[tracing] use SHA1 instead of base::Hash for memory dump GUIDs

base/trace_event/memory_allocator_dump_guid.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/memory_allocator_dump_guid.h"
 
 #include "base/format_macros.h"
-#include "base/hash.h"
+#include "base/sha1.h"
 #include "base/strings/stringprintf.h"
 
 namespace base {
 namespace trace_event {
 
+namespace {
+uint64 HashString(const std::string& str) {
+  unsigned char sha1_bytes[kSHA1Length];
+  SHA1HashBytes(reinterpret_cast<const unsigned char*>(str.data()), str.size(),
+                &sha1_bytes[0]);
+  uint64 hash = 0;
+  for (size_t i = 0; i < sizeof(uint64); ++i) {

[petrcermak, 2015/06/23 13:13:43]

nit: This relies on the fact that sizeof(uint64) <= kSHA1Length. I think it
would be a good idea to add a static assertion like here:
https://code.google.com/p/chromium/codesearch#chromium/src/components/variati...

[Primiano Tucci (use gerrit), 2015/06/23 13:52:07]

Ehh, if either uint64 or SHA1 change their size, England will fall! :P

+    hash |= sha1_bytes[i];
+    hash <<= 8;

[petrcermak, 2015/06/23 13:13:43]

This order "wastes" 1 byte completely, because the last 8 bits will always be
zero in the returned value. I think you should reverse the two operations:

  hash <<= 8;
  hash |= sha1_bytes[i];

[Primiano Tucci (use gerrit), 2015/06/23 13:52:07]

Actually picksi@ had a better idea here about using an array.

+  }

[picksi, 2015/06/23 13:35:51]

Apart from reversing the bytes, isn't this just the same as making a union of
the sha1_bytes array and a uint64, and just returning the uint64?

+  return hash;
+}
+}  // namespace
+
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(uint64 guid) : guid_(guid) {
 }
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid()
     : MemoryAllocatorDumpGuid(0u) {
 }
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(const std::string& guid_str)
-    : MemoryAllocatorDumpGuid(Hash(guid_str)) {
+    : MemoryAllocatorDumpGuid(HashString(guid_str)) {
 }
 
 std::string MemoryAllocatorDumpGuid::ToString() const {
   return StringPrintf("%" PRIx64, guid_);
 }
 
 }  // namespace trace_event
 }  // namespace base