[sparkplug][cleanup] Add helper to en-/decode VLQ.

VLQ encoding was implemented in TranslationArray and Sparkplug PC <->
bytecode mapping.
This CL introduces new VLQ helper methods used in both.

Bug: v8:11429
Change-Id: I89d9777eab4ad28f08e5957421b63df07e37f9cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704674Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73054}

BUILD.gn

@@ -4396,6 +4396,7 @@ v8_component("v8_libbase") {
     "src/base/utils/random-number-generator.h",
     "src/base/vlq-base64.cc",
     "src/base/vlq-base64.h",
+    "src/base/vlq.h",
   ]
 
   configs = [ ":internal_config_base" ]

src/base/vlq.h

+// Copyright 2021 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.
+
+#ifndef V8_BASE_VLQ_H_
+#define V8_BASE_VLQ_H_
+
+#include <limits>
+#include <vector>
+
+#include "src/base/memory.h"
+
+namespace v8 {
+namespace base {
+
+static constexpr uint32_t kContinueShift = 7;
+static constexpr uint32_t kContinueMask = 1 << kContinueShift;
+static constexpr uint32_t kDataMask = kContinueMask - 1;
+
+// Encodes an unsigned value using variable-length encoding and stores it using
+// the passed process_byte function.
+inline void VLQEncodeUnsigned(const std::function<void(byte)>& process_byte,
+                              uint32_t value) {
+  bool has_next;
+  do {
+    byte cur_byte = value & kDataMask;
+    value >>= kContinueShift;
+    has_next = value != 0;
+    // The most significant bit is set when we are not done with the value yet.
+    cur_byte |= static_cast<uint32_t>(has_next) << kContinueShift;
+    process_byte(cur_byte);
+  } while (has_next);
+}
+
+// Encodes value using variable-length encoding and stores it using the passed
+// process_byte function.
+inline void VLQEncode(const std::function<void(byte)>& process_byte,
+                      int32_t value) {
+  // This wouldn't handle kMinInt correctly if it ever encountered it.
+  DCHECK_NE(value, std::numeric_limits<int32_t>::min());
+  bool is_negative = value < 0;
+  // Encode sign in least significant bit.
+  uint32_t bits = static_cast<uint32_t>((is_negative ? -value : value) << 1) |
+                  static_cast<uint32_t>(is_negative);
+  VLQEncodeUnsigned(process_byte, bits);
+}
+
+// Wrapper of VLQEncode for std::vector backed storage containers.
+template <typename A>
+inline void VLQEncode(std::vector<byte, A>* data, int32_t value) {
+  VLQEncode([data](byte value) { data->push_back(value); }, value);
+}
+
+// Wrapper of VLQEncodeUnsigned for std::vector backed storage containers.
+template <typename A>
+inline void VLQEncodeUnsigned(std::vector<byte, A>* data, uint32_t value) {
+  VLQEncodeUnsigned([data](byte value) { data->push_back(value); }, value);
+}
+
+// Decodes a variable-length encoded unsigned value stored in contiguous memory
+// starting at data_start + index, updating index to where the next encoded
+// value starts.
+inline uint32_t VLQDecodeUnsigned(byte* data_start, int* index) {
+  uint32_t bits = 0;
+  for (int shift = 0; true; shift += kContinueShift) {
+    byte cur_byte = data_start[(*index)++];
+    bits += (cur_byte & kDataMask) << shift;
+    if ((cur_byte & kContinueMask) == 0) break;
+  }
+  return bits;
+}
+
+// Decodes a variable-length encoded value stored in contiguous memory starting
+// at data_start + index, updating index to where the next encoded value starts.
+inline int32_t VLQDecode(byte* data_start, int* index) {
+  uint32_t bits = VLQDecodeUnsigned(data_start, index);
+  bool is_negative = (bits & 1) == 1;
+  int32_t result = bits >> 1;
+  return is_negative ? -result : result;
+}
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_VLQ_H_

