Revert of [tracing] Turn StackFrame into struct.

base/trace_event/heap_profiler_allocation_context.cc

 // Copyright 2015 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.
 
 #include "base/trace_event/heap_profiler_allocation_context.h"
 
 #include <cstring>
 
 #include "base/hash.h"
 #include "base/macros.h"
 
 namespace base {
 namespace trace_event {
 
-bool operator < (const StackFrame& lhs, const StackFrame& rhs) {
-  return lhs.value < rhs.value;
+// Constructor that does not initialize members.
+AllocationContext::AllocationContext() {}
+
+// static
+AllocationContext AllocationContext::Empty() {
+  AllocationContext ctx;
+
+  for (size_t i = 0; i < arraysize(ctx.backtrace.frames); i++)
+    ctx.backtrace.frames[i] = nullptr;
+
+  ctx.type_name = nullptr;
+
+  return ctx;
 }
 
-bool operator == (const StackFrame& lhs, const StackFrame& rhs) {
-  return lhs.value == rhs.value;
+bool operator==(const Backtrace& lhs, const Backtrace& rhs) {
+  // Pointer equality of the stack frames is assumed, so instead of doing a deep
+  // string comparison on all of the frames, a |memcmp| suffices.
+  return std::memcmp(lhs.frames, rhs.frames, sizeof(lhs.frames)) == 0;
 }
 
-bool operator != (const StackFrame& lhs, const StackFrame& rhs) {
-  return !(lhs.value == rhs.value);
-}
-
-Backtrace::Backtrace(): frame_count(0) {}
-
-bool operator==(const Backtrace& lhs, const Backtrace& rhs) {
-  if (lhs.frame_count != rhs.frame_count) return false;
-  return std::equal(lhs.frames, lhs.frames + lhs.frame_count, rhs.frames);
-}
-
-AllocationContext::AllocationContext(): type_name(nullptr) {}
-
 bool operator==(const AllocationContext& lhs, const AllocationContext& rhs) {
   return (lhs.backtrace == rhs.backtrace) && (lhs.type_name == rhs.type_name);
 }
 
 }  // namespace trace_event
 }  // namespace base
 
 namespace BASE_HASH_NAMESPACE {
 using base::trace_event::AllocationContext;
 using base::trace_event::Backtrace;
-using base::trace_event::StackFrame;
-
-size_t hash<StackFrame>::operator()(const StackFrame& frame) const {
-  return hash<const void*>()(frame.value);
-}
 
 size_t hash<Backtrace>::operator()(const Backtrace& backtrace) const {
-  const void* values[Backtrace::kMaxFrameCount];
-  for (size_t i = 0; i != backtrace.frame_count; ++i) {
-    values[i] = backtrace.frames[i].value;
-  }
-  return base::SuperFastHash(reinterpret_cast<const char*>(values),
-                             backtrace.frame_count * sizeof(*values));
+  return base::SuperFastHash(reinterpret_cast<const char*>(backtrace.frames),
+                             sizeof(backtrace.frames));
 }
 
 size_t hash<AllocationContext>::operator()(const AllocationContext& ctx) const {
   size_t backtrace_hash = hash<Backtrace>()(ctx.backtrace);
 
   // Multiplicative hash from [Knuth 1998]. Works best if |size_t| is 32 bits,
   // because the magic number is a prime very close to 2^32 / golden ratio, but
   // will still redistribute keys bijectively on 64-bit architectures because
   // the magic number is coprime to 2^64.
   size_t type_hash = reinterpret_cast<size_t>(ctx.type_name) * 2654435761;
 
   // Multiply one side to break the commutativity of +. Multiplication with a
   // number coprime to |numeric_limits<size_t>::max() + 1| is bijective so
   // randomness is preserved.
   return (backtrace_hash * 3) + type_hash;
 }
 
 }  // BASE_HASH_NAMESPACE