src/v8/src/codegen/safepoint-table.cc

// Copyright 2011 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/codegen/safepoint-table.h"

#include "src/codegen/assembler-inl.h"
#include "src/codegen/macro-assembler.h"
#include "src/deoptimizer/deoptimizer.h"
#include "src/diagnostics/disasm.h"
#include "src/execution/frames-inl.h"
#include "src/utils/ostreams.h"

namespace v8 {
namespace internal {

SafepointTable::SafepointTable(Address instruction_start,
                               size_t safepoint_table_offset,
                               uint32_t stack_slots, bool has_deopt)
    : instruction_start_(instruction_start),
      stack_slots_(stack_slots),
      has_deopt_(has_deopt) {
  Address header = instruction_start_ + safepoint_table_offset;
  length_ = Memory<uint32_t>(header + kLengthOffset);
  entry_size_ = Memory<uint32_t>(header + kEntrySizeOffset);
  pc_and_deoptimization_indexes_ = header + kHeaderSize;
  entries_ = pc_and_deoptimization_indexes_ + (length_ * kFixedEntrySize);
}

SafepointTable::SafepointTable(Code code)
    : SafepointTable(code.InstructionStart(), code.safepoint_table_offset(),
                     code.stack_slots(), true) {}

unsigned SafepointTable::find_return_pc(unsigned pc_offset) {
  for (unsigned i = 0; i < length(); i++) {
    if (GetTrampolinePcOffset(i) == static_cast<int>(pc_offset)) {
      return GetPcOffset(i);
    } else if (GetPcOffset(i) == pc_offset) {
      return pc_offset;
    }
  }
  UNREACHABLE();
  return 0;
}

SafepointEntry SafepointTable::FindEntry(Address pc) const {
  unsigned pc_offset = static_cast<unsigned>(pc - instruction_start_);
  // We use kMaxUInt32 as sentinel value, so check that we don't hit that.
  DCHECK_NE(kMaxUInt32, pc_offset);
  unsigned len = length();
  CHECK_GT(len, 0);
  // If pc == kMaxUInt32, then this entry covers all call sites in the function.
  if (len == 1 && GetPcOffset(0) == kMaxUInt32) return GetEntry(0);
  for (unsigned i = 0; i < len; i++) {
    // TODO(kasperl): Replace the linear search with binary search.
    if (GetPcOffset(i) == pc_offset ||
        (has_deopt_ &&
         GetTrampolinePcOffset(i) == static_cast<int>(pc_offset))) {
      return GetEntry(i);
    }
  }
  UNREACHABLE();
  return SafepointEntry();
}

void SafepointTable::PrintEntry(unsigned index,
                                std::ostream& os) const {  // NOLINT
  disasm::NameConverter converter;
  SafepointEntry entry = GetEntry(index);
  uint8_t* bits = entry.bits();

  // Print the stack slot bits.
  if (entry_size_ > 0) {
    const int first = 0;
    int last = entry_size_ - 1;
    for (int i = first; i < last; i++) PrintBits(os, bits[i], kBitsPerByte);
    int last_bits = stack_slots_ - ((last - first) * kBitsPerByte);
    PrintBits(os, bits[last], last_bits);
  }
}

void SafepointTable::PrintBits(std::ostream& os,  // NOLINT
                               uint8_t byte, int digits) {
  DCHECK(digits >= 0 && digits <= kBitsPerByte);
  for (int i = 0; i < digits; i++) {
    os << (((byte & (1 << i)) == 0) ? "0" : "1");
  }
}

Safepoint SafepointTableBuilder::DefineSafepoint(
    Assembler* assembler, Safepoint::DeoptMode deopt_mode) {
  deoptimization_info_.push_back(
      DeoptimizationInfo(zone_, assembler->pc_offset()));
  if (deopt_mode == Safepoint::kNoLazyDeopt) {
    last_lazy_safepoint_ = deoptimization_info_.size();
  }
  DeoptimizationInfo& new_info = deoptimization_info_.back();
  return Safepoint(new_info.indexes);
}

void SafepointTableBuilder::RecordLazyDeoptimizationIndex(int index) {
  for (auto it = deoptimization_info_.Find(last_lazy_safepoint_);
       it != deoptimization_info_.end(); it++, last_lazy_safepoint_++) {
    it->deopt_index = index;
  }
}

unsigned SafepointTableBuilder::GetCodeOffset() const {
  DCHECK(emitted_);
  return offset_;
}

int SafepointTableBuilder::UpdateDeoptimizationInfo(int pc, int trampoline,
                                                    int start) {
  int index = start;
  for (auto it = deoptimization_info_.Find(start);
       it != deoptimization_info_.end(); it++, index++) {
    if (static_cast<int>(it->pc) == pc) {
      it->trampoline = trampoline;
      return index;
    }
  }
  UNREACHABLE();
}

void SafepointTableBuilder::Emit(Assembler* assembler, int bits_per_entry) {
  RemoveDuplicates();

