Add a x86_64::cmpxchg16b intrinsic

This intrinsic isn't actually specified by Intel, but it's something
gated with CPUID and can otherwise be a useful thing to have when
building primitives!

There exists an `AtomicU128` type in the standard library but it's only
exposed currently (and it's unstable) when a platform fully supports
128-bit atomics. The x86_64 architecture does not support it *unless*
the `cmpxchg16b` instruction is available, and it isn't always available!

This commit is also a proposal for how we can include support for
128-bit atomics in the standard library on relevant platforms. I'm
thinking that we'll expose this one low-level intrinsic in
`std::arch::x86_64`, and then if desired a crate on crates.io can build
`AtomicU128` from this API.

In any case this is all unstable regardless!

coresimd/x86_64/cmpxchg16b.rs

@@ -0,0 +1,68 @@
+use sync::atomic::Ordering;
+
+#[cfg(test)]
+use stdsimd_test::assert_instr;
+
+/// Compare and exchange 16 bytes (128 bits) of data atomically.
+///
+/// This intrinsic corresponds to the `cmpxchg16b` instruction on x86_64
+/// processors. It performs an atomic compare-and-swap, updating the `ptr`
+/// memory location to `val` if the current value in memory equals `old`.
+///
+/// # Return value
+///
+/// This function returns the previous value at the memory location. If it is
+/// equal to `old` then the memory was updated to `new`.
+///
+/// # Memory Orderings
+///
+/// This atomic operations has the same semantics of memory orderings as
+/// `AtomicUsize::compare_exchange` does, only operating on 16 bytes of memory
+/// instead of just a pointer.
+///
+/// For more information on memory orderings here see the `compare_exchange`
+/// documentation for other `Atomic*` types in the standard library.
+///
+/// # Unsafety
+///
+/// This method is unsafe because it takes a raw pointer and will attempt to
+/// read and possibly write the memory at the pointer. The pointer must also be
+/// aligned on a 16-byte boundary.
+///
+/// This method also requires the `cmpxchg16b` CPU feature to be available at
+/// runtime to work correctly. If the CPU running the binary does not actually
+/// support `cmpxchg16b` and the program enters an execution path that
+/// eventually would reach this function the behavior is undefined.
+#[inline]
+#[cfg_attr(test, assert_instr(cmpxchg16b, success = Ordering::SeqCst, failure = Ordering::SeqCst))]
+#[target_feature(enable = "cmpxchg16b")]
+pub unsafe fn cmpxchg16b(
+    dst: *mut u128,
+    old: u128,
+    new: u128,
+    success: Ordering,
+    failure: Ordering,
+) -> u128 {
+    use intrinsics;
+    use sync::atomic::Ordering::*;
+
+    debug_assert!(dst as usize % 16 == 0);
+
+    let (val, _ok) = match (success, failure) {
+        (Acquire, Acquire) => intrinsics::atomic_cxchg_acq(dst, old, new),
+        (Release, Relaxed) => intrinsics::atomic_cxchg_rel(dst, old, new),
+        (AcqRel, Acquire) => intrinsics::atomic_cxchg_acqrel(dst, old, new),
+        (Relaxed, Relaxed) => intrinsics::atomic_cxchg_relaxed(dst, old, new),
+        (SeqCst, SeqCst) => intrinsics::atomic_cxchg(dst, old, new),
+        (Acquire, Relaxed) => intrinsics::atomic_cxchg_acq_failrelaxed(dst, old, new),
+        (AcqRel, Relaxed) => intrinsics::atomic_cxchg_acqrel_failrelaxed(dst, old, new),
+        (SeqCst, Relaxed) => intrinsics::atomic_cxchg_failrelaxed(dst, old, new),
+        (SeqCst, Acquire) => intrinsics::atomic_cxchg_failacq(dst, old, new),
+
+        // The above block is all copied from libcore, and this statement is
+        // also copied from libcore except that it's a panic in libcore and we
+        // have a little bit more of a lightweight panic here.
+        _ => ::coresimd::x86::ud2(),
+    };
+    val
+}

coresimd/x86_64/mod.rs

@@ -38,3 +38,6 @@ pub use self::bswap::*;
 
 mod rdrand;
 pub use self::rdrand::*;
+
+mod cmpxchg16b;
+pub use self::cmpxchg16b::*;

crates/coresimd/src/lib.rs

@@ -33,6 +33,7 @@
     sse4a_target_feature,
     arm_target_feature,
     aarch64_target_feature,
+    cmpxchg16b_target_feature,
     avx512_target_feature,
     mips_target_feature,
     powerpc_target_feature,
@@ -67,6 +68,7 @@
     test(attr(allow(dead_code, deprecated, unused_variables, unused_mut)))
 )]
 
