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// 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/backend/instruction.h"
#include "src/zone/zone-containers.h"
namespace v8 {
namespace internal {
namespace compiler {
// A set of flags describing properties of the instructions so that the
// scheduler is aware of dependencies between instructions.
enum ArchOpcodeFlags {
kNoOpcodeFlags = 0,
kHasSideEffect = 1, // The instruction has some side effects (memory
// store, function call...)
kIsLoadOperation = 2, // The instruction is a memory load.
kMayNeedDeoptOrTrapCheck = 4, // The instruction may be associated with a
// deopt or trap check which must be run before
// instruction e.g. div on Intel platform which
// will raise an exception when the divisor is
// zero.
class InstructionScheduler final : public ZoneObject {
V8_EXPORT_PRIVATE InstructionScheduler(Zone* zone,
InstructionSequence* sequence);
V8_EXPORT_PRIVATE void StartBlock(RpoNumber rpo);
V8_EXPORT_PRIVATE void EndBlock(RpoNumber rpo);
V8_EXPORT_PRIVATE void AddInstruction(Instruction* instr);
V8_EXPORT_PRIVATE void AddTerminator(Instruction* instr);
static bool SchedulerSupported();
// A scheduling graph node.
// Represent an instruction and their dependencies.
class ScheduleGraphNode : public ZoneObject {
ScheduleGraphNode(Zone* zone, Instruction* instr);
// Mark the instruction represented by 'node' as a dependecy of this one.
// The current instruction will be registered as an unscheduled predecessor
// of 'node' (i.e. it must be scheduled before 'node').
void AddSuccessor(ScheduleGraphNode* node);
// Check if all the predecessors of this instruction have been scheduled.
bool HasUnscheduledPredecessor() {
return unscheduled_predecessors_count_ != 0;
// Record that we have scheduled one of the predecessors of this node.
void DropUnscheduledPredecessor() {
DCHECK_LT(0, unscheduled_predecessors_count_);
Instruction* instruction() { return instr_; }
ZoneDeque<ScheduleGraphNode*>& successors() { return successors_; }
int latency() const { return latency_; }
int total_latency() const { return total_latency_; }
void set_total_latency(int latency) { total_latency_ = latency; }
int start_cycle() const { return start_cycle_; }
void set_start_cycle(int start_cycle) { start_cycle_ = start_cycle; }
Instruction* instr_;
ZoneDeque<ScheduleGraphNode*> successors_;
// Number of unscheduled predecessors for this node.
int unscheduled_predecessors_count_;
// Estimate of the instruction latency (the number of cycles it takes for
// instruction to complete).
int latency_;
// The sum of all the latencies on the path from this node to the end of
// the graph (i.e. a node with no successor).
int total_latency_;
// The scheduler keeps a nominal cycle count to keep track of when the
// result of an instruction is available. This field is updated by the
// scheduler to indicate when the value of all the operands of this
// instruction will be available.
int start_cycle_;
// Keep track of all nodes ready to be scheduled (i.e. all their dependencies
// have been scheduled. Note that this class is inteded to be extended by
// concrete implementation of the scheduling queue which define the policy
// to pop node from the queue.
class SchedulingQueueBase {
explicit SchedulingQueueBase(InstructionScheduler* scheduler)
: scheduler_(scheduler), nodes_(scheduler->zone()) {}
void AddNode(ScheduleGraphNode* node);
bool IsEmpty() const { return nodes_.empty(); }
InstructionScheduler* scheduler_;
ZoneLinkedList<ScheduleGraphNode*> nodes_;
// A scheduling queue which prioritize nodes on the critical path (we look
// for the instruction with the highest latency on the path to reach the end
// of the graph).
class CriticalPathFirstQueue : public SchedulingQueueBase {
explicit CriticalPathFirstQueue(InstructionScheduler* scheduler)
: SchedulingQueueBase(scheduler) {}
// Look for the best candidate to schedule, remove it from the queue and
// return it.
ScheduleGraphNode* PopBestCandidate(int cycle);
// A queue which pop a random node from the queue to perform stress tests on
// the scheduler.
class StressSchedulerQueue : public SchedulingQueueBase {
explicit StressSchedulerQueue(InstructionScheduler* scheduler)
: SchedulingQueueBase(scheduler) {}
ScheduleGraphNode* PopBestCandidate(int cycle);
Isolate* isolate() { return scheduler_->isolate(); }
// Perform scheduling for the current block specifying the queue type to
// use to determine the next best candidate.
template <typename QueueType>
void ScheduleBlock();
// Return the scheduling properties of the given instruction.
V8_EXPORT_PRIVATE int GetInstructionFlags(const Instruction* instr) const;
int GetTargetInstructionFlags(const Instruction* instr) const;
// Check whether the given instruction has side effects (e.g. function call,
// memory store).
bool HasSideEffect(const Instruction* instr) const {
return (GetInstructionFlags(instr) & kHasSideEffect) != 0;
// Return true if the instruction is a memory load.
bool IsLoadOperation(const Instruction* instr) const {
return (GetInstructionFlags(instr) & kIsLoadOperation) != 0;
// The scheduler will not move the following instructions before the last
// deopt/trap check:
// * loads (this is conservative)
// * instructions with side effect
// * other deopts/traps
// Any other instruction can be moved, apart from those that raise exceptions
// on specific inputs - these are filtered out by the deopt/trap check.
bool MayNeedDeoptOrTrapCheck(const Instruction* instr) const {
return (GetInstructionFlags(instr) & kMayNeedDeoptOrTrapCheck) != 0;
// Return true if the instruction cannot be moved before the last deopt or
// trap point we encountered.
bool DependsOnDeoptOrTrap(const Instruction* instr) const {
return MayNeedDeoptOrTrapCheck(instr) || instr->IsDeoptimizeCall() ||
instr->IsTrap() || HasSideEffect(instr) || IsLoadOperation(instr);
// Identify nops used as a definition point for live-in registers at
// function entry.
bool IsFixedRegisterParameter(const Instruction* instr) const {
return (instr->arch_opcode() == kArchNop) && (instr->OutputCount() == 1) &&
(instr->OutputAt(0)->IsUnallocated()) &&
->HasFixedRegisterPolicy() ||
void ComputeTotalLatencies();
static int GetInstructionLatency(const Instruction* instr);
Zone* zone() { return zone_; }
InstructionSequence* sequence() { return sequence_; }
Isolate* isolate() { return sequence()->isolate(); }
Zone* zone_;
InstructionSequence* sequence_;
ZoneVector<ScheduleGraphNode*> graph_;
friend class InstructionSchedulerTester;
// Last side effect instruction encountered while building the graph.
ScheduleGraphNode* last_side_effect_instr_;
// Set of load instructions encountered since the last side effect instruction
// which will be added as predecessors of the next instruction with side
// effects.
ZoneVector<ScheduleGraphNode*> pending_loads_;
// Live-in register markers are nop instructions which are emitted at the
// beginning of a basic block so that the register allocator will find a
// defining instruction for live-in values. They must not be moved.
// All these nops are chained together and added as a predecessor of every
// other instructions in the basic block.
ScheduleGraphNode* last_live_in_reg_marker_;
// Last deoptimization or trap instruction encountered while building the
// graph.
ScheduleGraphNode* last_deopt_or_trap_;
// Keep track of definition points for virtual registers. This is used to
// record operand dependencies in the scheduling graph.
ZoneMap<int32_t, ScheduleGraphNode*> operands_map_;
} // namespace compiler
} // namespace internal
} // namespace v8