v8/src/hydrogen-bch.cc

403 lines
15 KiB
C++

// Copyright 2013 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following
// disclaimer in the documentation and/or other materials provided
// with the distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived
// from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "hydrogen-bch.h"
namespace v8 {
namespace internal {
/*
* This class is a table with one element for eack basic block.
*
* It is used to check if, inside one loop, all execution paths contain
* a bounds check for a particular [index, length] combination.
* The reason is that if there is a path that stays in the loop without
* executing a check then the check cannot be hoisted out of the loop (it
* would likely fail and cause a deopt for no good reason).
* We also check is there are paths that exit the loop early, and if yes we
* perform the hoisting only if graph()->use_optimistic_licm() is true.
* The reason is that such paths are realtively common and harmless (like in
* a "search" method that scans an array until an element is found), but in
* some cases they could cause a deopt if we hoist the check so this is a
* situation we need to detect.
*/
class InductionVariableBlocksTable BASE_EMBEDDED {
public:
class Element {
public:
static const int kNoBlock = -1;
HBasicBlock* block() { return block_; }
void set_block(HBasicBlock* block) { block_ = block; }
bool is_start() { return is_start_; }
bool is_proper_exit() { return is_proper_exit_; }
bool is_in_loop() { return is_in_loop_; }
bool has_check() { return has_check_; }
void set_has_check() { has_check_ = true; }
InductionVariableLimitUpdate* additional_limit() {
return &additional_limit_;
}
/*
* Initializes the table element for a given loop (identified by its
* induction variable).
*/
void InitializeLoop(InductionVariableData* data) {
ASSERT(data->limit() != NULL);
HLoopInformation* loop = data->phi()->block()->current_loop();
is_start_ = (block() == loop->loop_header());
is_proper_exit_ = (block() == data->induction_exit_target());
is_in_loop_ = loop->IsNestedInThisLoop(block()->current_loop());
has_check_ = false;
}
// Utility methods to iterate over dominated blocks.
void ResetCurrentDominatedBlock() { current_dominated_block_ = kNoBlock; }
HBasicBlock* CurrentDominatedBlock() {
ASSERT(current_dominated_block_ != kNoBlock);
return current_dominated_block_ < block()->dominated_blocks()->length() ?
block()->dominated_blocks()->at(current_dominated_block_) : NULL;
}
HBasicBlock* NextDominatedBlock() {
current_dominated_block_++;
return CurrentDominatedBlock();
}
Element()
: block_(NULL), is_start_(false), is_proper_exit_(false),
has_check_(false), additional_limit_(),
current_dominated_block_(kNoBlock) {}
private:
HBasicBlock* block_;
bool is_start_;
bool is_proper_exit_;
bool is_in_loop_;
bool has_check_;
InductionVariableLimitUpdate additional_limit_;
int current_dominated_block_;
};
HGraph* graph() const { return graph_; }
Counters* counters() const { return graph()->isolate()->counters(); }
HBasicBlock* loop_header() const { return loop_header_; }
Element* at(int index) const { return &(elements_.at(index)); }
Element* at(HBasicBlock* block) const { return at(block->block_id()); }
void AddCheckAt(HBasicBlock* block) {
at(block->block_id())->set_has_check();
}
/*
* Initializes the table for a given loop (identified by its induction
* variable).
*/
void InitializeLoop(InductionVariableData* data) {
for (int i = 0; i < graph()->blocks()->length(); i++) {
at(i)->InitializeLoop(data);
}
loop_header_ = data->phi()->block()->current_loop()->loop_header();
}
enum Hoistability {
HOISTABLE,
OPTIMISTICALLY_HOISTABLE,
NOT_HOISTABLE
};
/*
* This method checks if it is appropriate to hoist the bounds checks on an
* induction variable out of the loop.
* The problem is that in the loop code graph there could be execution paths
* where the check is not performed, but hoisting the check has the same
* semantics as performing it at every loop iteration, which could cause
* unnecessary check failures (which would mean unnecessary deoptimizations).
* The method returns OK if there are no paths that perform an iteration
* (loop back to the header) without meeting a check, or UNSAFE is set if
* early exit paths are found.
