Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 605 insertions, 0 deletions

diff --git a/js/src/jit/Snapshots.cpp b/js/src/jit/Snapshots.cpp
new file mode 100644
index 0000000000..2b6c2a945b
--- /dev/null
+++ b/js/src/jit/Snapshots.cpp
@@ -0,0 +1,605 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "jit/Snapshots.h"
+
+#include "jit/JitSpewer.h"
+#ifdef TRACK_SNAPSHOTS
+#  include "jit/LIR.h"
+#endif
+#include "jit/MIR.h"
+#include "jit/Recover.h"
+#include "js/Printer.h"
+
+using namespace js;
+using namespace js::jit;
+
+// [SMDOC] IonMonkey Snapshot encoding
+//
+// Encodings:
+//   [ptr] A fixed-size pointer.
+//   [vwu] A variable-width unsigned integer.
+//   [vws] A variable-width signed integer.
+//    [u8] An 8-bit unsigned integer.
+//   [u8'] An 8-bit unsigned integer which is potentially extended with packed
+//         data.
+//   [u8"] Packed data which is stored and packed in the previous [u8'].
+//  [vwu*] A list of variable-width unsigned integers.
+//   [pld] Payload of Recover Value Allocation:
+//         PAYLOAD_NONE:
+//           There is no payload.
+//
+//         PAYLOAD_INDEX:
+//           [vwu] Index, such as the constant pool index.
+//
+//         PAYLOAD_STACK_OFFSET:
+//           [vws] Stack offset based on the base of the Ion frame.
+//
+//         PAYLOAD_GPR:
+//            [u8] Code of the general register.
+//
+//         PAYLOAD_FPU:
+//            [u8] Code of the FPU register.
+//
+//         PAYLOAD_PACKED_TAG:
+//           [u8"] Bits 5-7: JSValueType is encoded on the low bits of the Mode
+//                           of the RValueAllocation.
+//
+// Snapshot header:
+//
+//   [vwu] bits ((n+1)-31]: recover instruction offset
+//         bits [0,n): bailout kind (n = SNAPSHOT_BAILOUTKIND_BITS)
+//
+// Snapshot body, repeated "frame count" times, from oldest frame to newest
+// frame. Note that the first frame doesn't have the "parent PC" field.
+//
+//   [ptr] Debug only: JSScript*
+//   [vwu] pc offset
+//   [vwu] # of RVA's indexes, including nargs
+//  [vwu*] List of indexes to R(ecover)ValueAllocation table. Contains
+//         nargs + nfixed + stackDepth items.
+//
+// Recover value allocations are encoded at the end of the Snapshot buffer, and
+// they are padded on ALLOCATION_TABLE_ALIGNMENT.  The encoding of each
+// allocation is determined by the RValueAllocation::Layout, which can be
+// obtained from the RValueAllocation::Mode with layoutFromMode function.  The
+// layout structure list the type of payload which are used to serialized /
+// deserialized / dumped the content of the allocations.
+//
+// R(ecover)ValueAllocation items:
+//   [u8'] Mode, which defines the type of the payload as well as the
+//         interpretation.
+//   [pld] first payload (packed tag, index, stack offset, register, ...)
+//   [pld] second payload (register, stack offset, none)
+//
+//       Modes:
+//         CONSTANT [INDEX]
+//           Index into the constant pool.
+//
+//         CST_UNDEFINED []
+//           Constant value which correspond to the "undefined" JS value.
+//
+//         CST_NULL []
+//           Constant value which correspond to the "null" JS value.
+//
+//         DOUBLE_REG [FPU_REG]
+//           Double value stored in a FPU register.
+//
+//         ANY_FLOAT_REG [FPU_REG]
+//           Any Float value (float32, simd) stored in a FPU register.
+//
+//         ANY_FLOAT_STACK [STACK_OFFSET]
+//           Any Float value (float32, simd) stored on the stack.
