Add atomic support for pre-ARMv6 on Linux

Cargo.toml

@@ -35,4 +35,7 @@ utest-macros = { git = "https://github.com/japaric/utest" }
 name = "intrinsics"
 required-features = ["c", "compiler-builtins"]
 
+# Support for atomic operations on Linux for pre-ARMv6 systems
+arm_linux_atomic = []
+
 [workspace]

src/arm_linux.rs

@@ -0,0 +1,170 @@
+use core::intrinsics;
+use core::mem;
+
+// Kernel-provided user-mode helper functions:
+// https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
+unsafe fn __kuser_cmpxchg(oldval: u32, newval: u32, ptr: *mut u32) -> bool {
+    let out: u32;
+    // FIXME: we can't use BLX on ARMv4
+    asm!("blx ${0}"
+         : "={r0}" (out)
+         : "r" (0xffff0fc0u32)
+           "{r0}" (oldval),
+           "{r1}" (newval),
+           "{r2}" (ptr)
+         : "r3", "r12", "lr", "cc", "memory");
+    out == 0
+}
+unsafe fn __kuser_memory_barrier() {
+    // FIXME: we can't use BLX on ARMv4
+    asm!("blx ${0}"
+         :
+         : "r" (0xffff0fa0u32)
+         : "lr", "memory");
+}
+
+// Word-align a pointer
+fn align_ptr<T>(ptr: *mut T) -> *mut u32 {
+    // This gives us a mask of 0 when T == u32 since the pointer is already
+    // supposed to be aligned, which avoids any masking in that case.
+    let ptr_mask = 3 & (4 - mem::size_of::<T>());
+    (ptr as usize & !ptr_mask) as *mut u32
+}
+
+// Calculate the shift and mask of a value inside an aligned word
+fn get_shift_mask<T>(ptr: *mut T) -> (u32, u32) {
+    // Mask to get the low byte/halfword/word
+    let mask = match mem::size_of::<T>() {
+        1 => 0xff,
+        2 => 0xffff,
+        4 => 0xffffffff,
+        _ => unreachable!(),
+    };
+
+    // If we are on big-endian then we need to adjust the shift accordingly
+    let endian_adjust = if cfg!(target_endian = "little") {
+        0
+    } else {
+        4 - mem::size_of::<T>() as u32
+    };
+
+    // Shift to get the desired element in the word
+    let ptr_mask = 3 & (4 - mem::size_of::<T>());
+    let shift = ((ptr as usize & ptr_mask) as u32 ^ endian_adjust) * 8;
+
+    (shift, mask)
+}
+
+// Extract a value from an aligned word
+fn extract_aligned(aligned: u32, shift: u32, mask: u32) -> u32 {
+    (aligned >> shift) & mask
+}
+
+// Insert a value into an aligned word
+fn insert_aligned(aligned: u32, val: u32, shift: u32, mask: u32) -> u32 {
+    (aligned & !(mask << shift)) | ((val & mask) << shift)
+}
+
+// Generic atomic read-modify-write operation
+unsafe fn atomic_rmw<T, F: Fn(u32) -> u32>(ptr: *mut T, f: F) -> u32 {
+    let aligned_ptr = align_ptr(ptr);
+    let (shift, mask) = get_shift_mask(ptr);
+
+    loop {
+        let curval_aligned = intrinsics::atomic_load_unordered(aligned_ptr);
+        let curval = extract_aligned(curval_aligned, shift, mask);
+        let newval = f(curval);
+        let newval_aligned = insert_aligned(curval_aligned, newval, shift, mask);
+        if __kuser_cmpxchg(curval_aligned, newval_aligned, aligned_ptr) {
+            return curval;
+        }
+    }
+}
+
+// Generic atomic compare-exchange operation
+unsafe fn atomic_cmpxchg<T>(oldval: u32, newval: u32, ptr: *mut T) -> u32 {
+    let aligned_ptr = align_ptr(ptr);
+    let (shift, mask) = get_shift_mask(ptr);
+
+    loop {
+        let curval_aligned = intrinsics::atomic_load_unordered(aligned_ptr);
+        let curval = extract_aligned(curval_aligned, shift, mask);
+        if curval != oldval {
+            return curval;
+        }
+        let newval_aligned = insert_aligned(curval_aligned, newval, shift, mask);
+        if __kuser_cmpxchg(curval_aligned, newval_aligned, aligned_ptr) {
+            return oldval;
+        }
+    }
+}
+