*/
Hoistability CheckHoistability() {
for (int i = 0; i < elements_.length(); i++) {
at(i)->ResetCurrentDominatedBlock();
}
bool unsafe = false;
HBasicBlock* current = loop_header();
while (current != NULL) {
HBasicBlock* next;
if (at(current)->has_check() || !at(current)->is_in_loop()) {
// We found a check or we reached a dominated block out of the loop,
// therefore this block is safe and we can backtrack.
next = NULL;
} else {
for (int i = 0; i < current->end()->SuccessorCount(); i ++) {
Element* successor = at(current->end()->SuccessorAt(i));
if (!successor->is_in_loop()) {
if (!successor->is_proper_exit()) {
// We found a path that exits the loop early, and is not the exit
// related to the induction limit, therefore hoisting checks is
// an optimistic assumption.
unsafe = true;
}
}
if (successor->is_start()) {
// We found a path that does one loop iteration without meeting any
// check, therefore hoisting checks would be likely to cause
// unnecessary deopts.
return NOT_HOISTABLE;
}
}
next = at(current)->NextDominatedBlock();
}
// If we have no next block we need to backtrack the tree traversal.
while (next == NULL) {
current = current->dominator();
if (current != NULL) {
next = at(current)->NextDominatedBlock();
} else {
// We reached the root: next stays NULL.
next = NULL;
break;
}
}
current = next;
}
return unsafe ? OPTIMISTICALLY_HOISTABLE : HOISTABLE;
}
explicit InductionVariableBlocksTable(HGraph* graph)
: graph_(graph), loop_header_(NULL),
elements_(graph->blocks()->length(), graph->zone()) {
for (int i = 0; i < graph->blocks()->length(); i++) {
Element element;
element.set_block(graph->blocks()->at(i));
elements_.Add(element, graph->zone());
ASSERT(at(i)->block()->block_id() == i);
}
}
// Tries to hoist a check out of its induction loop.
void ProcessRelatedChecks(
InductionVariableData::InductionVariableCheck* check,
InductionVariableData* data) {
HValue* length = check->check()->length();
check->set_processed();
HBasicBlock* header =
data->phi()->block()->current_loop()->loop_header();
HBasicBlock* pre_header = header->predecessors()->at(0);
// Check that the limit is defined in the loop preheader.
if (!data->limit()->IsInteger32Constant()) {
HBasicBlock* limit_block = data->limit()->block();
if (limit_block != pre_header &&
!limit_block->Dominates(pre_header)) {
return;
}
}
// Check that the length and limit have compatible representations.
if (!(data->limit()->representation().Equals(
length->representation()) ||
data->limit()->IsInteger32Constant())) {
return;
}
// Check that the length is defined in the loop preheader.
if (check->check()->length()->block() != pre_header &&
!check->check()->length()->block()->Dominates(pre_header)) {
return;
}
// Add checks to the table.
for (InductionVariableData::InductionVariableCheck* current_check = check;
current_check != NULL;
current_check = current_check->next()) {
if (current_check->check()->length() != length) continue;
AddCheckAt(current_check->check()->block());
current_check->set_processed();
}
// Check that we will not cause unwanted deoptimizations.
Hoistability hoistability = CheckHoistability();
if (hoistability == NOT_HOISTABLE ||
(hoistability == OPTIMISTICALLY_HOISTABLE &&
!graph()->use_optimistic_licm())) {
return;
}
// We will do the hoisting, but we must see if the limit is "limit" or if
// all checks are done on constants: if all check are done against the same
// constant limit we will use that instead of the induction limit.
bool has_upper_constant_limit = true;
int32_t upper_constant_limit =
check != NULL && check->HasUpperLimit() ? check->upper_limit() : 0;
for (InductionVariableData::InductionVariableCheck* current_check = check;
current_check != NULL;
current_check = current_check->next()) {
has_upper_constant_limit =
has_upper_constant_limit &&
check->HasUpperLimit() &&
check->upper_limit() == upper_constant_limit;
counters()->bounds_checks_eliminated()->Increment();
current_check->check()->set_skip_check();
}
// Choose the appropriate limit.
