New Objective-C runtime features

crates/objc2/Cargo.toml

@@ -65,6 +65,20 @@ unstable-c-unwind = []
 # For better documentation on docs.rs
 unstable-docsrs = []
 
+# Enable some new features available on ARM64 on:
+# - macOS 13.0
+# - iOS 16.0
+# - tvOS 16.0
+# - watchOS 9.0
+#
+# See https://developer.apple.com/videos/play/wwdc2022/110363/ for an overview
+# of the features.
+#
+# Currently untested, might be unsound or lead to confusing compiler errors.
+#
+# Additionally, the message sending improvements is not yet implemented.
+unstable-apple-new = ["apple"]
+
 # Runtime selection. See `objc-sys` for details.
 apple = ["objc-sys/apple"]
 gnustep-1-7 = ["objc-sys/gnustep-1-7"]

crates/objc2/src/rc/allocated.rs

@@ -3,7 +3,7 @@ use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
 use core::ptr::NonNull;
 
-use crate::ffi;
+use crate::runtime::objc_release_fast;
 use crate::Message;
 
 /// A marker type that can be used to indicate that the object has been
@@ -77,8 +77,8 @@ impl<T: ?Sized> Drop for Allocated<T> {
         // destructors are written to take into account that the object may
         // not have been initialized.
         //
-        // Rest is same as `Id`.
-        unsafe { ffi::objc_release(self.ptr.as_ptr().cast()) };
+        // Rest is same as `Id`'s `Drop`.
+        unsafe { objc_release_fast(self.ptr.as_ptr().cast()) };
     }
 }
 

crates/objc2/src/rc/id.rs

@@ -7,6 +7,7 @@ use core::ptr::{self, NonNull};
 
 use super::AutoreleasePool;
 use crate::mutability::{IsIdCloneable, IsMutable};
+use crate::runtime::{objc_release_fast, objc_retain_fast};
 use crate::{ffi, ClassType, Message};
 
 /// A reference counted pointer type for Objective-C objects.
@@ -321,7 +322,7 @@ impl<T: Message> Id<T> {
     #[inline]
     pub unsafe fn retain(ptr: *mut T) -> Option<Id<T>> {
         // SAFETY: The caller upholds that the pointer is valid
-        let res: *mut T = unsafe { ffi::objc_retain(ptr.cast()) }.cast();
+        let res: *mut T = unsafe { objc_retain_fast(ptr.cast()) }.cast();
         debug_assert_eq!(res, ptr, "objc_retain did not return the same pointer");
         // SAFETY: We just retained the object, so it has +1 retain count
         unsafe { Self::new(res) }
@@ -404,8 +405,14 @@ impl<T: Message> Id<T> {
             };
 
             // Supported since macOS 10.10.
-            #[cfg(target_arch = "aarch64")]
+            //
+            // On macOS 13.0 / iOS 16.0 / tvOS 16.0 / watchOS 9.0, the runtime
+            // instead checks the return pointer address, so we no longer need
+            // to emit these extra instructions, see this video from WWDC22:
+            // https://developer.apple.com/videos/play/wwdc2022/110363/
+            #[cfg(all(target_arch = "aarch64", not(feature = "unstable-apple-new")))]
             unsafe {
+                // Same as `mov x29, x29`
                 core::arch::asm!("mov fp, fp", options(nomem, preserves_flags, nostack))
             };
 
@@ -650,7 +657,7 @@ impl<T: ?Sized> Drop for Id<T> {
 
         // SAFETY: The `ptr` is guaranteed to be valid and have at least one
         // retain count.
-        unsafe { ffi::objc_release(self.ptr.as_ptr().cast()) };
+        unsafe { objc_release_fast(self.ptr.as_ptr().cast()) };
     }
 }
 

crates/objc2/src/runtime/mod.rs

@@ -38,8 +38,10 @@ mod nsobject;
 mod nsproxy;
 mod nszone;
 mod protocol_object;
+mod retain_release_fast;
 
 pub(crate) use self::method_encoding_iter::{EncodingParseError, MethodEncodingIter};
+pub(crate) use self::retain_release_fast::{objc_release_fast, objc_retain_fast};
 use crate::encode::__unstable::{EncodeArguments, EncodeConvertReturn, EncodeReturn};
 use crate::encode::{Encode, Encoding, OptionEncode, RefEncode};
 use crate::verify::{verify_method_signature, Inner};