+//
+//         UNTYPED_REG   [GPR_REG]
+//         UNTYPED_STACK [STACK_OFFSET]
+//         UNTYPED_REG_REG     [GPR_REG,      GPR_REG]
+//         UNTYPED_REG_STACK   [GPR_REG,      STACK_OFFSET]
+//         UNTYPED_STACK_REG   [STACK_OFFSET, GPR_REG]
+//         UNTYPED_STACK_STACK [STACK_OFFSET, STACK_OFFSET]
+//           Value with dynamically known type. On 32 bits architecture, the
+//           first register/stack-offset correspond to the holder of the type,
+//           and the second correspond to the payload of the JS Value.
+//
+//         RECOVER_INSTRUCTION [INDEX]
+//           Index into the list of recovered instruction results.
+//
+//         RI_WITH_DEFAULT_CST [INDEX] [INDEX]
+//           The first payload is the index into the list of recovered
+//           instruction results.  The second payload is the index in the
+//           constant pool.
+//
+//         TYPED_REG [PACKED_TAG, GPR_REG]:
+//           Value with statically known type, which payload is stored in a
+//           register.
+//
+//         TYPED_STACK [PACKED_TAG, STACK_OFFSET]:
+//           Value with statically known type, which payload is stored at an
+//           offset on the stack.
+//
+
+const RValueAllocation::Layout& RValueAllocation::layoutFromMode(Mode mode) {
+  switch (mode) {
+    case CONSTANT: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_INDEX,
+                                                      PAYLOAD_NONE, "constant"};
+      return layout;
+    }
+
+    case CST_UNDEFINED: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_NONE, PAYLOAD_NONE, "undefined"};
+      return layout;
+    }
+
+    case CST_NULL: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_NONE,
+                                                      PAYLOAD_NONE, "null"};
+      return layout;
+    }
+
+    case DOUBLE_REG: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_FPU, PAYLOAD_NONE,
+                                                      "double"};
+      return layout;
+    }
+    case ANY_FLOAT_REG: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_FPU, PAYLOAD_NONE,
+                                                      "float register content"};
+      return layout;
+    }
+    case ANY_FLOAT_STACK: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_STACK_OFFSET, PAYLOAD_NONE, "float register content"};
+      return layout;
+    }
+#if defined(JS_NUNBOX32)
+    case UNTYPED_REG_REG: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_GPR, PAYLOAD_GPR,
+                                                      "value"};
+      return layout;
+    }
+    case UNTYPED_REG_STACK: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_GPR, PAYLOAD_STACK_OFFSET, "value"};
+      return layout;
+    }
+    case UNTYPED_STACK_REG: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_STACK_OFFSET,
+                                                      PAYLOAD_GPR, "value"};
+      return layout;
+    }
+    case UNTYPED_STACK_STACK: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_STACK_OFFSET, PAYLOAD_STACK_OFFSET, "value"};
+      return layout;
+    }
+#elif defined(JS_PUNBOX64)
+    case UNTYPED_REG: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_GPR, PAYLOAD_NONE,
+                                                      "value"};
+      return layout;
+    }
+    case UNTYPED_STACK: {
+      static const RValueAllocation::Layout layout = {PAYLOAD_STACK_OFFSET,
+                                                      PAYLOAD_NONE, "value"};
+      return layout;
+    }
+#endif
+    case RECOVER_INSTRUCTION: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_INDEX, PAYLOAD_NONE, "instruction"};
+      return layout;
+    }
+    case RI_WITH_DEFAULT_CST: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_INDEX, PAYLOAD_INDEX, "instruction with default"};
+      return layout;
+    }
+
+    default: {
+      static const RValueAllocation::Layout regLayout = {
+          PAYLOAD_PACKED_TAG, PAYLOAD_GPR, "typed value"};
+
+      static const RValueAllocation::Layout stackLayout = {
+          PAYLOAD_PACKED_TAG, PAYLOAD_STACK_OFFSET, "typed value"};
+
+      if (mode >= TYPED_REG_MIN && mode <= TYPED_REG_MAX) {
+        return regLayout;
+      }
+      if (mode >= TYPED_STACK_MIN && mode <= TYPED_STACK_MAX) {
+        return stackLayout;
+      }
+    }
+  }
+
+  MOZ_CRASH_UNSAFE_PRINTF("Unexpected mode: 0x%x", uint32_t(mode));
+}
+
+// Pad serialized RValueAllocations by a multiple of X bytes in the allocation
+// buffer.  By padding serialized value allocations, we are building an
+// indexable table of elements of X bytes, and thus we can safely divide any
+// offset within the buffer by X to obtain an index.