src/baseline/baseline-compiler.h

@@ -13,6 +13,7 @@
 
 #include "src/base/logging.h"
 #include "src/base/threaded-list.h"
+#include "src/base/vlq.h"
 #include "src/baseline/baseline-assembler.h"
 #include "src/handles/handles.h"
 #include "src/interpreter/bytecode-array-iterator.h"
@@ -33,8 +34,14 @@ namespace baseline {
 class BytecodeOffsetTableBuilder {
  public:
   void AddPosition(size_t pc_offset, size_t bytecode_offset) {
-    WriteUint(pc_offset - previous_pc_);
-    WriteUint(bytecode_offset - previous_bytecode_);
+    size_t pc_diff = pc_offset - previous_pc_;
+    size_t bytecode_diff = bytecode_offset - previous_bytecode_;
+    DCHECK_GE(pc_diff, 0);
+    DCHECK_LE(pc_diff, std::numeric_limits<uint32_t>::max());
+    DCHECK_GE(bytecode_diff, 0);
+    DCHECK_LE(bytecode_diff, std::numeric_limits<uint32_t>::max());
+    base::VLQEncodeUnsigned(&bytes_, static_cast<uint32_t>(pc_diff));
+    base::VLQEncodeUnsigned(&bytes_, static_cast<uint32_t>(bytecode_diff));
     previous_pc_ = pc_offset;
     previous_bytecode_ = bytecode_offset;
   }
@@ -43,17 +50,6 @@ class BytecodeOffsetTableBuilder {
   Handle<ByteArray> ToBytecodeOffsetTable(LocalIsolate* isolate);
 
  private:
-  void WriteUint(size_t value) {
-    bool has_next;
-    do {
-      uint8_t byte = value & ((1 << 7) - 1);
-      value >>= 7;
-      has_next = value != 0;
-      byte |= (has_next << 7);
-      bytes_.push_back(byte);
-    } while (has_next);
-  }
-
   size_t previous_pc_ = 0;
   size_t previous_bytecode_ = 0;
   std::vector<byte> bytes_;

src/deoptimizer/translation-array.cc

@@ -4,6 +4,7 @@
 
 #include "src/deoptimizer/translation-array.h"
 
+#include "src/base/vlq.h"
 #include "src/deoptimizer/translated-state.h"
 #include "src/objects/fixed-array-inl.h"
 #include "third_party/zlib/google/compression_utils_portable.h"
@@ -56,19 +57,9 @@ int32_t TranslationArrayIterator::Next() {
   if (V8_UNLIKELY(FLAG_turbo_compress_translation_arrays)) {
     return uncompressed_contents_[index_++];
   } else {
-    // Run through the bytes until we reach one with a least significant
-    // bit of zero (marks the end).
-    uint32_t bits = 0;
-    for (int i = 0; true; i += 7) {
-      DCHECK(HasNext());
-      uint8_t next = buffer_.get(index_++);
-      bits |= (next >> 1) << i;
-      if ((next & 1) == 0) break;
-    }
-    // The bits encode the sign in the least significant bit.
-    bool is_negative = (bits & 1) == 1;
-    int32_t result = bits >> 1;
-    return is_negative ? -result : result;
+    int32_t value = base::VLQDecode(buffer_.GetDataStartAddress(), &index_);
+    DCHECK_LE(index_, buffer_.length());
+    return value;
   }
 }
 
@@ -84,19 +75,7 @@ void TranslationArrayBuilder::Add(int32_t value) {
   if (V8_UNLIKELY(FLAG_turbo_compress_translation_arrays)) {
     contents_for_compression_.push_back(value);
   } else {
-    // This wouldn't handle kMinInt correctly if it ever encountered it.
-    DCHECK_NE(value, kMinInt);
-    // Encode the sign bit in the least significant bit.
-    bool is_negative = (value < 0);
-    uint32_t bits = (static_cast<uint32_t>(is_negative ? -value : value) << 1) |
-                    static_cast<uint32_t>(is_negative);
-    // Encode the individual bytes using the least significant bit of
-    // each byte to indicate whether or not more bytes follow.
-    do {
-      uint32_t next = bits >> 7;
-      contents_.push_back(((bits << 1) & 0xFF) | (next != 0));
-      bits = next;
-    } while (bits != 0);
+    base::VLQEncode(&contents_, value);
   }
 }
 

src/objects/code-inl.h

@@ -6,6 +6,7 @@
 #define V8_OBJECTS_CODE_INL_H_
 
 #include "src/base/memory.h"
+#include "src/base/vlq.h"
 #include "src/codegen/code-desc.h"
 #include "src/common/assert-scope.h"
 #include "src/execution/isolate.h"
@@ -329,24 +330,6 @@ CodeKind Code::kind() const {
   return KindField::decode(ReadField<uint32_t>(kFlagsOffset));
 }
 
