Add an option to print the resource ids that Chrome loads.

ui/base/resource/data_pack.cc

Index: ui/base/resource/data_pack.cc
diff --git a/ui/base/resource/data_pack.cc b/ui/base/resource/data_pack.cc
index 6de25681749a0bb68e2177e5405eeb6466b4c7bf..eba2489834d4c6b24cc2c04b7bbb850e1103333b 100644
--- a/ui/base/resource/data_pack.cc
+++ b/ui/base/resource/data_pack.cc
@@ -5,14 +5,17 @@
 #include "ui/base/resource/data_pack.h"
 
 #include <errno.h>
+#include <set>
 #include <utility>
 
+#include "base/command_line.h"
 #include "base/files/file_util.h"
 #include "base/files/memory_mapped_file.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/stl_util.h"
 #include "base/strings/string_piece.h"
 
 // For details of the file layout, see
@@ -24,7 +27,7 @@ static const uint32_t kFileFormatVersion = 4;
 // Length of file header: version, entry count and text encoding type.
 static const size_t kHeaderLength = 2 * sizeof(uint32_t) + sizeof(uint8_t);
 
-#pragma pack(push,2)
+#pragma pack(push, 2)
 struct DataPackEntry {
   uint16_t resource_id;
   uint32_t file_offset;
@@ -65,6 +68,32 @@ void LogDataPackError(LoadErrors error) {
   UMA_HISTOGRAM_ENUMERATION("DataPack.Load", error, LOAD_ERRORS_COUNT);
 }
 
+// Prints the given resource id the first time it's loaded if Chrome has been
+// started with --print-resource-ids. This output is then used to generate a
+// more optimal resource renumbering to improve startup speed. See
+// tools/gritsettings/README.md for more info.
+void MaybePrintResourceId(uint16_t resource_id) {
+  // This code is run in other binaries than Chrome which do not initialize the
+  // CommandLine object. Early return in those cases.
+  if (!base::CommandLine::InitializedForCurrentProcess())
+    return;
+
+  // Note: This switch isn't in ui/base/ui_base_switches.h because ui/base
+  // depends on ui/base/resource and thus it would cause a circular dependency.
+  static bool print_resource_ids =
+      base::CommandLine::ForCurrentProcess()->HasSwitch("print-resource-ids");
+  if (!print_resource_ids)
+    return;
+
+  // Note: This is leaked intentionally. However, it's only allocated if the
+  // above command line is specified, so it shouldn't affect regular users.
+  static std::set<uint16_t>* resource_ids_logged = new std::set<uint16_t>();
+  if (!base::ContainsKey(*resource_ids_logged, resource_id)) {

[Nico, 2017/02/17 21:45:58]

is it guaranteed that we only load resources from a single thread?

[Alexei Svitkine (slow), 2017/02/17 22:30:43]

Good point. I had originally assumed yes, but looking at the header, there is no
such guarantee provided and looking at the implementation, all the functions are
const and just read the data - so it should in theory be useable from multiple
threads after initial construction.

So given the above, I've now added a lock to this function to safeguard against
potential multi-thread access.

+    printf("Resource=%d\n", resource_id);
+    resource_ids_logged->insert(resource_id);
+  }
+}
+
 }  // namespace
 
 namespace ui {
@@ -258,6 +287,7 @@ bool DataPack::GetStringPiece(uint16_t resource_id,
     return false;
   }
 
+  MaybePrintResourceId(resource_id);
   size_t length = next_entry->file_offset - target->file_offset;
   data->set(reinterpret_cast<const char*>(data_source_->GetData() +
                                           target->file_offset),