+#[macro_use]
 extern crate core as _core;
 #[cfg(test)]
 #[macro_use]
@@ -122,6 +124,8 @@ use _core::result;
 #[allow(unused_imports)]
 use _core::slice;
 #[allow(unused_imports)]
+use _core::sync;
+#[allow(unused_imports)]
 use _core::u128;
 #[allow(unused_imports)]
 use _core::u8;

crates/stdsimd-test/src/disassembly.rs

@@ -101,7 +101,9 @@ fn parse_objdump(output: &str) -> HashMap<String, Vec<Function>> {
                 .skip_while(|s| {
                     s.len() == expected_len
                         && usize::from_str_radix(s, 16).is_ok()
-                }).map(|s| s.to_string())
+                })
+                .skip_while(|s| *s == "lock") // skip x86-specific prefix
+                .map(|s| s.to_string())
                 .collect::<Vec<String>>();
             instructions.push(Instruction { parts });
         }
@@ -198,6 +200,7 @@ fn parse_dumpbin(output: &str) -> HashMap<String, Vec<Function>> {
                 .skip_while(|s| {
                     s.len() == 2 && usize::from_str_radix(s, 16).is_ok()
                 }).map(|s| s.to_string())
+                .skip_while(|s| *s == "lock") // skip x86-specific prefix
                 .collect::<Vec<String>>();
             instructions.push(Instruction { parts });
         }

crates/stdsimd-verify/src/lib.rs

@@ -124,7 +124,9 @@ fn to_type(t: &syn::Type) -> proc_macro2::TokenStream {
             "u16" => quote! { &U16 },
             "u32" => quote! { &U32 },
             "u64" => quote! { &U64 },
+            "u128" => quote! { &U128 },
             "u8" => quote! { &U8 },
+            "Ordering" => quote! { &ORDERING },
             "CpuidResult" => quote! { &CPUID },
 
             // arm ...

crates/stdsimd-verify/tests/x86-intel.rs

@@ -37,7 +37,9 @@ static I8: Type = Type::PrimSigned(8);
 static U16: Type = Type::PrimUnsigned(16);
 static U32: Type = Type::PrimUnsigned(32);
 static U64: Type = Type::PrimUnsigned(64);
+static U128: Type = Type::PrimUnsigned(128);
 static U8: Type = Type::PrimUnsigned(8);
+static ORDERING: Type = Type::Ordering;
 
 static M64: Type = Type::M64;
 static M128: Type = Type::M128;
@@ -75,6 +77,7 @@ enum Type {
     Tuple,
     CpuidResult,
     Never,
+    Ordering,
 }
 
 stdsimd_verify::x86_functions!(static FUNCTIONS);
@@ -145,6 +148,8 @@ fn verify_all_signatures() {
             "__cpuid_count" |
             "__cpuid" |
             "__get_cpuid_max" |
+            // Not listed with intel, but manually verified
+            "cmpxchg16b" |
             // The UD2 intrinsic is not defined by Intel, but it was agreed on
             // in the RFC Issue 2512:
             // https://github.com/rust-lang/rfcs/issues/2512

stdsimd/arch/detect/arch/x86.rs

@@ -226,6 +226,10 @@ macro_rules! is_x86_feature_detected {
         cfg!(target_feature = "xsavec") || $crate::arch::detect::check_for(
             $crate::arch::detect::Feature::xsavec)
     };
+    ("cmpxchg16b") => {
+        cfg!(target_feature = "cmpxchg16b") || $crate::arch::detect::check_for(
+            $crate::arch::detect::Feature::cmpxchg16b)
+    };
     ($t:tt) => {
         compile_error!(concat!("unknown target feature: ", $t))
     };
@@ -316,4 +320,6 @@ pub enum Feature {
     xsaves,
     /// XSAVEC (Save Processor Extended States Compacted)
     xsavec,
+    /// CMPXCH16B, a 16-byte compare-and-swap instruction
+    cmpxchg16b,
 }

stdsimd/arch/detect/os/x86.rs

@@ -116,6 +116,7 @@ fn detect_features() -> cache::Initializer {
 
         enable(proc_info_ecx, 0, Feature::sse3);
         enable(proc_info_ecx, 9, Feature::ssse3);
+        enable(proc_info_ecx, 13, Feature::cmpxchg16b);
         enable(proc_info_ecx, 19, Feature::sse4_1);
         enable(proc_info_ecx, 20, Feature::sse4_2);
         enable(proc_info_ecx, 23, Feature::popcnt);
@@ -288,6 +289,7 @@ mod tests {
         println!("xsaveopt: {:?}", is_x86_feature_detected!("xsaveopt"));
         println!("xsaves: {:?}", is_x86_feature_detected!("xsaves"));
         println!("xsavec: {:?}", is_x86_feature_detected!("xsavec"));
+        println!("cmpxchg16b: {:?}", is_x86_feature_detected!("cmpxchg16b"));
     }
 
     #[test]
@@ -344,5 +346,9 @@ mod tests {
             is_x86_feature_detected!("xsaves"),
             information.xsaves_xrstors_and_ia32_xss()
         );
+        assert_eq!(
+            is_x86_feature_detected!("cmpxchg16b"),
+            information.cmpxchg16b(),
+        );
     }
 }