Zone* zone = graph()->zone();
HValue* context = graph()->GetInvalidContext();
HValue* limit = data->limit();
if (has_upper_constant_limit) {
HConstant* new_limit = HConstant::New(zone, context,
upper_constant_limit);
new_limit->InsertBefore(pre_header->end());
limit = new_limit;
}
// If necessary, redefine the limit in the preheader.
if (limit->IsInteger32Constant() &&
limit->block() != pre_header &&
!limit->block()->Dominates(pre_header)) {
HConstant* new_limit = HConstant::New(zone, context,
limit->GetInteger32Constant());
new_limit->InsertBefore(pre_header->end());
limit = new_limit;
}
// Do the hoisting.
HBoundsCheck* hoisted_check = HBoundsCheck::New(
zone, context, limit, check->check()->length());
hoisted_check->InsertBefore(pre_header->end());
hoisted_check->set_allow_equality(true);
counters()->bounds_checks_hoisted()->Increment();
}
void CollectInductionVariableData(HBasicBlock* bb) {
bool additional_limit = false;
for (int i = 0; i < bb->phis()->length(); i++) {
HPhi* phi = bb->phis()->at(i);
phi->DetectInductionVariable();
}
additional_limit = InductionVariableData::ComputeInductionVariableLimit(
bb, at(bb)->additional_limit());
if (additional_limit) {
at(bb)->additional_limit()->updated_variable->
UpdateAdditionalLimit(at(bb)->additional_limit());
}
for (HInstruction* i = bb->first(); i != NULL; i = i->next()) {
if (!i->IsBoundsCheck()) continue;
HBoundsCheck* check = HBoundsCheck::cast(i);
InductionVariableData::BitwiseDecompositionResult decomposition;
InductionVariableData::DecomposeBitwise(check->index(), &decomposition);
if (!decomposition.base->IsPhi()) continue;
HPhi* phi = HPhi::cast(decomposition.base);
if (!phi->IsInductionVariable()) continue;
InductionVariableData* data = phi->induction_variable_data();
// For now ignore loops decrementing the index.
if (data->increment() <= 0) continue;
if (!data->LowerLimitIsNonNegativeConstant()) continue;
// TODO(mmassi): skip OSR values for check->length().
if (check->length() == data->limit() ||
check->length() == data->additional_upper_limit()) {
counters()->bounds_checks_eliminated()->Increment();
check->set_skip_check();
continue;
}
if (!phi->IsLimitedInductionVariable()) continue;
int32_t limit = data->ComputeUpperLimit(decomposition.and_mask,
decomposition.or_mask);
phi->induction_variable_data()->AddCheck(check, limit);
}
for (int i = 0; i < bb->dominated_blocks()->length(); i++) {
CollectInductionVariableData(bb->dominated_blocks()->at(i));
}
if (additional_limit) {
at(bb->block_id())->additional_limit()->updated_variable->
UpdateAdditionalLimit(at(bb->block_id())->additional_limit());
}
}
void EliminateRedundantBoundsChecks(HBasicBlock* bb) {
for (int i = 0; i < bb->phis()->length(); i++) {
HPhi* phi = bb->phis()->at(i);
if (!phi->IsLimitedInductionVariable()) continue;
InductionVariableData* induction_data = phi->induction_variable_data();
InductionVariableData::ChecksRelatedToLength* current_length_group =
induction_data->checks();
while (current_length_group != NULL) {
current_length_group->CloseCurrentBlock();
InductionVariableData::InductionVariableCheck* current_base_check =
current_length_group->checks();
InitializeLoop(induction_data);
while (current_base_check != NULL) {
ProcessRelatedChecks(current_base_check, induction_data);
while (current_base_check != NULL &&
current_base_check->processed()) {
current_base_check = current_base_check->next();
}
}
current_length_group = current_length_group->next();
}
}
}
private:
HGraph* graph_;
HBasicBlock* loop_header_;
ZoneList<Element> elements_;
};
void HBoundsCheckHoistingPhase::HoistRedundantBoundsChecks() {
InductionVariableBlocksTable table(graph());
table.CollectInductionVariableData(graph()->entry_block());
for (int i = 0; i < graph()->blocks()->length(); i++) {
table.EliminateRedundantBoundsChecks(graph()->blocks()->at(i));
}
}
} } // namespace v8::internal