+macro_rules! atomic_rmw {
+    ($name:ident, $ty:ty, $op:expr) => {
+        #[cfg_attr(not(feature = "mangled-names"), no_mangle)]
+        pub unsafe extern "C" fn $name(ptr: *mut $ty, val: $ty) -> $ty {
+            atomic_rmw(ptr, |x| $op(x as $ty, val) as u32) as $ty
+        }
+    }
+}
+macro_rules! atomic_cmpxchg {
+    ($name:ident, $ty:ty) => {
+        #[cfg_attr(not(feature = "mangled-names"), no_mangle)]
+        pub unsafe extern "C" fn $name(oldval: $ty, newval: $ty, ptr: *mut $ty) -> $ty {
+            atomic_cmpxchg(oldval as u32, newval as u32, ptr) as $ty
+        }
+    }
+}
+
+atomic_rmw!(__sync_fetch_and_add_1, u8, |a: u8, b: u8| a.wrapping_add(b));
+atomic_rmw!(__sync_fetch_and_add_2, u16, |a: u16, b: u16| a.wrapping_add(b));
+atomic_rmw!(__sync_fetch_and_add_4, u32, |a: u32, b: u32| a.wrapping_add(b));
+
+atomic_rmw!(__sync_fetch_and_sub_1, u8, |a: u8, b: u8| a.wrapping_sub(b));
+atomic_rmw!(__sync_fetch_and_sub_2, u16, |a: u16, b: u16| a.wrapping_sub(b));
+atomic_rmw!(__sync_fetch_and_sub_4, u32, |a: u32, b: u32| a.wrapping_sub(b));
+
+atomic_rmw!(__sync_fetch_and_and_1, u8, |a: u8, b: u8| a & b);
+atomic_rmw!(__sync_fetch_and_and_2, u16, |a: u16, b: u16| a & b);
+atomic_rmw!(__sync_fetch_and_and_4, u32, |a: u32, b: u32| a & b);
+
+atomic_rmw!(__sync_fetch_and_or_1, u8, |a: u8, b: u8| a | b);
+atomic_rmw!(__sync_fetch_and_or_2, u16, |a: u16, b: u16| a | b);
+atomic_rmw!(__sync_fetch_and_or_4, u32, |a: u32, b: u32| a | b);
+
+atomic_rmw!(__sync_fetch_and_xor_1, u8, |a: u8, b: u8| a ^ b);
+atomic_rmw!(__sync_fetch_and_xor_2, u16, |a: u16, b: u16| a ^ b);
+atomic_rmw!(__sync_fetch_and_xor_4, u32, |a: u32, b: u32| a ^ b);
+
+atomic_rmw!(__sync_fetch_and_nand_1, u8, |a: u8, b: u8| !a & b);
+atomic_rmw!(__sync_fetch_and_nand_2, u16, |a: u16, b: u16| !a & b);
+atomic_rmw!(__sync_fetch_and_nand_4, u32, |a: u32, b: u32| !a & b);
+
+atomic_rmw!(__sync_fetch_and_max_1, i8, |a: i8, b: i8| if a > b { a } else { b });
+atomic_rmw!(__sync_fetch_and_max_2, i16, |a: i16, b: i16| if a > b { a } else { b });
+atomic_rmw!(__sync_fetch_and_max_4, i32, |a: i32, b: i32| if a > b { a } else { b });
+
+atomic_rmw!(__sync_fetch_and_umax_1, u8, |a: u8, b: u8| if a > b { a } else { b });
+atomic_rmw!(__sync_fetch_and_umax_2, u16, |a: u16, b: u16| if a > b { a } else { b });
+atomic_rmw!(__sync_fetch_and_umax_4, u32, |a: u32, b: u32| if a > b { a } else { b });
+
+atomic_rmw!(__sync_fetch_and_min_1, i8, |a: i8, b: i8| if a < b { a } else { b });
+atomic_rmw!(__sync_fetch_and_min_2, i16, |a: i16, b: i16| if a < b { a } else { b });
+atomic_rmw!(__sync_fetch_and_min_4, i32, |a: i32, b: i32| if a < b { a } else { b });
+
+atomic_rmw!(__sync_fetch_and_umin_1, u8, |a: u8, b: u8| if a < b { a } else { b });
+atomic_rmw!(__sync_fetch_and_umin_2, u16, |a: u16, b: u16| if a < b { a } else { b });
+atomic_rmw!(__sync_fetch_and_umin_4, u32, |a: u32, b: u32| if a < b { a } else { b });
+
+atomic_rmw!(__sync_lock_test_and_set_1, u8, |_: u8, b: u8| b);
+atomic_rmw!(__sync_lock_test_and_set_2, u16, |_: u16, b: u16| b);
+atomic_rmw!(__sync_lock_test_and_set_4, u32, |_: u32, b: u32| b);
+
+atomic_cmpxchg!(__sync_val_compare_and_swap_1, u8);
+atomic_cmpxchg!(__sync_val_compare_and_swap_2, u16);
+atomic_cmpxchg!(__sync_val_compare_and_swap_4, u32);
+
+#[cfg_attr(not(feature = "mangled-names"), no_mangle)]
+pub unsafe extern "C" fn __sync_synchronize() {
+    __kuser_memory_barrier();
+}

src/lib.rs

@@ -54,6 +54,10 @@ pub mod arm;
 #[cfg(target_arch = "x86")]
 pub mod x86;
 
+#[cfg(all(feature = "arm_linux_atomic", target_os = "linux", target_arch = "arm"))]
+pub mod arm_linux;
+
+
 #[cfg(target_arch = "x86_64")]
 pub mod x86_64;