tracing v2: Add proto_utils.h

components/tracing/core/proto_utils.h

+// Copyright 2016 The Chromium 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 COMPONENTS_TRACING_CORE_PROTO_UTILS_H_
+#define COMPONENTS_TRACING_CORE_PROTO_UTILS_H_
+
+#include <inttypes.h>
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "components/tracing/tracing_export.h"
+
+namespace tracing {
+namespace v2 {
+namespace proto {
+
+// See https://developers.google.com/protocol-buffers/docs/encoding wire types.
+
+enum : uint32_t {
+  kFieldTypeVarInt = 0,
+  kFieldTypeFixed64 = 1,
+  kFieldTypeLengthDelimited = 2,
+  kFieldTypeFixed32 = 5,
+};
+
+// Variable-length field types: (s)int32, (s)int64, bool, enum.
+inline uint32_t MakeTagVarInt(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeVarInt;
+}
+
+// Length-limited field types: string, bytes, embedded messages.
+inline uint32_t MakeTagLengthLimited(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeLengthDelimited;
+}
+
+// 32-bit fixed-length field types: fixed32, sfixed32, float.
+inline uint32_t MakeTagFixed32(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeFixed32;
+}
+
+// 64-bit fixed-length field types: fixed64, sfixed64, double.
+inline uint32_t MakeTagFixed64(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeFixed64;
+}
+
+template <typename T>
+inline uint8_t* WriteVarIntInternal(T value, uint8_t* target) {

[alph, 2016/07/06 17:29:52]

Maybe add a static check that T is unsigned?

[Primiano Tucci (use gerrit), 2016/07/07 12:59:25]

Done.

+  while (value >= 0x80) {
+    *target = static_cast<uint8_t>(value | 0x80);

[alph, 2016/07/06 17:29:52]

nit: *target++ =

[Primiano Tucci (use gerrit), 2016/07/07 12:59:25]

Done.

+    value >>= 7;
+    ++target;
+  }
+  *target = static_cast<uint8_t>(value);
+  return target + 1;
+}
+
+inline uint8_t* WriteVarIntU32(uint32_t value, uint8_t* target) {
+  return WriteVarIntInternal<uint32_t>(value, target);
+}
+
+inline uint8_t* WriteVarIntU64(uint64_t value, uint8_t* target) {
+  return WriteVarIntInternal<uint64_t>(value, target);
+}
+
+// TODO(kraynov): add support for signed integers and zig-zag encoding.
+
+// Writes a fixed-size redundant encoding of the given |value|. This is
+// used to backfill fixed-size reservations for the length field using a
+// non-canonical varint encoding (e.g. \x81\x80\x80\x00 instead of \x01).
+// See https://github.com/google/protobuf/issues/1530.
+// Concretely this is used when writing a nested message, where the size of
+// nested message is not known until it is ended. In this case a fixed amount of
+// bytes are reserved to encode its size field and backfilled at the end.
+inline void WriteRedundantVarIntUnsigned(uint32_t value,
+                                         size_t length,

[alph, 2016/07/06 17:29:52]

As it seems to be a hot path, I'd better make it a template with length
parameter. So the compiler could inline and unroll the loop.

[Primiano Tucci (use gerrit), 2016/07/07 12:59:25]

I was planning to do a final perf-optimization pass (tracked in
crbug.com/624311) and address this as part of that. But both you and kraynov
asked for the same, so I'll do this one here now :)

+                                         uint8_t* buf) {
+  for (size_t i = 0; i < length; ++i) {
+    const uint8_t msb = (i < length - 1) ? 0x80 : 0;
+    buf[i] = static_cast<uint8_t>((value & 0x7F) | msb);
+    value >>= 7;
+  }
+  DCHECK_EQ(0u, value) << "Buffer too short to encode the given value";
+}
+
+}  // namespace proto
+}  // namespace v2
+}  // namespace tracing
+
+#endif  // COMPONENTS_TRACING_CORE_PROTO_UTILS_H_