+//
+// By padding, we are loosing space within the allocation buffer, but we
+// multiple by X the number of indexes that we can store on one byte in each
+// snapshots.
+//
+// Some value allocations are taking more than X bytes to be encoded, in which
+// case we will pad to a multiple of X, and we are wasting indexes. The choice
+// of X should be balanced between the wasted padding of serialized value
+// allocation, and the saving made in snapshot indexes.
+static const size_t ALLOCATION_TABLE_ALIGNMENT = 2; /* bytes */
+
+void RValueAllocation::readPayload(CompactBufferReader& reader,
+                                   PayloadType type, uint8_t* mode,
+                                   Payload* p) {
+  switch (type) {
+    case PAYLOAD_NONE:
+      break;
+    case PAYLOAD_INDEX:
+      p->index = reader.readUnsigned();
+      break;
+    case PAYLOAD_STACK_OFFSET:
+      p->stackOffset = reader.readSigned();
+      break;
+    case PAYLOAD_GPR:
+      p->gpr = Register::FromCode(reader.readByte());
+      break;
+    case PAYLOAD_FPU:
+      p->fpu.data = reader.readByte();
+      break;
+    case PAYLOAD_PACKED_TAG:
+      p->type = JSValueType(*mode & PACKED_TAG_MASK);
+      *mode = *mode & ~PACKED_TAG_MASK;
+      break;
+  }
+}
+
+RValueAllocation RValueAllocation::read(CompactBufferReader& reader) {
+  uint8_t mode = reader.readByte();
+  const Layout& layout = layoutFromMode(Mode(mode & MODE_BITS_MASK));
+  Payload arg1, arg2;
+
+  readPayload(reader, layout.type1, &mode, &arg1);
+  readPayload(reader, layout.type2, &mode, &arg2);
+  return RValueAllocation(Mode(mode), arg1, arg2);
+}
+
+void RValueAllocation::writePayload(CompactBufferWriter& writer,
+                                    PayloadType type, Payload p) {
+  switch (type) {
+    case PAYLOAD_NONE:
+      break;
+    case PAYLOAD_INDEX:
+      writer.writeUnsigned(p.index);
+      break;
+    case PAYLOAD_STACK_OFFSET:
+      writer.writeSigned(p.stackOffset);
+      break;
+    case PAYLOAD_GPR:
+      static_assert(Registers::Total <= 0x100,
+                    "Not enough bytes to encode all registers.");
+      writer.writeByte(p.gpr.code());
+      break;
+    case PAYLOAD_FPU:
+      static_assert(FloatRegisters::Total <= 0x100,
+                    "Not enough bytes to encode all float registers.");
+      writer.writeByte(p.fpu.code());
+      break;
+    case PAYLOAD_PACKED_TAG: {
+      // This code assumes that the PACKED_TAG payload is following the
+      // writeByte of the mode.
+      if (!writer.oom()) {
+        MOZ_ASSERT(writer.length());
+        uint8_t* mode = writer.buffer() + (writer.length() - 1);
+        MOZ_ASSERT((*mode & PACKED_TAG_MASK) == 0 &&
+                   (p.type & ~PACKED_TAG_MASK) == 0);
+        *mode = *mode | p.type;
+      }
+      break;
+    }
+  }
+}
+
+void RValueAllocation::writePadding(CompactBufferWriter& writer) {
+  // Write 0x7f in all padding bytes.
