memref) != 1)
continue;
// Skip 'srcNode' if it has out edges on 'memref' other than 'dstId'.
if (mdg->getOutEdgeCount(srcNode->id, memref) != 1)
continue;
// Skip 'srcNode' if it has in dependence edges. NOTE: This is overly
// TODO(andydavis) Track dependence type with edges, and just check
// for WAW dependence edge here.
if (mdg->getInEdgeCount(srcNode->id, memref) != 0)
continue;
// Skip if 'srcNode' has out edges to other memrefs after 'dstId'.
if (mdg->getMinOutEdgeNodeId(srcNode->id) != dstId)
continue;
// Get unique 'srcNode' store op.
auto *srcStoreOpInst = srcNode->stores.front();
// Check if fusion would be profitable.
unsigned dstLoopDepth;
mlir::ComputationSliceState sliceState;
if (!isFusionProfitable(srcStoreOpInst, dstLoadOpInsts, &sliceState,
&dstLoopDepth))
continue;
// Fuse computation slice of 'srcLoopNest' into 'dstLoopNest'.
auto *sliceLoopNest = mlir::insertBackwardComputationSlice(
srcStoreOpInst, dstLoadOpInsts[0], dstLoopDepth, &sliceState);
if (sliceLoopNest != nullptr) {
// Remove edges between 'srcNode' and 'dstNode' and remove 'srcNode'
mdg->updateEdgesAndRemoveSrcNode(srcNode->id, dstNode->id);
// Record all load/store accesses in 'sliceLoopNest' at 'dstPos'.
LoopNestStateCollector collector;
collector.walkForInst(sliceLoopNest);
mdg->addToNode(dstId, collector.loadOpInsts,
collector.storeOpInsts);
// Add new load ops to current Node load op list 'loads' to
// continue fusing based on new operands.
for (auto *loadOpInst : collector.loadOpInsts)
loads.push_back(loadOpInst);
// Promote single iteration loops to single IV value.
for (auto *forInst : collector.forInsts) {
promoteIfSingleIteration(forInst);
}
// Remove old src loop nest.
cast<ForInst>(srcNode->inst)->erase();
}
}
}
}
}
};
} // end anonymous namespace
PassResult LoopFusion::runOnFunction(Function *f) {
MemRefDependenceGraph g;
if (g.init(f))
GreedyFusion(&g).run();
return success();
}
static PassRegistration<LoopFusion> pass("loop-fusion", "Fuse loop nests");
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