style: Use atom handles in favour of atom pointers in style system code.

Differential Revision: https://phabricator.services.mozilla.com/D15078

components/style/gecko_string_cache/mod.rs

@@ -15,6 +15,7 @@ use crate::gecko_bindings::bindings::Gecko_Atomize;
 use crate::gecko_bindings::bindings::Gecko_Atomize16;
 use crate::gecko_bindings::bindings::Gecko_ReleaseAtom;
 use crate::gecko_bindings::structs::{nsAtom, nsDynamicAtom, nsStaticAtom};
+use crate::gecko_bindings::structs::root::mozilla::detail::gGkAtoms;
 use nsstring::{nsAString, nsStr};
 use precomputed_hash::PrecomputedHash;
 use std::borrow::{Borrow, Cow};
@@ -43,9 +44,12 @@ macro_rules! local_name {
     };
 }
 
-/// A strong reference to a Gecko atom.
+/// A handle to a Gecko atom.
+///
+/// This is either a strong reference to a dynamic atom (an nsAtom pointer),
+/// or an offset from gGkAtoms to the nsStaticAtom object.
 #[derive(Eq, PartialEq)]
-pub struct Atom(*mut WeakAtom);
+pub struct Atom(usize);
 
 /// An atom *without* a strong reference.
 ///
@@ -53,12 +57,31 @@ pub struct Atom(*mut WeakAtom);
 /// where `'a` is the lifetime of something that holds a strong reference to that atom.
 pub struct WeakAtom(nsAtom);
 
+#[inline]
+fn valid_static_atom_addr(addr: usize) -> bool {
+    unsafe {
+        let start = gGkAtoms.mAtoms.get_unchecked(0) as *const _;
+        let end = gGkAtoms.mAtoms.get_unchecked(gGkAtoms.mAtoms.len()) as *const _;
+        let in_range = addr >= start as usize && addr < end as usize;
+        let aligned = addr % mem::align_of::<nsStaticAtom>() == 0;
+        in_range && aligned
+    }
+}
+
 impl Deref for Atom {
     type Target = WeakAtom;
 
     #[inline]
     fn deref(&self) -> &WeakAtom {
-        unsafe { &*self.0 }
+        unsafe {
+            let addr = if self.is_static() {
+                (&gGkAtoms as *const _ as usize) + (self.0 >> 1)
+            } else {
+                self.0
+            };
+            debug_assert!(!self.is_static() || valid_static_atom_addr(addr));
+            WeakAtom::new(addr as *const nsAtom)
+        }
     }
 }
 
@@ -277,50 +300,73 @@ impl fmt::Display for WeakAtom {
     }
 }
 
+#[inline]
+unsafe fn make_handle(ptr: *const nsAtom) -> usize {
+    debug_assert!(!ptr.is_null());
+    if !WeakAtom::new(ptr).is_static() {
+        ptr as usize
+    } else {
+        make_static_handle(ptr as *mut nsStaticAtom)
+    }
+}
+
+#[inline]
+unsafe fn make_static_handle(ptr: *const nsStaticAtom) -> usize {
+    // FIXME(heycam): Use offset_from once it's stabilized.
+    // https://github.com/rust-lang/rust/issues/41079
+    debug_assert!(valid_static_atom_addr(ptr as usize));
+    let base = &gGkAtoms as *const _;
+    let offset = ptr as usize - base as usize;
+    (offset << 1) | 1
+}
+
 impl Atom {
+    #[inline]
+    fn is_static(&self) -> bool {
+        self.0 & 1 == 1
+    }
+
     /// Execute a callback with the atom represented by `ptr`.
     pub unsafe fn with<F, R>(ptr: *const nsAtom, callback: F) -> R
     where
         F: FnOnce(&Atom) -> R,
     {
-        let atom = Atom(WeakAtom::new(ptr));
+        let atom = Atom(make_handle(ptr as *mut nsAtom));
         let ret = callback(&atom);
         mem::forget(atom);
         ret
     }
 
