[Tracing] Make heap profiler type info a string

base/trace_event/heap_profiler_allocation_context.cc

Index: base/trace_event/heap_profiler_allocation_context.cc
diff --git a/base/trace_event/heap_profiler_allocation_context.cc b/base/trace_event/heap_profiler_allocation_context.cc
index ce8e50d1c777130cb2751b995229c5fb03fcb7d2..038c083f0fa83241f9fef46c0ed239572d282d62 100644
--- a/base/trace_event/heap_profiler_allocation_context.cc
+++ b/base/trace_event/heap_profiler_allocation_context.cc
@@ -22,7 +22,7 @@ AllocationContext AllocationContext::Empty() {
   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;
 }
@@ -34,7 +34,7 @@ bool operator==(const Backtrace& lhs, const Backtrace& rhs) {
 }
 
 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
@@ -50,13 +50,18 @@ size_t hash<Backtrace>::operator()(const Backtrace& backtrace) const {
 }
 
 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