+  while (writer.length() % ALLOCATION_TABLE_ALIGNMENT) {
+    writer.writeByte(0x7f);
+  }
+}
+
+void RValueAllocation::write(CompactBufferWriter& writer) const {
+  const Layout& layout = layoutFromMode(mode());
+  MOZ_ASSERT(layout.type2 != PAYLOAD_PACKED_TAG);
+  MOZ_ASSERT(writer.length() % ALLOCATION_TABLE_ALIGNMENT == 0);
+
+  writer.writeByte(mode_);
+  writePayload(writer, layout.type1, arg1_);
+  writePayload(writer, layout.type2, arg2_);
+  writePadding(writer);
+}
+
+HashNumber RValueAllocation::hash() const {
+  HashNumber res = 0;
+  res = HashNumber(mode_);
+  res = arg1_.index + (res << 6) + (res << 16) - res;
+  res = arg2_.index + (res << 6) + (res << 16) - res;
+  return res;
+}
+
+#ifdef JS_JITSPEW
+void RValueAllocation::dumpPayload(GenericPrinter& out, PayloadType type,
+                                   Payload p) {
+  switch (type) {
+    case PAYLOAD_NONE:
+      break;
+    case PAYLOAD_INDEX:
+      out.printf("index %u", p.index);
+      break;
+    case PAYLOAD_STACK_OFFSET:
+      out.printf("stack %d", p.stackOffset);
+      break;
+    case PAYLOAD_GPR:
+      out.printf("reg %s", p.gpr.name());
+      break;
+    case PAYLOAD_FPU:
+      out.printf("reg %s", p.fpu.name());
+      break;
+    case PAYLOAD_PACKED_TAG:
+      out.printf("%s", ValTypeToString(p.type));
+      break;
+  }
+}
+
+void RValueAllocation::dump(GenericPrinter& out) const {
+  const Layout& layout = layoutFromMode(mode());
+  out.printf("%s", layout.name);
+
+  if (layout.type1 != PAYLOAD_NONE) {
+    out.printf(" (");
+  }
+  dumpPayload(out, layout.type1, arg1_);
+  if (layout.type2 != PAYLOAD_NONE) {
+    out.printf(", ");
+  }
+  dumpPayload(out, layout.type2, arg2_);
+  if (layout.type1 != PAYLOAD_NONE) {
+    out.printf(")");
+  }
+}
+#endif  // JS_JITSPEW
+
+SnapshotReader::SnapshotReader(const uint8_t* snapshots, uint32_t offset,
+                               uint32_t RVATableSize, uint32_t listSize)
+    : reader_(snapshots + offset, snapshots + listSize),
+      allocReader_(snapshots + listSize, snapshots + listSize + RVATableSize),
+      allocTable_(snapshots + listSize),
+      allocRead_(0) {
+  if (!snapshots) {
+    return;
+  }
+  JitSpew(JitSpew_IonSnapshots, "Creating snapshot reader");
+  readSnapshotHeader();
+}
+
+#define COMPUTE_SHIFT_AFTER_(name) (name##_BITS + name##_SHIFT)
+#define COMPUTE_MASK_(name) ((uint32_t(1 << name##_BITS) - 1) << name##_SHIFT)
+
+// Details of snapshot header packing.