-    /// Creates an atom from an static atom pointer without checking in release
-    /// builds.
-    ///
-    /// Right now it's only used by the atom macro, and ideally it should keep
-    /// that way, now we have sugar for is_static, creating atoms using
-    /// Atom::from_raw should involve almost no overhead.
+    /// Creates a static atom from its index in the static atom table, without
+    /// checking in release builds.
     #[inline]
-    pub unsafe fn from_static(ptr: *const nsStaticAtom) -> Self {
-        let atom = Atom(ptr as *mut WeakAtom);
-        debug_assert!(
-            atom.is_static(),
-            "Called from_static for a non-static atom!"
-        );
+    pub unsafe fn from_index(index: u16) -> Self {
+        let ptr = gGkAtoms.mAtoms.get_unchecked(index as usize) as *const _;
+        let handle = make_static_handle(ptr);
+        let atom = Atom(handle);
+        debug_assert!(valid_static_atom_addr(ptr as usize));
+        debug_assert!(atom.is_static());
+        debug_assert!((*atom).is_static());
+        debug_assert!(handle == make_handle(atom.as_ptr()));
         atom
     }
 
     /// Creates an atom from an atom pointer.
     #[inline(always)]
     pub unsafe fn from_raw(ptr: *mut nsAtom) -> Self {
-        let atom = Atom(ptr as *mut WeakAtom);
+        let atom = Atom(make_handle(ptr));
         if !atom.is_static() {
             Gecko_AddRefAtom(ptr);
         }
         atom
     }
 
-    /// Creates an atom from a dynamic atom pointer that has already had AddRef
-    /// called on it.
+    /// Creates an atom from an atom pointer that has already had AddRef
+    /// called on it. This may be a static or dynamic atom.
     #[inline]
     pub unsafe fn from_addrefed(ptr: *mut nsAtom) -> Self {
         assert!(!ptr.is_null());
-        Atom(WeakAtom::new(ptr))
+        Atom(make_handle(ptr))
     }
 
     /// Convert this atom into an addrefed nsAtom pointer.
@@ -353,7 +399,13 @@ impl Hash for WeakAtom {
 impl Clone for Atom {
     #[inline(always)]
     fn clone(&self) -> Atom {
-        unsafe { Atom::from_raw(self.as_ptr()) }
+        unsafe {
+            let atom = Atom(self.0);
+            if !atom.is_static() {
+                Gecko_AddRefAtom(atom.as_ptr());
+            }
+            atom
+        }
     }
 }
 
@@ -377,13 +429,13 @@ impl Default for Atom {
 
 impl fmt::Debug for Atom {
     fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
-        write!(w, "Gecko Atom({:p}, {})", self.0, self)
+        write!(w, "Atom(0x{:08x}, {})", self.0, self)
     }
 }
 
 impl fmt::Display for Atom {
     fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
-        unsafe { (&*self.0).fmt(w) }
+        self.deref().fmt(w)
     }
 }
 
@@ -392,10 +444,10 @@ impl<'a> From<&'a str> for Atom {
     fn from(string: &str) -> Atom {
         debug_assert!(string.len() <= u32::max_value() as usize);
         unsafe {
-            Atom(WeakAtom::new(Gecko_Atomize(
+            Atom::from_addrefed(Gecko_Atomize(
                 string.as_ptr() as *const _,
                 string.len() as u32,
-            )))
+            ))
         }
     }
 }
@@ -410,7 +462,7 @@ impl<'a> From<&'a [u16]> for Atom {
 impl<'a> From<&'a nsAString> for Atom {
     #[inline]
     fn from(string: &nsAString) -> Atom {
-        unsafe { Atom(WeakAtom::new(Gecko_Atomize16(string))) }
+        unsafe { Atom::from_addrefed(Gecko_Atomize16(string)) }
     }
 }