style: Add a very simple use counter implementation.

As simple as I could make it, for now. We can improve on this.

Differential Revision: https://phabricator.services.mozilla.com/D3827

components/style/use_counters/mod.rs

@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Various stuff for CSS property use counters.
+
+use properties::{NonCustomPropertyId, NON_CUSTOM_PROPERTY_ID_COUNT};
+// FIXME(emilio): We need AtomicU32 on stable ASAP...
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering;
+
+#[cfg(target_pointer_width = "64")]
+const BITS_PER_ENTRY: usize = 64;
+
+#[cfg(target_pointer_width = "32")]
+const BITS_PER_ENTRY: usize = 32;
+
+/// One bit per each non-custom CSS property.
+#[derive(Default)]
+pub struct NonCustomPropertyUseCounters {
+    storage: [AtomicUsize; (NON_CUSTOM_PROPERTY_ID_COUNT - 1 + BITS_PER_ENTRY) / BITS_PER_ENTRY],
+}
+
+impl NonCustomPropertyUseCounters {
+    /// Record that a given non-custom property ID has been parsed.
+    #[inline]
+    pub fn record(&self, id: NonCustomPropertyId) {
+        let bit = id.bit();
+        let bucket = bit / BITS_PER_ENTRY;
+        let bit_in_bucket = bit % BITS_PER_ENTRY;
+        self.storage[bucket].fetch_or(1 << bit_in_bucket, Ordering::Relaxed);
+    }
+}
+
+/// The use-counter data related to a given document we want to store.
+pub struct UseCounters {
+    /// The counters for non-custom properties that have been parsed in the
+    /// document's stylesheets.
+    pub non_custom_properties: NonCustomPropertyUseCounters,
+}