Add TBI helpers for AArch64

Adds a trait to `core_arch` for AArch64 to set a value in the top byte of a pointer (which should logically become the canonical address of the allocation), and to retrieve the value stored in the top byte of a pointer, if any.

Author: dheaton-arm

crates/core_arch/src/aarch64/mod.rs

@@ -6,6 +6,10 @@
 //! [arm_ref]: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073a/IHI0073A_arm_neon_intrinsics_ref.pdf
 //! [arm_dat]: https://developer.arm.com/technologies/neon/intrinsics
 
+mod tbi;
+#[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+pub use self::tbi::*;
+
 // NEON intrinsics are currently broken on big-endian, so don't expose them. (#1484)
 #[cfg(target_endian = "little")]
 mod neon;

crates/core_arch/src/aarch64/tbi.rs

@@ -0,0 +1,94 @@
+/// Provides additional methods on pointers to get and set any values set in the top byte of the pointer,
+/// as per AArch64's Top-Byte Ignore (TBI) feature.
+#[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+pub trait TBIPointer: Sized {
+    /// Returns a new pointer with the top byte set to a given value, invalidating the original pointer.
+    ///
+    /// Continuing to use the pointer passed in to this function is Undefined Behavior; you should replace it with the returned pointer instead.
+    #[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+    fn with_top_byte(self, b: u8) -> Self;
+
+    /// Returns the value (if any) stored in the top byte of the pointer.
+    #[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+    fn top_byte(&self) -> u8;
+}
+
+macro_rules! tbi_ptr_impl {
+    () => {
+        fn with_top_byte(self, b: u8) -> Self {
+            // We can't actually call `realloc` here as we don't have `std`, and the pointer may not be valid for `realloc` anyway (if the value's stored on the stack, for example).
+            let addr = self.addr() & 0x00ffffffffffffff;
+            let p = addr | ((b as usize) << 56);
+            self.with_addr(p)
+        }
+
+        fn top_byte(&self) -> u8 {
+            (self.addr() >> 56) as u8
+        }
+    };
+}
+
+#[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+impl<T> TBIPointer for *const T {
+    tbi_ptr_impl!();
+}
+
+#[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+impl<T> TBIPointer for *mut T {
+    tbi_ptr_impl!();
+}
+
+#[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+impl<T> TBIPointer for *const [T] {
+    tbi_ptr_impl!();
+}
+
+#[unstable(feature = "stdarch_aarch64_tbi", issue = "none")]
+impl<T> TBIPointer for *mut [T] {
+    tbi_ptr_impl!();
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn tbi_const() {
+        let value: u32 = 10;
+        let mut address = &value as *const u32;
+        address = address.with_top_byte(0x80);
+        assert_eq!(address.top_byte(), 0x80);
+        assert_eq!(unsafe { *address }, 10);
+    }
+
+    #[test]
+    fn tbi_mut() {
+        let mut value: u32 = 10;
+        let mut address = &mut value as *mut u32;
+        address = address.with_top_byte(0x80);
+        assert_eq!(address.top_byte(), 0x80);
+        assert_eq!(unsafe { *address }, 10);
+        unsafe { *address = 255 };
+        assert_eq!(unsafe { *address }, 255);
+    }
+
+    #[test]
+    fn tbi_const_array() {
+        let value: [u32; 4] = [10, 255, 65535, 0xffffffff];
+        let mut address = &value as *const [u32; 4];
+        address = address.with_top_byte(0x80);
+        assert_eq!(address.top_byte(), 0x80);
+        assert_eq!(unsafe { *address }, [10, 255, 65535, 0xffffffff]);
+    }
+
+    #[test]
+    fn tbi_mut_array() {
+        let mut value: [u32; 4] = [10, 255, 65535, 0xffffffff];
+        let mut address = &mut value as *mut [u32; 4];
+        address = address.with_top_byte(0x80);
+        assert_eq!(address.top_byte(), 0x80);
+        assert_eq!(unsafe { *address }, [10, 255, 65535, 0xffffffff]);
+        unsafe { (*address)[0] = 25 };
+        assert_eq!(unsafe { *address }, [25, 255, 65535, 0xffffffff]);
+    }
+}

crates/core_arch/src/lib.rs

@@ -35,7 +35,9 @@
     generic_arg_infer,
     asm_experimental_arch,
     sha512_sm_x86,
-    f16
+    f16,
+    ptr_metadata,
+    strict_provenance
 )]
 #![cfg_attr(test, feature(test, abi_vectorcall, stdarch_internal))]
 #![deny(clippy::missing_inline_in_public_items)]