nvptx: fix _syncthreads to use unaligned barrier

* Deprecate _syncthreads (the CUDA name) in favor of new
_barrier_sync (NVPTX name barrier.sync).

* The: barrier.sync instruction is equivalent to barrier.sync.aligned
prior to sm_70, and will lead to errors/deadlock if passes (such as MIR
JumpThreading) lose the aligned property.
rust-lang/rust#137086
https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#parallel-synchronization-and-communication-instructions-bar

* Since: MIR does not currently have a way to apply something like
LLVM's convergent attribute (and because convergent does not preserve
alignment, which can be broken by inlining), we cannot prevent loss of
alignment, and thus we require target feature sm_70.

https://llvm.org/docs/ConvergentOperations.html

WIP: compile-time dependency on ptx60

Author: jedbrown

crates/core_arch/src/nvptx/mod.rs

@@ -20,8 +20,8 @@ pub use packed::*;
 
 #[allow(improper_ctypes)]
 unsafe extern "C" {
-    #[link_name = "llvm.nvvm.barrier0"]
-    fn syncthreads() -> ();
+    #[link_name = "llvm.nvvm.barrier.sync"]
+    fn barrier_sync(_: u32) -> ();
     #[link_name = "llvm.nvvm.read.ptx.sreg.ntid.x"]
     fn block_dim_x() -> i32;
     #[link_name = "llvm.nvvm.read.ptx.sreg.ntid.y"]
@@ -49,10 +49,52 @@ unsafe extern "C" {
 }
 
 /// Synchronizes all threads in the block.
+///
+/// The argument `a` is a logical barrier resource with value `0` through `15`.
+///
+/// This does not require textual alignment, so the following code is valid.
+///
+/// ```
+/// if tid % 2 == 0 {
+///     shared[tid] *= 2;
+///     _barrier_sync(0);
+///     myval += shared[tid + 1];
+/// } else {
+///     shared[tid] *= 4;
+///     _barrier_sync(0);
+/// }
+/// ```
+///
+/// This intrinsic has different execution semantics prior to `sm_70`, and thus
+/// it requires the `sm_70` target feature for correct behavior. The instruction
+/// was introduced in PTX 6.0, so its use has a compile-time dependency on the
+/// `ptx60` target feature.
+///
+/// TODO: The more restrictive "aligned" semantics of
+/// `llvm.nvvm.barrier.sync.aligned` are [currently
+/// miscompiled](https://github.com/rust-lang/rust/issues/137086) due to MIR
+/// JumpThreading and lack of `convergent` attribute propagated to LLVM. Once
+/// resolved, a `_barrier_sync_aligned` intrinsic can be exposed at all target
+/// features.
+///
+#[inline]
+#[cfg(target_feature = "ptx60")]
+#[target_feature(enable = "sm_70", enable = "ptx60")]
+#[unstable(feature = "stdarch_nvptx", issue = "111199")]
+pub unsafe fn _barrier_sync(a: u32) -> () {
+    barrier_sync(a)
+}
+
+/// Synchronizes all threads in the block.
+///
+/// Deprecated alias for [`_barrier_sync`].
 #[inline]
+#[cfg(target_feature = "ptx60")]
+#[target_feature(enable = "sm_70", enable = "ptx60")]
 #[unstable(feature = "stdarch_nvptx", issue = "111199")]
+#[deprecated(since = "1.88.0", note = "use _barrier_sync(0)")]
 pub unsafe fn _syncthreads() -> () {
-    syncthreads()
+    _barrier_sync(0)
 }
 
 /// x-th thread-block dimension.