blob: 845081b711163e472e96dd24f50aaaf1e6f69344 [file] [log] [blame]
// Copyright 2016 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/source-position-table.h"
#include "src/log.h"
#include "src/objects-inl.h"
#include "src/objects.h"
namespace v8 {
namespace internal {
// We'll use a simple encoding scheme to record the source positions.
// Conceptually, each position consists of:
// - code_offset: An integer index into the BytecodeArray or code.
// - source_position: An integer index into the source string.
// - position type: Each position is either a statement or an expression.
//
// The basic idea for the encoding is to use a variable-length integer coding,
// where each byte contains 7 bits of payload data, and 1 'more' bit that
// determines whether additional bytes follow. Additionally:
// - we record the difference from the previous position,
// - we just stuff one bit for the type into the code offset,
// - we write least-significant bits first,
// - we use zig-zag encoding to encode both positive and negative numbers.
namespace {
// Each byte is encoded as MoreBit | ValueBits.
class MoreBit : public BitField8<bool, 7, 1> {};
class ValueBits : public BitField8<unsigned, 0, 7> {};
// Helper: Add the offsets from 'other' to 'value'. Also set is_statement.
void AddAndSetEntry(PositionTableEntry& value,
const PositionTableEntry& other) {
value.code_offset += other.code_offset;
value.source_position += other.source_position;
value.is_statement = other.is_statement;
}
// Helper: Substract the offsets from 'other' from 'value'.
void SubtractFromEntry(PositionTableEntry& value,
const PositionTableEntry& other) {
value.code_offset -= other.code_offset;
value.source_position -= other.source_position;
}
// Helper: Encode an integer.
template <typename T>
void EncodeInt(ZoneVector<byte>& bytes, T value) {
// Zig-zag encoding.
static const int kShift = sizeof(T) * kBitsPerByte - 1;
value = ((value << 1) ^ (value >> kShift));
DCHECK_GE(value, 0);
auto encoded = static_cast<typename std::make_unsigned<T>::type>(value);
bool more;
do {
more = encoded > ValueBits::kMax;
byte current =
MoreBit::encode(more) | ValueBits::encode(encoded & ValueBits::kMask);
bytes.push_back(current);
encoded >>= ValueBits::kSize;
} while (more);
}
// Encode a PositionTableEntry.
void EncodeEntry(ZoneVector<byte>& bytes, const PositionTableEntry& entry) {
// We only accept ascending code offsets.
DCHECK(entry.code_offset >= 0);
// Since code_offset is not negative, we use sign to encode is_statement.
EncodeInt(bytes,
entry.is_statement ? entry.code_offset : -entry.code_offset - 1);
EncodeInt(bytes, entry.source_position);
}
// Helper: Decode an integer.
template <typename T>
T DecodeInt(ByteArray* bytes, int* index) {
byte current;
int shift = 0;
T decoded = 0;
bool more;
do {
current = bytes->get((*index)++);
decoded |= static_cast<typename std::make_unsigned<T>::type>(
ValueBits::decode(current))
<< shift;
more = MoreBit::decode(current);
shift += ValueBits::kSize;
} while (more);
DCHECK_GE(decoded, 0);
decoded = (decoded >> 1) ^ (-(decoded & 1));
return decoded;
}
void DecodeEntry(ByteArray* bytes, int* index, PositionTableEntry* entry) {
int tmp = DecodeInt<int>(bytes, index);
if (tmp >= 0) {
entry->is_statement = true;
entry->code_offset = tmp;
} else {
entry->is_statement = false;
entry->code_offset = -(tmp + 1);
}
entry->source_position = DecodeInt<int64_t>(bytes, index);
}
} // namespace
SourcePositionTableBuilder::SourcePositionTableBuilder(
Zone* zone, SourcePositionTableBuilder::RecordingMode mode)
: mode_(mode),
bytes_(zone),
#ifdef ENABLE_SLOW_DCHECKS
raw_entries_(zone),
#endif
previous_() {
}
void SourcePositionTableBuilder::AddPosition(size_t code_offset,
SourcePosition source_position,
bool is_statement) {
if (Omit()) return;
DCHECK(source_position.IsKnown());
int offset = static_cast<int>(code_offset);
AddEntry({offset, source_position.raw(), is_statement});
}
void SourcePositionTableBuilder::AddEntry(const PositionTableEntry& entry) {
PositionTableEntry tmp(entry);
SubtractFromEntry(tmp, previous_);
EncodeEntry(bytes_, tmp);
previous_ = entry;
#ifdef ENABLE_SLOW_DCHECKS
raw_entries_.push_back(entry);
#endif
}
Handle<ByteArray> SourcePositionTableBuilder::ToSourcePositionTable(
Isolate* isolate, Handle<AbstractCode> code) {
if (bytes_.empty()) return isolate->factory()->empty_byte_array();
DCHECK(!Omit());
Handle<ByteArray> table = isolate->factory()->NewByteArray(
static_cast<int>(bytes_.size()), TENURED);
MemCopy(table->GetDataStartAddress(), &*bytes_.begin(), bytes_.size());
LOG_CODE_EVENT(isolate, CodeLinePosInfoRecordEvent(*code, *table));
#ifdef ENABLE_SLOW_DCHECKS
// Brute force testing: Record all positions and decode
// the entire table to verify they are identical.
auto raw = raw_entries_.begin();
for (SourcePositionTableIterator encoded(*table); !encoded.done();
encoded.Advance(), raw++) {
DCHECK(raw != raw_entries_.end());
DCHECK_EQ(encoded.code_offset(), raw->code_offset);
DCHECK_EQ(encoded.source_position().raw(), raw->source_position);
DCHECK_EQ(encoded.is_statement(), raw->is_statement);
}
DCHECK(raw == raw_entries_.end());
// No additional source positions after creating the table.
mode_ = OMIT_SOURCE_POSITIONS;
#endif
return table;
}
SourcePositionTableIterator::SourcePositionTableIterator(ByteArray* byte_array)
: table_(byte_array), index_(0), current_() {
Advance();
}
void SourcePositionTableIterator::Advance() {
DCHECK(!done());
DCHECK(index_ >= 0 && index_ <= table_->length());
if (index_ == table_->length()) {
index_ = kDone;
} else {
PositionTableEntry tmp;
DecodeEntry(table_, &index_, &tmp);
AddAndSetEntry(current_, tmp);
}
}
} // namespace internal
} // namespace v8