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// 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/interpreter/source-position-table.h"
#include "src/objects-inl.h"
#include "src/objects.h"
namespace v8 {
namespace internal {
namespace interpreter {
// We'll use a simple encoding scheme to record the source positions.
// Conceptually, each position consists of:
// - bytecode_offset: An integer index into the BytecodeArray
// - 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 bytecode offset,
// - we write least-significant bits first,
// - negative numbers occur only rarely, so we use a denormalized
// most-significant byte (a byte with all zeros, which normally wouldn't
// make any sense) to encode a negative sign, so that we 'pay' nothing for
// positive numbers, but have to pay a full byte for negative integers.
namespace {
// A zero-value in the most-significant byte is used to mark negative numbers.
const int kNegativeSignMarker = 0;
// Each byte is encoded as MoreBit | ValueBits.
class MoreBit : public BitField8<bool, 7, 1> {};
class ValueBits : public BitField8<int, 0, 7> {};
// Helper: Add the offsets from 'other' to 'value'. Also set is_statement.
void AddAndSetEntry(PositionTableEntry& value,
const PositionTableEntry& other) {
value.bytecode_offset += other.bytecode_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.bytecode_offset -= other.bytecode_offset;
value.source_position -= other.source_position;
}
// Helper: Encode an integer.
void EncodeInt(ZoneVector<byte>& bytes, int value) {
bool sign = false;
if (value < 0) {
sign = true;
value = -value;
}
bool more;
do {
more = value > ValueBits::kMax;
bytes.push_back(MoreBit::encode(more || sign) |
ValueBits::encode(value & ValueBits::kMax));
value >>= ValueBits::kSize;
} while (more);
if (sign) {
bytes.push_back(MoreBit::encode(false) |
ValueBits::encode(kNegativeSignMarker));
}
}
// Encode a PositionTableEntry.
void EncodeEntry(ZoneVector<byte>& bytes, const PositionTableEntry& entry) {
// 1 bit for sign + is_statement each, which leaves 30b for the value.
DCHECK(abs(entry.bytecode_offset) < (1 << 30));
EncodeInt(bytes, (entry.is_statement ? 1 : 0) | (entry.bytecode_offset << 1));
EncodeInt(bytes, entry.source_position);
}
// Helper: Decode an integer.
void DecodeInt(ByteArray* bytes, int* index, int* v) {
byte current;
int n = 0;
int value = 0;
bool more;
do {
current = bytes->get((*index)++);
value |= ValueBits::decode(current) << (n * ValueBits::kSize);
n++;
more = MoreBit::decode(current);
} while (more);
if (ValueBits::decode(current) == kNegativeSignMarker) {
value = -value;
}
*v = value;
}
void DecodeEntry(ByteArray* bytes, int* index, PositionTableEntry* entry) {
int tmp;
DecodeInt(bytes, index, &tmp);
entry->is_statement = (tmp & 1);
// Note that '>>' needs to be arithmetic shift in order to handle negative
// numbers properly.
entry->bytecode_offset = (tmp >> 1);
DecodeInt(bytes, index, &entry->source_position);
}
} // namespace
void SourcePositionTableBuilder::AddStatementPosition(size_t bytecode_offset,
int source_position) {
int offset = static_cast<int>(bytecode_offset);
AddEntry({offset, source_position, true});
}
void SourcePositionTableBuilder::AddExpressionPosition(size_t bytecode_offset,
int source_position) {
int offset = static_cast<int>(bytecode_offset);
AddEntry({offset, source_position, false});
}
void SourcePositionTableBuilder::AddEntry(const PositionTableEntry& entry) {
// Don't encode a new entry if this bytecode already has a source position
// assigned.
if (candidate_.bytecode_offset == entry.bytecode_offset) {
if (entry.is_statement) candidate_ = entry;
return;
}
CommitEntry();
candidate_ = entry;
}
void SourcePositionTableBuilder::CommitEntry() {
if (candidate_.bytecode_offset == kUninitializedCandidateOffset) return;
PositionTableEntry tmp(candidate_);
SubtractFromEntry(tmp, previous_);
EncodeEntry(bytes_, tmp);
previous_ = candidate_;
if (candidate_.is_statement) {
LOG_CODE_EVENT(isolate_, CodeLinePosInfoAddStatementPositionEvent(
jit_handler_data_, candidate_.bytecode_offset,
candidate_.source_position));
}
LOG_CODE_EVENT(isolate_, CodeLinePosInfoAddPositionEvent(
jit_handler_data_, candidate_.bytecode_offset,
candidate_.source_position));
#ifdef ENABLE_SLOW_DCHECKS
raw_entries_.push_back(candidate_);
#endif
}
Handle<ByteArray> SourcePositionTableBuilder::ToSourcePositionTable() {
CommitEntry();
if (bytes_.empty()) return isolate_->factory()->empty_byte_array();
Handle<ByteArray> table = isolate_->factory()->NewByteArray(
static_cast<int>(bytes_.size()), TENURED);
MemCopy(table->GetDataStartAddress(), &*bytes_.begin(), bytes_.size());
#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.bytecode_offset(), raw->bytecode_offset);
DCHECK_EQ(encoded.source_position(), raw->source_position);
DCHECK_EQ(encoded.is_statement(), raw->is_statement);
}
DCHECK(raw == raw_entries_.end());
#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 interpreter
} // namespace internal
} // namespace v8
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