+static const uint32_t SNAPSHOT_BAILOUTKIND_SHIFT = 0;
+static const uint32_t SNAPSHOT_BAILOUTKIND_BITS = 6;
+static const uint32_t SNAPSHOT_BAILOUTKIND_MASK =
+    COMPUTE_MASK_(SNAPSHOT_BAILOUTKIND);
+
+static_assert((1 << SNAPSHOT_BAILOUTKIND_BITS) - 1 >=
+                  uint8_t(BailoutKind::Limit),
+              "Not enough bits for BailoutKinds");
+
+static const uint32_t SNAPSHOT_ROFFSET_SHIFT =
+    COMPUTE_SHIFT_AFTER_(SNAPSHOT_BAILOUTKIND);
+static const uint32_t SNAPSHOT_ROFFSET_BITS = 32 - SNAPSHOT_ROFFSET_SHIFT;
+static const uint32_t SNAPSHOT_ROFFSET_MASK = COMPUTE_MASK_(SNAPSHOT_ROFFSET);
+
+#undef COMPUTE_MASK_
+#undef COMPUTE_SHIFT_AFTER_
+
+void SnapshotReader::readSnapshotHeader() {
+  uint32_t bits = reader_.readUnsigned();
+
+  bailoutKind_ = BailoutKind((bits & SNAPSHOT_BAILOUTKIND_MASK) >>
+                             SNAPSHOT_BAILOUTKIND_SHIFT);
+  recoverOffset_ = (bits & SNAPSHOT_ROFFSET_MASK) >> SNAPSHOT_ROFFSET_SHIFT;
+
+  JitSpew(JitSpew_IonSnapshots, "Read snapshot header with bailout kind %u",
+          uint32_t(bailoutKind_));
+
+#ifdef TRACK_SNAPSHOTS
+  readTrackSnapshot();
+#endif
+}
+
+#ifdef TRACK_SNAPSHOTS
+void SnapshotReader::readTrackSnapshot() {
+  pcOpcode_ = reader_.readUnsigned();
+  mirOpcode_ = reader_.readUnsigned();
+  mirId_ = reader_.readUnsigned();
+  lirOpcode_ = reader_.readUnsigned();
+  lirId_ = reader_.readUnsigned();
+}
+
+void SnapshotReader::spewBailingFrom() const {
+#  ifdef JS_JITSPEW
+  if (JitSpewEnabled(JitSpew_IonBailouts)) {
+    JitSpewHeader(JitSpew_IonBailouts);
+    Fprinter& out = JitSpewPrinter();
+    out.printf(" bailing from bytecode: %s, MIR: ", CodeName(JSOp(pcOpcode_)));
+    MDefinition::PrintOpcodeName(out, MDefinition::Opcode(mirOpcode_));
+    out.printf(" [%u], LIR: ", mirId_);
+    LInstruction::printName(out, LInstruction::Opcode(lirOpcode_));
+    out.printf(" [%u]", lirId_);
+    out.printf("\n");
+  }
+#  endif
+}
+#endif
+
+uint32_t SnapshotReader::readAllocationIndex() {
+  allocRead_++;
+  return reader_.readUnsigned();
+}
+
+RValueAllocation SnapshotReader::readAllocation() {
+  JitSpew(JitSpew_IonSnapshots, "Reading slot %u", allocRead_);
+  uint32_t offset = readAllocationIndex() * ALLOCATION_TABLE_ALIGNMENT;
+  allocReader_.seek(allocTable_, offset);
+  return RValueAllocation::read(allocReader_);
+}
+
+SnapshotWriter::SnapshotWriter()
+    // Based on the measurements made in Bug 962555 comment 20, this length
+    // should be enough to prevent the reallocation of the hash table for at
+    // least half of the compilations.
+    : allocMap_(32) {}
+
+RecoverReader::RecoverReader(SnapshotReader& snapshot, const uint8_t* recovers,
+                             uint32_t size)
+    : reader_(nullptr, nullptr), numInstructions_(0), numInstructionsRead_(0) {
+  if (!recovers) {
+    return;
+  }
+  reader_ =
+      CompactBufferReader(recovers + snapshot.recoverOffset(), recovers + size);
+  readRecoverHeader();
+  readInstruction();
+}
+
+RecoverReader::RecoverReader(const RecoverReader& rr)
+    : reader_(rr.reader_),
+      numInstructions_(rr.numInstructions_),
+      numInstructionsRead_(rr.numInstructionsRead_) {
+  if (reader_.currentPosition()) {
+    rr.instruction()->cloneInto(&rawData_);
+  }
+}
+
+RecoverReader& RecoverReader::operator=(const RecoverReader& rr) {
+  reader_ = rr.reader_;
+  numInstructions_ = rr.numInstructions_;
