tracing v2: Introduce TraceBufferReader to read-back the trace buffer

components/tracing/core/string_interning.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_STRING_INTERNING_H_
+#define COMPONENTS_TRACING_CORE_STRING_INTERNING_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <limits>
+
+#include "base/logging.h"
+#include "components/tracing/tracing_export.h"
+
+// All non-copied strings in tracing are, by API contract, long lived
+// const char* pointers. In order to save trace buffer size (and performance),
+// tracing doesn't copy those strings, but keeps only an index for each event.
+// The way these strings are interned is pretty straightforward: we encode the
+// ptr distance from a known constant in the binary (kInternedStringBase) and
+// use that as string index.
+// There are two use cases for interned strings:
+// 1. In the most common cases they are looked up in batch when the trace is
+//    finalized and trace metadata is written, to produce the final string
+//    table. This case always works (as long as the strings are long lived)
+//    in both component and static builds. We just need to do the reverse math
+//    to work out the const char* from the offset at finalization time.
+// 2. When supporting the recovery of non-finalized traces from a crahed
+//    process, the same principle can be applied, but only in the case of
+//    non-component builds. In a monolithic build the string offset is constatnt

[DmitrySkiba, 2016/11/29 18:09:56]

constatnt -> constant

+//    regardless of ASLR.
+
+namespace tracing {
+namespace v2 {
+
+TRACING_EXPORT extern const char kInternedStringBase[];
+
+inline int64_t InternString(const char* str) {

[DmitrySkiba, 2016/11/29 18:09:56]

Maybe rename "intern" to "index" or "stable offset", or something? In my head
"interning" means this:
http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#intern%28%29

I.e. there is a pool of strings, and "interning" means that we take arbitrary
string and return either its counterpart from the pool, or add the string to the
pool (and return it).

+  // keeping the diff of two arbitrary pointers into a int64_t is safe as the
+  // virtual address space of all the 64-bit architectures we care about is
+  // << 64 bits (MMUs are typically limited at 38 bits).
+  ptrdiff_t offset = reinterpret_cast<intptr_t>(str) -
+                     reinterpret_cast<intptr_t>(kInternedStringBase);
+  return static_cast<int64_t>(offset);
+}
+
+inline const char* GetInternedStringValue(int64_t offset) {
+  intptr_t ptr = reinterpret_cast<intptr_t>(kInternedStringBase) + offset;
+  return reinterpret_cast<const char*>(ptr);
+}
+
+}  // namespace v2
+}  // namespace tracing
+
+#endif  // COMPONENTS_TRACING_CORE_STRING_INTERNING_H_