[identity-map] Change resize heuristic

Change the resizing behaviour on insert to match that of the hash map
in base. Specifically, resize when hitting 80% occupancy.

Locally, I measure a ~6% improvement in serialization time from this
change.

Change-Id: I3fe84de39b2337859fe75fa6b3848198b82071ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2448798
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70324}

src/utils/identity-map.cc

@@ -64,24 +64,30 @@ int IdentityMapBase::ScanKeysFor(Address address, uint32_t hash) const {
 std::pair<int, bool> IdentityMapBase::InsertKey(Address address,
                                                 uint32_t hash) {
   DCHECK_EQ(gc_counter_, heap_->gc_count());
+
+  // Grow the map if we reached >= 80% occupancy.
+  if (size_ + size_ / 4 >= capacity_) {
+    Resize(capacity_ * kResizeFactor);
+  }
+
   Address not_mapped = ReadOnlyRoots(heap_).not_mapped_symbol().ptr();
+
+  int start = hash & mask_;
+  // Guaranteed to terminate since size_ < capacity_, there must be at least
+  // one empty slot.
+  int index = start;
   while (true) {
-    int start = hash & mask_;
-    int limit = capacity_ / 2;
-    // Search up to {limit} entries.
-    for (int index = start; --limit > 0; index = (index + 1) & mask_) {
-      if (keys_[index] == address) return {index, true};  // Found.
-      if (keys_[index] == not_mapped) {                   // Free entry.
-        size_++;
-        DCHECK_LE(size_, capacity_);
-        keys_[index] = address;
-        return {index, false};
-      }
+    if (keys_[index] == address) return {index, true};  // Found.
+    if (keys_[index] == not_mapped) {                   // Free entry.
+      size_++;
+      DCHECK_LE(size_, capacity_);
+      keys_[index] = address;
+      return {index, false};
     }
-    // Should only have to resize once, since we grow 4x.
-    Resize(capacity_ * kResizeFactor);
+    index = (index + 1) & mask_;
+    // We should never loop back to the start.
+    DCHECK_NE(index, start);
   }
-  UNREACHABLE();
 }
 
 bool IdentityMapBase::DeleteIndex(int index, uintptr_t* deleted_value) {