  // Make sure the safepoint table is properly aligned. Pad with nops.
  assembler->Align(kIntSize);
  assembler->RecordComment(";;; Safepoint table.");
  offset_ = assembler->pc_offset();

  // Compute the number of bytes per safepoint entry.
  int bytes_per_entry =
      RoundUp(bits_per_entry, kBitsPerByte) >> kBitsPerByteLog2;

  // Emit the table header.
  STATIC_ASSERT(SafepointTable::kLengthOffset == 0 * kIntSize);
  STATIC_ASSERT(SafepointTable::kEntrySizeOffset == 1 * kIntSize);
  STATIC_ASSERT(SafepointTable::kHeaderSize == 2 * kIntSize);
  int length = static_cast<int>(deoptimization_info_.size());
  assembler->dd(length);
  assembler->dd(bytes_per_entry);

  // Emit sorted table of pc offsets together with additional info (i.e. the
  // deoptimization index or arguments count) and trampoline offsets.
  STATIC_ASSERT(SafepointTable::kPcOffset == 0 * kIntSize);
  STATIC_ASSERT(SafepointTable::kEncodedInfoOffset == 1 * kIntSize);
  STATIC_ASSERT(SafepointTable::kTrampolinePcOffset == 2 * kIntSize);
  STATIC_ASSERT(SafepointTable::kFixedEntrySize == 3 * kIntSize);
  for (const DeoptimizationInfo& info : deoptimization_info_) {
    assembler->dd(info.pc);
    assembler->dd(info.deopt_index);
    assembler->dd(info.trampoline);
  }

  // Emit table of bitmaps.
  ZoneVector<uint8_t> bits(bytes_per_entry, 0, zone_);
  for (const DeoptimizationInfo& info : deoptimization_info_) {
    ZoneChunkList<int>* indexes = info.indexes;
    std::fill(bits.begin(), bits.end(), 0);

    // Run through the indexes and build a bitmap.
    for (int idx : *indexes) {
      int index = bits_per_entry - 1 - idx;
      int byte_index = index >> kBitsPerByteLog2;
      int bit_index = index & (kBitsPerByte - 1);
      bits[byte_index] |= (1U << bit_index);
    }

    // Emit the bitmap for the current entry.
    for (int k = 0; k < bytes_per_entry; k++) {
      assembler->db(bits[k]);
    }
  }
  emitted_ = true;
}

void SafepointTableBuilder::RemoveDuplicates() {
  // If the table contains more than one entry, and all entries are identical
  // (except for the pc), replace the whole table by a single entry with pc =
  // kMaxUInt32. This especially compacts the table for wasm code without tagged
  // pointers and without deoptimization info.

  if (deoptimization_info_.size() < 2) return;

  // Check that all entries (1, size] are identical to entry 0.
  const DeoptimizationInfo& first_info = deoptimization_info_.front();
  for (auto it = deoptimization_info_.Find(1); it != deoptimization_info_.end();
       it++) {
    if (!IsIdenticalExceptForPc(first_info, *it)) return;
  }

  // If we get here, all entries were identical. Rewind the list to just one
  // entry, and set the pc to kMaxUInt32.
  deoptimization_info_.Rewind(1);
  deoptimization_info_.front().pc = kMaxUInt32;
}

bool SafepointTableBuilder::IsIdenticalExceptForPc(
    const DeoptimizationInfo& info1, const DeoptimizationInfo& info2) const {
  if (info1.deopt_index != info2.deopt_index) return false;

  ZoneChunkList<int>* indexes1 = info1.indexes;
  ZoneChunkList<int>* indexes2 = info2.indexes;
  if (indexes1->size() != indexes2->size()) return false;
  if (!std::equal(indexes1->begin(), indexes1->end(), indexes2->begin())) {
    return false;
  }

  return true;
}

}  // namespace internal
}  // namespace v8