+  numInstructionsRead_ = rr.numInstructionsRead_;
+  if (reader_.currentPosition()) {
+    rr.instruction()->cloneInto(&rawData_);
+  }
+  return *this;
+}
+
+void RecoverReader::readRecoverHeader() {
+  numInstructions_ = reader_.readUnsigned();
+  MOZ_ASSERT(numInstructions_);
+
+  JitSpew(JitSpew_IonSnapshots, "Read recover header with instructionCount %u",
+          numInstructions_);
+}
+
+void RecoverReader::readInstruction() {
+  MOZ_ASSERT(moreInstructions());
+  RInstruction::readRecoverData(reader_, &rawData_);
+  numInstructionsRead_++;
+}
+
+SnapshotOffset SnapshotWriter::startSnapshot(RecoverOffset recoverOffset,
+                                             BailoutKind kind) {
+  lastStart_ = writer_.length();
+  allocWritten_ = 0;
+
+  JitSpew(JitSpew_IonSnapshots,
+          "starting snapshot with recover offset %u, bailout kind %u",
+          recoverOffset, uint32_t(kind));
+
+  MOZ_ASSERT(uint32_t(kind) < (1 << SNAPSHOT_BAILOUTKIND_BITS));
+  MOZ_ASSERT(recoverOffset < (1 << SNAPSHOT_ROFFSET_BITS));
+  uint32_t bits = (uint32_t(kind) << SNAPSHOT_BAILOUTKIND_SHIFT) |
+                  (recoverOffset << SNAPSHOT_ROFFSET_SHIFT);
+
+  writer_.writeUnsigned(bits);
+  return lastStart_;
+}
+
+#ifdef TRACK_SNAPSHOTS
+void SnapshotWriter::trackSnapshot(uint32_t pcOpcode, uint32_t mirOpcode,
+                                   uint32_t mirId, uint32_t lirOpcode,
+                                   uint32_t lirId) {
+  writer_.writeUnsigned(pcOpcode);
+  writer_.writeUnsigned(mirOpcode);
+  writer_.writeUnsigned(mirId);
+  writer_.writeUnsigned(lirOpcode);
+  writer_.writeUnsigned(lirId);
+}
+#endif
+
+bool SnapshotWriter::add(const RValueAllocation& alloc) {
+  uint32_t offset;
+  RValueAllocMap::AddPtr p = allocMap_.lookupForAdd(alloc);
+  if (!p) {
+    offset = allocWriter_.length();
+    alloc.write(allocWriter_);
+    if (!allocMap_.add(p, alloc, offset)) {
+      allocWriter_.setOOM();
+      return false;
+    }
+  } else {
+    offset = p->value();
+  }
+
+#ifdef JS_JITSPEW
+  if (JitSpewEnabled(JitSpew_IonSnapshots)) {
+    JitSpewHeader(JitSpew_IonSnapshots);
+    Fprinter& out = JitSpewPrinter();
+    out.printf("    slot %u (%u): ", allocWritten_, offset);
+    alloc.dump(out);
+    out.printf("\n");
+  }
+#endif
+
+  allocWritten_++;
+  writer_.writeUnsigned(offset / ALLOCATION_TABLE_ALIGNMENT);
+  return true;
+}
+
+void SnapshotWriter::endSnapshot() {
+  // Place a sentinel for asserting on the other end.
+#ifdef DEBUG
+  writer_.writeSigned(-1);
+#endif
+
+  JitSpew(JitSpew_IonSnapshots,
+          "ending snapshot total size: %u bytes (start %u)",
+          uint32_t(writer_.length() - lastStart_), lastStart_);
+}
+
+RecoverOffset RecoverWriter::startRecover(uint32_t instructionCount) {
+  MOZ_ASSERT(instructionCount);
+  instructionCount_ = instructionCount;
+  instructionsWritten_ = 0;
+
+  JitSpew(JitSpew_IonSnapshots, "starting recover with %u instruction(s)",
+          instructionCount);
+
+  RecoverOffset recoverOffset = writer_.length();
+  writer_.writeUnsigned(instructionCount);
+  return recoverOffset;
+}
+
+void RecoverWriter::writeInstruction(const MNode* rp) {
+  if (!rp->writeRecoverData(writer_)) {
+    writer_.setOOM();
+  }
+  instructionsWritten_++;
+}
+
+void RecoverWriter::endRecover() {
+  MOZ_ASSERT(instructionCount_ == instructionsWritten_);
+}