Implement prefetch hints for aarch64 (#918)

Co-authored-by: Wang Maozhang <wangmaozhang@huawei.com>

library/stdarch/crates/core_arch/src/aarch64/mod.rs

@@ -21,6 +21,9 @@ pub use self::tme::*;
 mod crc;
 pub use self::crc::*;
 
+mod prefetch;
+pub use self::prefetch::*;
+
 pub use super::acle::*;
 
 #[cfg(test)]

library/stdarch/crates/core_arch/src/aarch64/prefetch.rs

@@ -0,0 +1,89 @@
+#[cfg(test)]
+use stdarch_test::assert_instr;
+
+extern "C" {
+    #[link_name = "llvm.prefetch"]
+    fn prefetch(p: *const i8, rw: i32, loc: i32, ty: i32);
+}
+
+/// See [`prefetch`](fn._prefetch.html).
+pub const _PREFETCH_READ: i32 = 0;
+
+/// See [`prefetch`](fn._prefetch.html).
+pub const _PREFETCH_WRITE: i32 = 1;
+
+/// See [`prefetch`](fn._prefetch.html).
+pub const _PREFETCH_LOCALITY0: i32 = 0;
+
+/// See [`prefetch`](fn._prefetch.html).
+pub const _PREFETCH_LOCALITY1: i32 = 1;
+
+/// See [`prefetch`](fn._prefetch.html).
+pub const _PREFETCH_LOCALITY2: i32 = 2;
+
+/// See [`prefetch`](fn._prefetch.html).
+pub const _PREFETCH_LOCALITY3: i32 = 3;
+
+/// Fetch the cache line that contains address `p` using the given `rw` and `locality`.
+///
+/// The `rw` must be one of:
+///
+/// * [`_PREFETCH_READ`](constant._PREFETCH_READ.html): the prefetch is preparing
+///   for a read.
+///
+/// * [`_PREFETCH_WRITE`](constant._PREFETCH_WRITE.html): the prefetch is preparing
+///   for a write.
+///
+/// The `locality` must be one of:
+///
+/// * [`_PREFETCH_LOCALITY0`](constant._PREFETCH_LOCALITY0.html): Streaming or
+///   non-temporal prefetch, for data that is used only once.
+///
+/// * [`_PREFETCH_LOCALITY1`](constant._PREFETCH_LOCALITY1.html): Fetch into level 3 cache.
+///
+/// * [`_PREFETCH_LOCALITY2`](constant._PREFETCH_LOCALITY2.html): Fetch into level 2 cache.
+///
+/// * [`_PREFETCH_LOCALITY3`](constant._PREFETCH_LOCALITY3.html): Fetch into level 1 cache.
+///
+/// The prefetch memory instructions signal to the memory system that memory accesses
+/// from a specified address are likely to occur in the near future. The memory system
+/// can respond by taking actions that are expected to speed up the memory access when
+/// they do occur, such as preloading the specified address into one or more caches.
+/// Because these signals are only hints, it is valid for a particular CPU to treat
+/// any or all prefetch instructions as a NOP.
+///
+///
+/// [Arm's documentation](https://developer.arm.com/documentation/den0024/a/the-a64-instruction-set/memory-access-instructions/prefetching-memory?lang=en)
+#[inline(always)]
+#[cfg_attr(test, assert_instr("prfm pldl1strm", rw = _PREFETCH_READ, locality = _PREFETCH_LOCALITY0))]
+#[cfg_attr(test, assert_instr("prfm pldl3keep", rw = _PREFETCH_READ, locality = _PREFETCH_LOCALITY1))]
+#[cfg_attr(test, assert_instr("prfm pldl2keep", rw = _PREFETCH_READ, locality = _PREFETCH_LOCALITY2))]
+#[cfg_attr(test, assert_instr("prfm pldl1keep", rw = _PREFETCH_READ, locality = _PREFETCH_LOCALITY3))]
+#[cfg_attr(test, assert_instr("prfm pstl1strm", rw = _PREFETCH_WRITE, locality = _PREFETCH_LOCALITY0))]
+#[cfg_attr(test, assert_instr("prfm pstl3keep", rw = _PREFETCH_WRITE, locality = _PREFETCH_LOCALITY1))]
+#[cfg_attr(test, assert_instr("prfm pstl2keep", rw = _PREFETCH_WRITE, locality = _PREFETCH_LOCALITY2))]
+#[cfg_attr(test, assert_instr("prfm pstl1keep", rw = _PREFETCH_WRITE, locality = _PREFETCH_LOCALITY3))]
+#[rustc_args_required_const(1, 2)]
+pub unsafe fn _prefetch(p: *const i8, rw: i32, locality: i32) {
+    // We use the `llvm.prefetch` instrinsic with `cache type` = 1 (data cache).
+    // `rw` and `strategy` are based on the function parameters.
+    macro_rules! pref {
+        ($rdwr:expr, $local:expr) => {
+            match ($rdwr, $local) {
+                (0, 0) => prefetch(p, 0, 0, 1),
+                (0, 1) => prefetch(p, 0, 1, 1),
+                (0, 2) => prefetch(p, 0, 2, 1),
+                (0, 3) => prefetch(p, 0, 3, 1),
+                (1, 0) => prefetch(p, 1, 0, 1),
+                (1, 1) => prefetch(p, 1, 1, 1),
+                (1, 2) => prefetch(p, 1, 2, 1),
+                (1, 3) => prefetch(p, 1, 3, 1),
+                (_, _) => panic!(
+                    "Illegal (rw, locality) pair in prefetch, value ({}, {}).",
+                    $rdwr, $local
+                ),
+            }
+        };
+    }
+    pref!(rw, locality);
+}

library/stdarch/crates/core_arch/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(
     const_fn,
     const_fn_union,
+    const_fn_transmute,
     const_generics,
     custom_inner_attributes,
     link_llvm_intrinsics,