[Tracing] Make heap profiler type info a string

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 {
 
 // 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_id = 0;
+  ctx.type_name = nullptr;
 
   return ctx;
 }
 
 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 AllocationContext& lhs, const AllocationContext& rhs) {
-  return (lhs.backtrace == rhs.backtrace) && (lhs.type_id == rhs.type_id);
+  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;
 
 size_t hash<Backtrace>::operator()(const Backtrace& backtrace) const {
   return base::SuperFastHash(reinterpret_cast<const char*>(backtrace.frames),
                              sizeof(backtrace.frames));
 }
 
 size_t hash<AllocationContext>::operator()(const AllocationContext& ctx) const {
-  size_t ctx_hash = hash<Backtrace>()(ctx.backtrace);
+  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. The type ID is assumed to be distributed randomly
-  // already so there is no need to hash it.
-  return (ctx_hash * 3) + static_cast<size_t>(ctx.type_id);
+  // randomness is preserved.
+  return (backtrace_hash * 3) + type_hash;
 }
 
 }  // BASE_HASH_NAMESPACE