add riscv64gc-unknown-openbsd support (target riscv64-unknown-openbsd on OpenBSD)

- add platform-support documentation
- add riscv64gc-unknown-openbsd spec
- do not try to link with -latomic on openbsd

compiler/rustc_llvm/build.rs

@@ -342,10 +342,10 @@ fn main() {
     };
 
     // RISC-V GCC erroneously requires libatomic for sub-word
-    // atomic operations. FreeBSD uses Clang as its system
+    // atomic operations. Some BSD uses Clang as its system
     // compiler and provides no libatomic in its base system so
     // does not want this.
-    if !target.contains("freebsd") && target.starts_with("riscv") {
+    if target.starts_with("riscv") && !target.contains("freebsd") && !target.contains("openbsd") {
         println!("cargo:rustc-link-lib=atomic");
     }
 

compiler/rustc_target/src/spec/mod.rs

@@ -895,6 +895,7 @@ supported_targets! {
     ("i686-unknown-openbsd", i686_unknown_openbsd),
     ("powerpc-unknown-openbsd", powerpc_unknown_openbsd),
     ("powerpc64-unknown-openbsd", powerpc64_unknown_openbsd),
+    ("riscv64gc-unknown-openbsd", riscv64gc_unknown_openbsd),
     ("sparc64-unknown-openbsd", sparc64_unknown_openbsd),
     ("x86_64-unknown-openbsd", x86_64_unknown_openbsd),
 

compiler/rustc_target/src/spec/riscv64gc_unknown_openbsd.rs

@@ -0,0 +1,18 @@
+use crate::spec::{CodeModel, Target, TargetOptions};
+
+pub fn target() -> Target {
+    Target {
+        llvm_target: "riscv64-unknown-openbsd".into(),
+        pointer_width: 64,
+        data_layout: "e-m:e-p:64:64-i64:64-i128:128-n64-S128".into(),
+        arch: "riscv64".into(),
+        options: TargetOptions {
+            code_model: Some(CodeModel::Medium),
+            cpu: "generic-rv64".into(),
+            features: "+m,+a,+f,+d,+c".into(),
+            llvm_abiname: "lp64d".into(),
+            max_atomic_width: Some(64),
+            ..super::openbsd_base::opts()
+        },
+    }
+}

src/bootstrap/native.rs

@@ -432,12 +432,13 @@ impl Step for Llvm {
             cfg.define("LLVM_LINK_LLVM_DYLIB", "ON");
         }
 
-        if target.starts_with("riscv") && !target.contains("freebsd") {
+        if target.starts_with("riscv") && !target.contains("freebsd") && !target.contains("openbsd")
+        {
             // RISC-V GCC erroneously requires linking against
             // `libatomic` when using 1-byte and 2-byte C++
             // atomics but the LLVM build system check cannot
             // detect this. Therefore it is set manually here.
-            // FreeBSD uses Clang as its system compiler and
+            // Some BSD uses Clang as its system compiler and
             // provides no libatomic in its base system so does
             // not want this.
             ldflags.exe.push(" -latomic");

src/doc/rustc/src/platform-support.md

@@ -285,6 +285,7 @@ target | std | host | notes
 `riscv32imc-esp-espidf` | ✓ |  | RISC-V ESP-IDF
 `riscv64gc-unknown-freebsd` |   |   | RISC-V FreeBSD
 `riscv64gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 4.20, musl 1.2.0)
+[`riscv64gc-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | OpenBSD/riscv64
 `s390x-unknown-linux-musl` |  |  | S390x Linux (kernel 3.2, MUSL)
 `sparc-unknown-linux-gnu` | ✓ |  | 32-bit SPARC Linux
 `sparc64-unknown-netbsd` | ✓ | ✓ | NetBSD/sparc64

src/doc/rustc/src/platform-support/openbsd.md

@@ -13,6 +13,7 @@ The target names follow this format: `$ARCH-unknown-openbsd`, where `$ARCH` spec
 | `aarch64-unknown-openbsd`      | libc++      | [64-bit ARM systems](https://www.openbsd.org/arm64.html)  |
 | `i686-unknown-openbsd`         | libc++      | [Standard PC and clones based on the Intel i386 architecture and compatible processors](https://www.openbsd.org/i386.html) |
 | `powerpc64-unknown-openbsd`    | libc++      | [IBM POWER-based PowerNV systems](https://www.openbsd.org/powerpc64.html) |
+| `riscv64gc-unknown-openbsd`    | libc++      | [64-bit RISC-V systems](https://www.openbsd.org/riscv64.html) |
 | `sparc64-unknown-openbsd`      | estdc++     | [Sun UltraSPARC and Fujitsu SPARC64 systems](https://www.openbsd.org/sparc64.html) |
 | `x86_64-unknown-openbsd`       | libc++      | [AMD64-based systems](https://www.openbsd.org/amd64.html) |