// Copyright 2015 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/compiler/common-operator.h"
#include "src/compiler/graph.h"
#include "src/compiler/loop-peeling.h"
#include "src/compiler/node.h"
#include "src/compiler/node-marker.h"
#include "src/compiler/node-properties.h"
#include "src/zone.h"
// Loop peeling is an optimization that copies the body of a loop, creating
// a new copy of the body called the "peeled iteration" that represents the
// first iteration. Beginning with a loop as follows:
// E
// | A
// | | (backedges)
// | +---------------|---------------------------------+
// | | +-------------|-------------------------------+ |
// | | | | +--------+ | |
// | | | | | +----+ | | |
// | | | | | | | | | |
// ( Loop )<-------- ( phiA ) | | | |
// | | | | | |
// ((======P=================U=======|=|=====)) | |
// (( | | )) | |
// (( X <---------------------+ | )) | |
// (( | )) | |
// (( body | )) | |
// (( | )) | |
// (( Y <-----------------------+ )) | |
// (( )) | |
// ((===K====L====M==========================)) | |
// | | | | |
// | | +-----------------------------------------+ |
// | +------------------------------------------------+
// |
// exit
// The body of the loop is duplicated so that all nodes considered "inside"
// the loop (e.g. {P, U, X, Y, K, L, M}) have a corresponding copies in the
// peeled iteration (e.g. {P', U', X', Y', K', L', M'}). What were considered
// backedges of the loop correspond to edges from the peeled iteration to
// the main loop body, with multiple backedges requiring a merge.
// Similarly, any exits from the loop body need to be merged with "exits"
// from the peeled iteration, resulting in the graph as follows:
// E
// | A
// | |
// ((=====P'================U'===============))
// (( ))
// (( X'<-------------+ ))
// (( | ))
// (( peeled iteration | ))
// (( | ))
// (( Y'<-----------+ | ))
// (( | | ))
// ((===K'===L'====M'======|=|===============))
// | | | | |
// +--------+ +-+ +-+ | |
// | | | | |
// | Merge <------phi
// | | |
// | +-----+ |
// | | | (backedges)
// | | +---------------|---------------------------------+
// | | | +-------------|-------------------------------+ |
// | | | | | +--------+ | |
// | | | | | | +----+ | | |
// | | | | | | | | | | |
// | ( Loop )<-------- ( phiA ) | | | |
// | | | | | | |
// | ((======P=================U=======|=|=====)) | |
// | (( | | )) | |
// | (( X <---------------------+ | )) | |
// | (( | )) | |
// | (( body | )) | |
// | (( | )) | |
// | (( Y <-----------------------+ )) | |
// | (( )) | |
// | ((===K====L====M==========================)) | |
// | | | | | |
// | | | +-----------------------------------------+ |
// | | +------------------------------------------------+
// | |
// | |
// +----+ +-+
// | |
// Merge
// |
// exit
// Note that the boxes ((===)) above are not explicitly represented in the
// graph, but are instead computed by the {LoopFinder}.
namespace v8 {
namespace internal {
namespace compiler {
struct Peeling {
// Maps a node to its index in the {pairs} vector.
NodeMarker<size_t> node_map;
// The vector which contains the mapped nodes.
NodeVector* pairs;
Peeling(Graph* graph, Zone* tmp_zone, size_t max, NodeVector* p)
: node_map(graph, static_cast<uint32_t>(max)), pairs(p) {}
Node* map(Node* node) {
if (node_map.Get(node) == 0) return node;
return pairs->at(node_map.Get(node));
}
void Insert(Node* original, Node* copy) {
node_map.Set(original, 1 + pairs->size());
pairs->push_back(original);
pairs->push_back(copy);
}
void CopyNodes(Graph* graph, Zone* tmp_zone, Node* dead, NodeRange nodes) {
NodeVector inputs(tmp_zone);
// Copy all the nodes first.
for (Node* node : nodes) {
inputs.clear();
for (Node* input : node->inputs()) inputs.push_back(map(input));
Insert(node, graph->NewNode(node->op(), node->InputCount(), &inputs[0]));
}
// Fix remaining inputs of the copies.
for (Node* original : nodes) {
Node* copy = pairs->at(node_map.Get(original));
for (int i = 0; i < copy->InputCount(); i++) {
copy->ReplaceInput(i, map(original->InputAt(i)));
}
}
}
bool Marked(Node* node) { return node_map.Get(node) > 0; }
};
class PeeledIterationImpl : public PeeledIteration {
public:
NodeVector node_pairs_;
explicit PeeledIterationImpl(Zone* zone) : node_pairs_(zone) {}
};
Node* PeeledIteration::map(Node* node) {
// TODO(turbofan): we use a simple linear search, since the peeled iteration
// is really only used in testing.
PeeledIterationImpl* impl = static_cast<PeeledIterationImpl*>(this);
for (size_t i = 0; i < impl->node_pairs_.size(); i += 2) {
if (impl->node_pairs_[i] == node) return impl->node_pairs_[i + 1];
}
return node;
}
static void FindLoopExits(LoopTree* loop_tree, LoopTree::Loop* loop,
NodeVector& exits, NodeVector& rets) {
// Look for returns and if projections that are outside the loop but whose
// control input is inside the loop.
for (Node* node : loop_tree->LoopNodes(loop)) {
for (Node* use : node->uses()) {
if (!loop_tree->Contains(loop, use)) {
if (IrOpcode::IsIfProjectionOpcode(use->opcode())) {
// This is a branch from inside the loop to outside the loop.
exits.push_back(use);
} else if (use->opcode() == IrOpcode::kReturn &&
loop_tree->Contains(loop,
NodeProperties::GetControlInput(use))) {
// This is a return from inside the loop.
rets.push_back(use);
}
}
}
}
}
bool LoopPeeler::CanPeel(LoopTree* loop_tree, LoopTree::Loop* loop) {
Zone zone(loop_tree->zone()->allocator());
NodeVector exits(&zone);
NodeVector rets(&zone);
FindLoopExits(loop_tree, loop, exits, rets);
return exits.size() <= 1u;
}
PeeledIteration* LoopPeeler::Peel(Graph* graph, CommonOperatorBuilder* common,
LoopTree* loop_tree, LoopTree::Loop* loop,
Zone* tmp_zone) {
//============================================================================
// Find the loop exit region to determine if this loop can be peeled.
