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,
+  kFieldTypeLengthLimited = 2,

[kraynov, 2016/07/05 11:25:09]

...LengthDelimited
For consistency with
https://developers.google.com/protocol-buffers/docs/encoding

[Primiano Tucci (use gerrit), 2016/07/06 16:52:55]

Done.

+  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) {

[kraynov, 2016/07/05 11:25:09]

ditto Delimited

[Primiano Tucci (use gerrit), 2016/07/06 16:52:55]

Done.

+  return (field_id << 3) | kFieldTypeLengthLimited;
+}
+
+// 64-bit fixed-length field types: fixed32, sfixed32, float.

[kraynov, 2016/06/22 16:38:32]

nit: 32

[Primiano Tucci (use gerrit), 2016/07/06 16:52:54]

Done.

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

[kraynov, 2016/06/22 16:38:32]

WriteVarIntS32 (S for zig-zag)
WriteVarIntS64
WriteVarInt32
WriteVarInt64

These names?

[Primiano Tucci (use gerrit), 2016/07/06 16:52:54]

Yup, correct. But no rush, we won't need them until much later.

+
+// 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.
+inline void WriteRedundantVarIntUnsigned(uint32_t value,

[oystein (OOO til 10th of July), 2016/06/15 08:45:44]

Can you expand a bit on why this is necessary? Fixed-size varints sounds
self-contradictory to me :). Is this because the proto wire format only supports
varint encoding for integers?

(It may also be pushing the limits of what should be an inlined function,
especially since it pulls in DCHECK etc).

[Primiano Tucci (use gerrit), 2016/06/15 09:02:12]

Right. So the problem here is that when you have length-limited fields, such as
strings or nested messages, the proto encoding format mandates that their size
must be written using VarInt (no option for fixed int).
The only thing that can be "fixed" are numeric fields, which can be fixed32/64,
but the size field of nested objects is varint.
It's not a problem for strings, as we can strlen them upfront. But for nested
messages, we cannot know the size upfront. Hence the trick of reserving 4 bytes
and using redundant encoding.

Apparently this technique is already used internally, for the same design
reasons.See go/hklai .
So, the reason why I made this inline is because IIRC is used only in one place
really. But happy to make it non-inline and hope/expect that the compiler will
do that.

+                                         size_t length,
+                                         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_