crates/objc2/src/runtime/retain_release_fast.rs

@@ -0,0 +1,104 @@
+//! Optimized versions of `objc_retain` and `objc_release`.
+//!
+//! On macOS 13.0 / iOS 16.0 / tvOS 16.0 / watchOS 9.0, on ARM64, optimized
+//! versions of these two functions that use a different calling convention
+//! than the usual C calling convention, are available.
+//!
+//! Specifically, the expected input register is changed. The output register
+//! is unchanged.
+//!
+//! As an example, if the object is stored in the `x19` register and we need
+//! to release it, we usually end up emitting an extra `mov` to get the object
+//! into the `x0` register first, as expected by the C calling convention:
+//!
+//! ```asm
+//! mov x0, x19
+//! bl _objc_release
+//! ```
+//!
+//! With this optimization though, since the expected register is encoded in
+//! the name of the function instead, we can avoid the move altogether.
+//!
+//! ```asm
+//! bl _objc_release_x19
+//! ```
+//!
+//!
+//!
+//! Safety of our two uses of the `asm!` macro:
+//!
+//! 1. We use the register class `reg`, with the modifier `x`, which on
+//!    Aarch64 is defined as `x[0-30]`, see [this][asm-reg-cls].
+//!
+//!    The functions are only available in the variants `x[0-15]` and
+//!    `x[19-28]` though, see [this][objc4-source], so if the register
+//!    allocator ends up using `x16`, `x17`, `x18`, `x29` or `x30`, we will
+//!    emit a call to e.g. `objc_retain_x29`, which will fail at link time.
+//!
+//!    TODO: Before this option can be stable, we need a way to prevent that!
+//!
+//! 2. We use the `clobber_abi("C")` since we're effectively calling a C
+//!    C function.
+//!
+//! [asm-reg-cls]: https://doc.rust-lang.org/nightly/reference/inline-assembly.html#register-operands
+//! [objc4-source]: https://github.com/apple-oss-distributions/objc4/blob/objc4-866.9/runtime/objc-abi.h#L442-L498
+use crate::ffi;
+
+/// A potentially faster version of `ffi::objc_retain`.
+///
+///
+/// # Safety
+///
+/// Same as `ffi::objc_retain`.
+#[inline]
+pub(crate) unsafe fn objc_retain_fast(obj: *mut ffi::objc_object) -> *mut ffi::objc_object {
+    #[cfg(all(feature = "unstable-apple-new", target_arch = "aarch64"))]
+    // SAFETY: See the file header.
+    //
+    // As per the ARM64 calling convention, the return value is put in `x0`.
+    //
+    // That the function itself is safe to call is upheld by the caller.
+    unsafe {
+        let result;
+        core::arch::asm!(
+            "bl _objc_retain_{obj:x}",
+            obj = in(reg) obj,
+            lateout("x0") result,
+            clobber_abi("C"),
+        );
+        result
+    }
+
+    #[cfg(not(all(feature = "unstable-apple-new", target_arch = "aarch64")))]
+    // SAFETY: Upheld by caller.
+    unsafe {
+        ffi::objc_retain(obj)
+    }
+}
+
+/// A potentially faster version of `ffi::objc_release`.
+///
+///
+/// # Safety
+///
+/// Same as `ffi::objc_release`.
+#[inline]
+pub(crate) unsafe fn objc_release_fast(obj: *mut ffi::objc_object) {
+    #[cfg(all(feature = "unstable-apple-new", target_arch = "aarch64"))]
+    // SAFETY: See the file header.
+    //
+    // That the function itself is safe to call is upheld by the caller.
+    unsafe {
+        core::arch::asm!(
+            "bl _objc_release_{obj:x}",
+            obj = in(reg) obj,
+            clobber_abi("C"),
+        )
+    }
+
+    #[cfg(not(all(feature = "unstable-apple-new", target_arch = "aarch64")))]
+    // SAFETY: Upheld by caller.
+    unsafe {
+        ffi::objc_release(obj)
+    }
+}