//============================================================================
NodeVector exits(tmp_zone);
NodeVector rets(tmp_zone);
FindLoopExits(loop_tree, loop, exits, rets);
if (exits.size() != 1) return nullptr; // not peelable currently.
//============================================================================
// Construct the peeled iteration.
//============================================================================
PeeledIterationImpl* iter = new (tmp_zone) PeeledIterationImpl(tmp_zone);
size_t estimated_peeled_size =
5 + (loop->TotalSize() + exits.size() + rets.size()) * 2;
Peeling peeling(graph, tmp_zone, estimated_peeled_size, &iter->node_pairs_);
Node* dead = graph->NewNode(common->Dead());
// Map the loop header nodes to their entry values.
for (Node* node : loop_tree->HeaderNodes(loop)) {
peeling.Insert(node, node->InputAt(kAssumedLoopEntryIndex));
}
// Copy all the nodes of loop body for the peeled iteration.
peeling.CopyNodes(graph, tmp_zone, dead, loop_tree->BodyNodes(loop));
//============================================================================
// Replace the entry to the loop with the output of the peeled iteration.
//============================================================================
Node* loop_node = loop_tree->GetLoopControl(loop);
Node* new_entry;
int backedges = loop_node->InputCount() - 1;
if (backedges > 1) {
// Multiple backedges from original loop, therefore multiple output edges
// from the peeled iteration.
NodeVector inputs(tmp_zone);
for (int i = 1; i < loop_node->InputCount(); i++) {
inputs.push_back(peeling.map(loop_node->InputAt(i)));
}
Node* merge =
graph->NewNode(common->Merge(backedges), backedges, &inputs[0]);
// Merge values from the multiple output edges of the peeled iteration.
for (Node* node : loop_tree->HeaderNodes(loop)) {
if (node->opcode() == IrOpcode::kLoop) continue; // already done.
inputs.clear();
for (int i = 0; i < backedges; i++) {
inputs.push_back(peeling.map(node->InputAt(1 + i)));
}
for (Node* input : inputs) {
if (input != inputs[0]) { // Non-redundant phi.
inputs.push_back(merge);
const Operator* op = common->ResizeMergeOrPhi(node->op(), backedges);
Node* phi = graph->NewNode(op, backedges + 1, &inputs[0]);
node->ReplaceInput(0, phi);
break;
}
}
}
new_entry = merge;
} else {
// Only one backedge, simply replace the input to loop with output of
// peeling.
for (Node* node : loop_tree->HeaderNodes(loop)) {
node->ReplaceInput(0, peeling.map(node->InputAt(0)));
}
new_entry = peeling.map(loop_node->InputAt(1));
}
loop_node->ReplaceInput(0, new_entry);
//============================================================================
// Duplicate the loop exit region and add a merge.
//============================================================================
// Currently we are limited to peeling loops with a single exit. The exit is
// the postdominator of the loop (ignoring returns).
Node* postdom = exits[0];
for (Node* node : rets) exits.push_back(node);
for (Node* use : postdom->uses()) {
if (NodeProperties::IsPhi(use)) exits.push_back(use);
}
NodeRange exit_range(&exits[0], &exits[0] + exits.size());
peeling.CopyNodes(graph, tmp_zone, dead, exit_range);
Node* merge = graph->NewNode(common->Merge(2), postdom, peeling.map(postdom));
postdom->ReplaceUses(merge);
merge->ReplaceInput(0, postdom); // input 0 overwritten by above line.
// Find and update all the edges into either the loop or exit region.
for (int i = 0; i < 2; i++) {
NodeRange range = i == 0 ? loop_tree->LoopNodes(loop) : exit_range;
ZoneVector<Edge> value_edges(tmp_zone);
ZoneVector<Edge> effect_edges(tmp_zone);
for (Node* node : range) {
// Gather value and effect edges from outside the region.
for (Edge edge : node->use_edges()) {
if (!peeling.Marked(edge.from())) {
// Edge from outside the loop into the region.
if (NodeProperties::IsValueEdge(edge) ||
NodeProperties::IsContextEdge(edge)) {
value_edges.push_back(edge);
} else if (NodeProperties::IsEffectEdge(edge)) {
effect_edges.push_back(edge);
} else {
// don't do anything for control edges.
// TODO(titzer): should update control edges to peeled?
}
}
}
// Update all the value and effect edges at once.
if (!value_edges.empty()) {
// TODO(titzer): machine type is wrong here.
Node* phi =
graph->NewNode(common->Phi(MachineRepresentation::kTagged, 2), node,
peeling.map(node), merge);
for (Edge edge : value_edges) edge.UpdateTo(phi);
value_edges.clear();
}
if (!effect_edges.empty()) {
Node* effect_phi = graph->NewNode(common->EffectPhi(2), node,
peeling.map(node), merge);
for (Edge edge : effect_edges) edge.UpdateTo(effect_phi);
effect_edges.clear();
}
}
}
return iter;
}
} // namespace compiler
} // namespace internal
} // namespace v8