servo: Merge #20433 - Statically allocate static atoms (from nnethercote:bug-1411469); r=froydnj

<!-- Please describe your changes on the following line: -->

This is the Servo part of bug 1411469.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix Mozilla bug 1411469

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because tests are on the Gecko side.

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r? @emilio

Source-Repo: https://github.com/servo/servo
Source-Revision: 10813436b36323711430ab56fad83234029c9f96

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 26652a48385035c413b0290768cbefee37b765e9

servo/components/style/gecko/generated/structs.rs

@@ -17188,26 +17188,25 @@ pub mod root {
     pub struct nsAtom {
         pub _bitfield_1: root::__BindgenBitfieldUnit<[u8; 4usize], u32>,
         pub mHash: u32,
-        pub mString: *const u16,
     }
     #[repr(u8)]
     #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
     pub enum nsAtom_AtomKind {
-        DynamicAtom = 0,
-        StaticAtom = 1,
-        HTML5Atom = 2,
+        Static = 0,
+        DynamicNormal = 1,
+        DynamicHTML5 = 2,
     }
     pub type nsAtom_HasThreadSafeRefCnt = root::mozilla::TrueType;
     #[test]
     fn bindgen_test_layout_nsAtom() {
         assert_eq!(
             ::std::mem::size_of::<nsAtom>(),
-            16usize,
+            8usize,
             concat!("Size of: ", stringify!(nsAtom))
         );
         assert_eq!(
             ::std::mem::align_of::<nsAtom>(),
-            8usize,
+            4usize,
             concat!("Alignment of ", stringify!(nsAtom))
         );
         assert_eq!(
@@ -17220,16 +17219,6 @@ pub mod root {
                 stringify!(mHash)
             )
         );
-        assert_eq!(
-            unsafe { &(*(::std::ptr::null::<nsAtom>())).mString as *const _ as usize },
-            8usize,
-            concat!(
-                "Offset of field: ",
-                stringify!(nsAtom),
-                "::",
-                stringify!(mString)
-            )
-        );
     }
     impl nsAtom {
         #[inline]
@@ -17278,19 +17267,55 @@ pub mod root {
     #[derive(Debug)]
     pub struct nsStaticAtom {
         pub _base: root::nsAtom,
+        pub mStringOffset: u32,
     }
     #[test]
     fn bindgen_test_layout_nsStaticAtom() {
         assert_eq!(
             ::std::mem::size_of::<nsStaticAtom>(),
-            16usize,
+            12usize,
             concat!("Size of: ", stringify!(nsStaticAtom))
         );
         assert_eq!(
             ::std::mem::align_of::<nsStaticAtom>(),
-            8usize,
+            4usize,
             concat!("Alignment of ", stringify!(nsStaticAtom))
         );
+        assert_eq!(
+            unsafe { &(*(::std::ptr::null::<nsStaticAtom>())).mStringOffset as *const _ as usize },
+            8usize,
+            concat!("Offset of field: ", stringify!(nsStaticAtom), "::", stringify!(mString))
+        );
+    }
+    #[repr(C)]
+    #[derive(Debug)]
+    pub struct nsDynamicAtom {
+        pub _base: root::nsAtom,
+        pub mRefCnt: root::mozilla::ThreadSafeAutoRefCnt,
+        pub mString: *const u16,
+    }
+    #[test]
+    fn bindgen_test_layout_nsDynamicAtom() {
+        assert_eq!(
+            ::std::mem::size_of::<nsDynamicAtom>(),
+            24usize,
+            concat!("Size of: ", stringify!(nsDynamicAtom))
+        );
+        assert_eq!(
+            ::std::mem::align_of::<nsDynamicAtom>(),
+            8usize,
+            concat!("Alignment of ", stringify!(nsDynamicAtom))
+        );
+        assert_eq!(
+            unsafe { &(*(::std::ptr::null::<nsDynamicAtom>())).mRefCnt as *const _ as usize },
+            8usize,
+            concat!("Offset of field: ", stringify!(nsDynamicAtom), "::", stringify!(mRefCnt))
+        );
+        assert_eq!(
+            unsafe { &(*(::std::ptr::null::<nsDynamicAtom>())).mString as *const _ as usize },
+            16usize,
+            concat!("Offset of field: ", stringify!(nsDynamicAtom), "::", stringify!(mString))
+        );
     }
     pub type nsLoadFlags = u32;
     #[repr(C)]

servo/components/style/gecko_string_cache/mod.rs

@@ -10,7 +10,7 @@ use gecko_bindings::bindings::Gecko_AddRefAtom;
 use gecko_bindings::bindings::Gecko_Atomize;
 use gecko_bindings::bindings::Gecko_Atomize16;
 use gecko_bindings::bindings::Gecko_ReleaseAtom;
-use gecko_bindings::structs::{nsAtom, nsAtom_AtomKind, nsStaticAtom};
+use gecko_bindings::structs::{nsAtom, nsAtom_AtomKind, nsDynamicAtom, nsStaticAtom};
 use nsstring::{nsAString, nsStr};
 use precomputed_hash::PrecomputedHash;
 use std::{mem, slice, str};
@@ -113,9 +113,19 @@ impl WeakAtom {
     /// Get the atom as a slice of utf-16 chars.
     #[inline]
     pub fn as_slice(&self) -> &[u16] {
-        unsafe {
-            slice::from_raw_parts((*self.as_ptr()).mString, self.len() as usize)
-        }
+        let string = if self.is_static() {
+           let atom_ptr = self.as_ptr() as *const nsStaticAtom;
+           let string_offset = unsafe { (*atom_ptr).mStringOffset };
+           let string_offset = -(string_offset as isize);
+           let u8_ptr = atom_ptr as *const u8;
+           // It is safe to use offset() here because both addresses are within
+           // the same struct, e.g. mozilla::detail::gGkAtoms.
+           unsafe { u8_ptr.offset(string_offset) as *const u16 }
+        } else {
+           let atom_ptr = self.as_ptr() as *const nsDynamicAtom;
+           unsafe { (*(atom_ptr)).mString }
+        };
+        unsafe { slice::from_raw_parts(string, self.len() as usize) }
     }
 
     // NOTE: don't expose this, since it's slow, and easy to be misused.
@@ -166,7 +176,7 @@ impl WeakAtom {
     #[inline]
     pub fn is_static(&self) -> bool {
         unsafe {
-            (*self.as_ptr()).mKind() == nsAtom_AtomKind::StaticAtom as u32
+            (*self.as_ptr()).mKind() == nsAtom_AtomKind::Static as u32
         }
     }