Use std::string_view in more PrefService methods.

This CL adds transparent hashing and transparent equality comparisons
to the std::unordered_map<std::string, Preference> that it keeps.

This should be a no-op for any interactions with the map that involve
a std::string:
- Comparisons just forward to std::string comparisons.
- Hashing for std::basic_string_view and std::basic_string is identical.

However, this CL allows to use a std::string_view when calling
std::unordered_map::find without requiring to convert the string_view
to a std::string first.

The CL takes advantage of that by replacing a few more const reference
to std::string by std::string_view. This avoids potential heap
allocations and reduces the binary size by ~3KB.

Bug: 349741884
Change-Id: Ifcfd7638ee27f641097ad3929bc7fa58c4260b3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5658436
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1320204}

components/prefs/pref_service.cc

@@ -236,7 +236,7 @@ base::FilePath PrefService::GetFilePath(std::string_view path) const {
   return *result;
 }
 
-bool PrefService::HasPrefPath(const std::string& path) const {
+bool PrefService::HasPrefPath(std::string_view path) const {
   const Preference* pref = FindPreference(path);
   return pref && !pref->IsDefaultValue();
 }
@@ -285,17 +285,19 @@ PrefService::GetPreferencesValueAndStore() const {
 }
 
 const PrefService::Preference* PrefService::FindPreference(
-    const std::string& pref_name) const {
+    std::string_view path) const {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  auto it = prefs_map_.find(pref_name);
+  auto it = prefs_map_.find(path);
   if (it != prefs_map_.end())
     return &(it->second);
   const base::Value* default_value = nullptr;
-  if (!pref_registry_->defaults()->GetValue(pref_name, &default_value))
+  if (!pref_registry_->defaults()->GetValue(path, &default_value)) {
     return nullptr;
+  }
   it = prefs_map_
            .insert(std::make_pair(
-               pref_name, Preference(this, pref_name, default_value->type())))
+               std::string(path),
+               Preference(this, std::string(path), default_value->type())))
            .first;
   return &(it->second);
 }
@@ -327,20 +329,18 @@ PrefService::GetAllPrefStoresInitializationStatus() const {
   return GetInitializationStatus();
 }
 
-bool PrefService::IsManagedPreference(const std::string& pref_name) const {
-  const Preference* pref = FindPreference(pref_name);
+bool PrefService::IsManagedPreference(std::string_view path) const {
+  const Preference* pref = FindPreference(path);
   return pref && pref->IsManaged();
 }
 
-bool PrefService::IsPreferenceManagedByCustodian(
-    const std::string& pref_name) const {
-  const Preference* pref = FindPreference(pref_name);
+bool PrefService::IsPreferenceManagedByCustodian(std::string_view path) const {
+  const Preference* pref = FindPreference(path);
   return pref && pref->IsManagedByCustodian();
 }
 
-bool PrefService::IsUserModifiablePreference(
-    const std::string& pref_name) const {
-  const Preference* pref = FindPreference(pref_name);
+bool PrefService::IsUserModifiablePreference(std::string_view path) const {
+  const Preference* pref = FindPreference(path);
   return pref && pref->IsUserModifiable();
 }
 

components/prefs/pref_service.h

@@ -13,6 +13,7 @@
 
 #include <stdint.h>
 
+#include <functional>
 #include <memory>
 #include <set>
 #include <string>
@@ -236,19 +237,19 @@ class COMPONENTS_PREFS_EXPORT PrefService {
 
   // Returns true if the preference for the given preference name is available
   // and is managed.
-  bool IsManagedPreference(const std::string& pref_name) const;
+  bool IsManagedPreference(std::string_view path) const;
 
   // Returns true if the preference for the given preference name is available
   // and is controlled by the parent/guardian of the child Account.
-  bool IsPreferenceManagedByCustodian(const std::string& pref_name) const;
+  bool IsPreferenceManagedByCustodian(std::string_view path) const;
 
   // Returns |true| if a preference with the given name is available and its
   // value can be changed by the user.
-  bool IsUserModifiablePreference(const std::string& pref_name) const;
+  bool IsUserModifiablePreference(std::string_view path) const;
 
   // Look up a preference.  Returns NULL if the preference is not
   // registered.
-  const PrefService::Preference* FindPreference(const std::string& path) const;
+  const PrefService::Preference* FindPreference(std::string_view path) const;
 
   // If the path is valid and the value at the end of the path matches the type
   // specified, it will return the specified value.  Otherwise, the default
@@ -334,7 +335,7 @@ class COMPONENTS_PREFS_EXPORT PrefService {
   // NOTE: this is NOT the same as FindPreference. In particular
   // FindPreference returns whether RegisterXXX has been invoked, where as
   // this checks if a value exists for the path.
-  bool HasPrefPath(const std::string& path) const;
+  bool HasPrefPath(std::string_view path) const;
 
   // Issues a callback for every preference value. The preferences must not be
   // mutated during iteration.
@@ -450,7 +451,11 @@ class COMPONENTS_PREFS_EXPORT PrefService {
   // string comparisons. Order is unimportant, and deletions are rare.
   // Confirmed on Android where this speeded Chrome startup by roughly 50ms
   // vs. std::map, and by roughly 180ms vs. std::set of Preference pointers.
-  typedef std::unordered_map<std::string, Preference> PreferenceMap;
+  struct StringViewHasher : public std::hash<std::string_view> {
+    using is_transparent = void;
+  };
+  using PreferenceMap = std::
+      unordered_map<std::string, Preference, StringViewHasher, std::equal_to<>>;
 
   // Give access to ReportUserPrefChanged() and GetMutableUserPref().
   friend class subtle::ScopedUserPrefUpdateBase;

components/sync_preferences/pref_service_syncable.cc

@@ -282,7 +282,7 @@ void PrefServiceSyncable::RemoveSyncedPrefObserver(
 
 void PrefServiceSyncable::AddRegisteredSyncablePreference(std::string_view path,
                                                           uint32_t flags) {
-  DCHECK(FindPreference(std::string(path)));
+  DCHECK(FindPreference(path));
   if (flags & user_prefs::PrefRegistrySyncable::SYNCABLE_PREF) {
     pref_sync_associator_.RegisterPref(path);
     return;