-namespace detail {
-
-// TODO(v8:11429): Extract out of header, to generic helper, and merge with
-// TranslationArray de/encoding.
-inline int ReadUint(ByteArray array, int* index) {
-  int byte = 0;
-  int value = 0;
-  int shift = 0;
-  do {
-    byte = array.get((*index)++);
-    value += (byte & ((1 << 7) - 1)) << shift;
-    shift += 7;
-  } while (byte & (1 << 7));
-  return value;
-}
-
-}  // namespace detail
-
 int Code::GetBytecodeOffsetForBaselinePC(Address baseline_pc) {
   DisallowGarbageCollection no_gc;
   CHECK(!is_baseline_prologue_builtin());
@@ -357,10 +340,12 @@ int Code::GetBytecodeOffsetForBaselinePC(Address baseline_pc) {
   Address pc = baseline_pc - InstructionStart();
   int index = 0;
   int offset = 0;
+  byte* data_start = data.GetDataStartAddress();
   while (pc > lookup_pc) {
-    lookup_pc += detail::ReadUint(data, &index);
-    offset += detail::ReadUint(data, &index);
+    lookup_pc += base::VLQDecodeUnsigned(data_start, &index);
+    offset += base::VLQDecodeUnsigned(data_start, &index);
   }
+  DCHECK_LE(index, data.Size());
   CHECK_EQ(pc, lookup_pc);
   return offset;
 }
@@ -375,13 +360,15 @@ uintptr_t Code::GetBaselinePCForBytecodeOffset(int bytecode_offset,
   int offset = 0;
   // TODO(v8:11429,cbruni): clean up
   // Return the offset for the last bytecode that matches
+  byte* data_start = data.GetDataStartAddress();
   while (offset < bytecode_offset && index < data.length()) {
-    int delta_pc = detail::ReadUint(data, &index);
-    int delta_offset = detail::ReadUint(data, &index);
+    int delta_pc = base::VLQDecodeUnsigned(data_start, &index);
+    int delta_offset = base::VLQDecodeUnsigned(data_start, &index);
     if (!precise && (bytecode_offset < offset + delta_offset)) break;
     pc += delta_pc;
     offset += delta_offset;
   }
+  DCHECK_LE(index, data.length());
   if (precise) {
     CHECK_EQ(offset, bytecode_offset);
   } else {

test/unittests/BUILD.gn

@@ -226,6 +226,7 @@ v8_source_set("unittests_sources") {
     "base/threaded-list-unittest.cc",
     "base/utils/random-number-generator-unittest.cc",
     "base/vlq-base64-unittest.cc",
+    "base/vlq-unittest.cc",
     "codegen/code-stub-assembler-unittest.cc",
     "codegen/code-stub-assembler-unittest.h",
     "codegen/register-configuration-unittest.cc",

