RFC: use some in-memory state in SimpleCache to quickly cache-miss some CantConditionalize cases

net/disk_cache/simple/simple_index.cc

Index: net/disk_cache/simple/simple_index.cc
diff --git a/net/disk_cache/simple/simple_index.cc b/net/disk_cache/simple/simple_index.cc
index c41c829bdefd808db4ea55dded03e54f4574b641..42b6eafded46cf43f62f7b83b94f04a9372b2c1a 100644
--- a/net/disk_cache/simple/simple_index.cc
+++ b/net/disk_cache/simple/simple_index.cc
@@ -60,7 +60,8 @@ EntryMetadata::EntryMetadata()
 
 EntryMetadata::EntryMetadata(base::Time last_used_time,
                              base::StrictNumeric<uint32_t> entry_size)
-    : last_used_time_seconds_since_epoch_(0), entry_size_(entry_size) {
+    : last_used_time_seconds_since_epoch_(0), entry_size_(0) {
+  SetEntrySize(entry_size);  // to round/pack properly.
   SetLastUsedTime(last_used_time);
 }
 
@@ -88,11 +89,23 @@ void EntryMetadata::SetLastUsedTime(const base::Time& last_used_time) {
 }
 
 uint32_t EntryMetadata::GetEntrySize() const {
-  return entry_size_;
+  return entry_size_ & 0xFFFFFF00;

[jkarlin, 2017/06/08 14:44:06]

I'd rather add a byte (or a uint32_t even) to the index than hide a byte within
the existing struct. Hiding just complicates the code's logic and makes
GetEntrySize less precise.

+}
+
+uint8_t EntryMetadata::GetOracleByte() const {
+  return entry_size_ & 0xFF;
 }
 
 void EntryMetadata::SetEntrySize(base::StrictNumeric<uint32_t> entry_size) {
-  entry_size_ = entry_size;
+  uint8_t save_oracle_byte = GetOracleByte();
+  // ### what happens if we overflow here?
+  entry_size_ = ((static_cast<uint32_t>(entry_size) + 255) & 0xFFFFFF00) |
+                save_oracle_byte;
+}
+
+void EntryMetadata::SetOracleByte(uint8_t oracle_byte) {
+  uint32_t save_size = GetEntrySize();
+  entry_size_ = (save_size | oracle_byte);
 }
 
 void EntryMetadata::Serialize(base::Pickle* pickle) const {
@@ -104,7 +117,8 @@ void EntryMetadata::Serialize(base::Pickle* pickle) const {
   pickle->WriteUInt64(entry_size_);
 }
 
-bool EntryMetadata::Deserialize(base::PickleIterator* it) {
+bool EntryMetadata::Deserialize(base::PickleIterator* it,
+                                bool has_oracle_byte) {
   DCHECK(it);
   int64_t tmp_last_used_time;
   uint64_t tmp_entry_size;
@@ -112,7 +126,12 @@ bool EntryMetadata::Deserialize(base::PickleIterator* it) {
       tmp_entry_size > std::numeric_limits<decltype(entry_size_)>::max())
     return false;
   SetLastUsedTime(base::Time::FromInternalValue(tmp_last_used_time));
-  entry_size_ = static_cast<uint32_t>(tmp_entry_size);
+  if (has_oracle_byte) {
+    entry_size_ = static_cast<uint32_t>(tmp_entry_size);
+  } else {
+    SetEntrySize(static_cast<uint32_t>(tmp_entry_size));
+    SetOracleByte(0);
+  }
   return true;
 }
 
@@ -281,6 +300,15 @@ bool SimpleIndex::Has(uint64_t hash) const {
   return !initialized_ || entries_set_.count(hash) > 0;
 }
 
+bool SimpleIndex::HasWithOracleByte(uint64_t entry_hash, uint8_t* oracle_byte) {
+  EntrySet::iterator it = entries_set_.find(entry_hash);
+  if (it == entries_set_.end())
+    // If not initialized, always return true, forcing it to go to the disk.
+    return !initialized_;
+  *oracle_byte = it->second.GetOracleByte();
+  return true;
+}
+
 bool SimpleIndex::UseIfExists(uint64_t entry_hash) {
   DCHECK(io_thread_checker_.CalledOnValidThread());
   // Always update the last used time, even if it is during initialization.
@@ -363,6 +391,16 @@ bool SimpleIndex::UpdateEntrySize(uint64_t entry_hash,
   return true;
 }
 
+void SimpleIndex::UpdateEntryOracleByte(uint64_t entry_hash,
+                                        uint8_t oracle_byte) {
+  DCHECK(io_thread_checker_.CalledOnValidThread());
+  EntrySet::iterator it = entries_set_.find(entry_hash);
+  if (it == entries_set_.end())
+    return;
+  it->second.SetOracleByte(oracle_byte);
+  PostponeWritingToDisk();
+}
+
 void SimpleIndex::EvictionDone(int result) {
   DCHECK(io_thread_checker_.CalledOnValidThread());
 
@@ -410,8 +448,9 @@ void SimpleIndex::UpdateEntryIteratorSize(
   DCHECK(io_thread_checker_.CalledOnValidThread());
   DCHECK_GE(cache_size_, (*it)->second.GetEntrySize());
   cache_size_ -= (*it)->second.GetEntrySize();
-  cache_size_ += static_cast<uint32_t>(entry_size);
   (*it)->second.SetEntrySize(entry_size);
+  // We use GetEntrySize to get consistent rounding.
+  cache_size_ += (*it)->second.GetEntrySize();
 }
 
 void SimpleIndex::MergeInitializingSet(