WIP attempt to implement user_fpsimd_struct.

Potentially fixes #2524, see the comments in the patch for details.

Author: Gankra

libc-test/semver/android-aarch64.txt

@@ -12,3 +12,4 @@ SYS_syscalls
 SYS_fcntl
 __system_property_wait
 user_regs_struct
+user_fpsimd_struct

src/fixed_width_ints.rs

@@ -18,3 +18,70 @@ pub type uint16_t = u16;
 pub type uint32_t = u32;
 #[deprecated(since = "0.2.55", note = "Use u64 instead.")]
 pub type uint64_t = u64;
+
+cfg_if! {
+    if #[cfg(all(target_arch = "aarch64",
+        any(target_os = "android", target_os="macos", target_os = "ios")))] {
+        // This introduces partial support for FFI with __int128 and
+        // equivalent types on platforms where Rust's definition is validated
+        // to match the standard C ABI of that platform.
+        //
+        // Rust does not guarantee u128/i128 are sound for FFI, and its
+        // definitions are in fact known to be incompatible. [0]
+        //
+        // However these problems aren't fundamental, and are just platform
+        // inconsistencies. Specifically at the time of this writing:
+        //
+        // * For x64 SysV ABIs (everything but Windows), the types are underaligned.
+        // * For all Windows ABIs, Microsoft doesn't actually officially define __int128,
+        //   and as a result different implementations don't actually agree on its ABI.
+        //
+        // But on the major aarch64 platforms (android, macos, ios) we have validated
+        // that rustc has the right ABI for these types. This is important because
+        // aarch64 uses these types in some fundamental OS types like user_fpsimd_struct,
+        // which represents saved simd registers.
+        //
+        // Any API which uses these types will need to `#[ignore(improper_ctypes)]`
+        // until the upstream rust issue is resolved, but this at least lets us make
+        // progress on platforms where this type is important.
+        //
+        // The supported architectures and OSes is intentionally very restricted,
+        // as careful work needs to be done to verify that a particular platform
+        // has a conformant ABI.
+        //
+        // [0]: https://github.com/rust-lang/rust/issues/54341
+
+        /// C `__int128` (a GCC extension that's part of many ABIs)
+        pub type __int128 = i128;
+        /// C `unsigned __int128` (a GCC extension that's part of many ABIs)
+        pub type __uint128 = u128;
+        /// C __int128_t (alternate name for [__int128][])
+        pub type __int128_t = i128;
+        /// C __uint128_t (alternate name for [__uint128][])
+        pub type __uint128_t = u128;
+
+        // NOTE: if you add more platforms to here, you made need to cfg
+        // these consts. They should always match the platform's values
+        // for `sizeof(__int128)` and `_Alignof(__int128)`.
+        const SIZE_128: usize = 16;
+        const ALIGN_128: usize = 16;
+
+        /// Since Rust doesn't officially guarantee that these types
+        /// have compatible ABIs, we const assert that these values have the
+        /// known size/align of the target platform's libc. If rustc ever
+        /// tries to regress things, it will cause a compilation error.
+        const _ASSERT_128_COMPAT: () = {
+            assert!(std::mem::size_of::<__int128>() == SIZE_128);
+            assert!(std::mem::align_of::<__int128>() == ALIGN_128);
+
+            assert!(std::mem::size_of::<__uint128>() == SIZE_128);
+            assert!(std::mem::align_of::<__uint128>() == ALIGN_128);
+
+            assert!(std::mem::size_of::<__int128_t>() == SIZE_128);
+            assert!(std::mem::align_of::<__int128_t>() == ALIGN_128);
+
+            assert!(std::mem::size_of::<__uint128_t>() == SIZE_128);
+            assert!(std::mem::align_of::<__uint128_t>() == ALIGN_128);
+        };
+    }
+}

src/unix/bsd/apple/b64/aarch64/align.rs

@@ -39,9 +39,8 @@ s! {
         pub __pad: u32,
     }
 
-    #[repr(align(16))]
     pub struct __darwin_arm_neon_state64 {
-        pub __v: [[u64; 2]; 32],
+        pub __v: [__uint128_t; 32],
         pub __fpsr: u32,
         pub __fpcr: u32,
     }

src/unix/linux_like/android/b64/aarch64/mod.rs

@@ -55,6 +55,12 @@ s! {
         pub pc: u64,
         pub pstate: u64,
     }
+
+    pub struct user_fpsimd_struct {
+        pub vregs: [__uint128_t; 32],
+        pub fpsr: u32,
+        pub fpcr: u32,
+    }
 }
 
 pub const O_DIRECT: ::c_int = 0x10000;