test/unittests/base/vlq-unittest.cc

+// Copyright 2021 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/base/vlq.h"
+
+#include <cmath>
+#include <limits>
+
+#include "src/base/memory.h"
+#include "test/unittests/test-utils.h"
+#include "testing/gtest-support.h"
+
+namespace v8 {
+namespace base {
+
+int ExpectedBytesUsed(int64_t value, bool is_signed) {
+  uint64_t bits = value;
+  if (is_signed) {
+    bits = (value < 0 ? -value : value) << 1;
+  }
+  int num_bits = 0;
+  while (bits != 0) {
+    num_bits++;
+    bits >>= 1;
+  }
+  return std::max(1, static_cast<int>(ceil(static_cast<float>(num_bits) / 7)));
+}
+
+void TestVLQUnsignedEquals(uint32_t value) {
+  std::vector<byte> buffer;
+  VLQEncodeUnsigned(&buffer, value);
+  byte* data_start = buffer.data();
+  int index = 0;
+  int expected_bytes_used = ExpectedBytesUsed(value, false);
+  EXPECT_EQ(buffer.size(), static_cast<size_t>(expected_bytes_used));
+  EXPECT_EQ(value, VLQDecodeUnsigned(data_start, &index));
+  EXPECT_EQ(index, expected_bytes_used);
+}
+
+void TestVLQEquals(int32_t value) {
+  std::vector<byte> buffer;
+  VLQEncode(&buffer, value);
+  byte* data_start = buffer.data();
+  int index = 0;
+  int expected_bytes_used = ExpectedBytesUsed(value, true);
+  EXPECT_EQ(buffer.size(), static_cast<size_t>(expected_bytes_used));
+  EXPECT_EQ(value, VLQDecode(data_start, &index));
+  EXPECT_EQ(index, expected_bytes_used);
+}
+
+TEST(VLQ, Unsigned) {
+  TestVLQUnsignedEquals(0);
+  TestVLQUnsignedEquals(1);
+  TestVLQUnsignedEquals(63);
+  TestVLQUnsignedEquals(64);
+  TestVLQUnsignedEquals(127);
+  TestVLQUnsignedEquals(255);
+  TestVLQUnsignedEquals(256);
+}
+
+TEST(VLQ, Positive) {
+  TestVLQEquals(0);
+  TestVLQEquals(1);
+  TestVLQEquals(63);
+  TestVLQEquals(64);
+  TestVLQEquals(127);
+  TestVLQEquals(255);
+  TestVLQEquals(256);
+}
+
+TEST(VLQ, Negative) {
+  TestVLQEquals(-1);
+  TestVLQEquals(-63);
+  TestVLQEquals(-64);
+  TestVLQEquals(-127);
+  TestVLQEquals(-255);
+  TestVLQEquals(-256);
+}
+
+TEST(VLQ, LimitsUnsigned) {
+  TestVLQEquals(std::numeric_limits<uint8_t>::max());
+  TestVLQEquals(std::numeric_limits<uint8_t>::max() - 1);
+  TestVLQEquals(std::numeric_limits<uint8_t>::max() + 1);
+  TestVLQEquals(std::numeric_limits<uint16_t>::max());
+  TestVLQEquals(std::numeric_limits<uint16_t>::max() - 1);
+  TestVLQEquals(std::numeric_limits<uint16_t>::max() + 1);
+  TestVLQEquals(std::numeric_limits<uint32_t>::max());
+  TestVLQEquals(std::numeric_limits<uint32_t>::max() - 1);
+}
+
+TEST(VLQ, LimitsSigned) {
+  TestVLQEquals(std::numeric_limits<int8_t>::max());
+  TestVLQEquals(std::numeric_limits<int8_t>::max() - 1);
+  TestVLQEquals(std::numeric_limits<int8_t>::max() + 1);
+  TestVLQEquals(std::numeric_limits<int16_t>::max());
+  TestVLQEquals(std::numeric_limits<int16_t>::max() - 1);
+  TestVLQEquals(std::numeric_limits<int16_t>::max() + 1);
+  TestVLQEquals(std::numeric_limits<int32_t>::max());
+  TestVLQEquals(std::numeric_limits<int32_t>::max() - 1);
+  TestVLQEquals(std::numeric_limits<int8_t>::min());
+  TestVLQEquals(std::numeric_limits<int8_t>::min() - 1);
+  TestVLQEquals(std::numeric_limits<int8_t>::min() + 1);
+  TestVLQEquals(std::numeric_limits<int16_t>::min());
+  TestVLQEquals(std::numeric_limits<int16_t>::min() - 1);
+  TestVLQEquals(std::numeric_limits<int16_t>::min() + 1);
+  // int32_t::min() is not supported.
+  TestVLQEquals(std::numeric_limits<int32_t>::min() + 1);
+}
+
+TEST(VLQ, Random) {
+  static constexpr int RANDOM_RUNS = 50;
+
+  base::RandomNumberGenerator rng(::testing::FLAGS_gtest_random_seed);
+  for (int i = 0; i < RANDOM_RUNS; ++i) {
+    TestVLQUnsignedEquals(rng.NextInt(std::numeric_limits<int32_t>::max()));
+  }
+  for (int i = 0; i < RANDOM_RUNS; ++i) {
+    TestVLQEquals(rng.NextInt());
+  }
+}
+}  // namespace base
+}  // namespace v8