draft: Add support for hexagon-unknown-none-elf as target

Still TODO: document usage details for new target

compiler/rustc_target/src/spec/hexagon_unknown_none_elf.rs

@@ -0,0 +1,31 @@
+use crate::spec::{Cc, LinkerFlavor, Lld, PanicStrategy, RelocModel, Target, TargetOptions};
+
+pub fn target() -> Target {
+    Target {
+        llvm_target: "hexagon-unknown-none-elf".into(),
+        pointer_width: 32,
+        data_layout: concat!(
+            "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32",
+            ":32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32",
+            ":32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048",
+            ":2048:2048"
+        )
+        .into(),
+        arch: "hexagon".into(),
+
+        options: TargetOptions {
+            cpu: "hexagonv60".into(),
+            linker: Some("clang".into()),
+            linker_flavor: LinkerFlavor::Gnu(Cc::Yes, Lld::No),
+            relocation_model: RelocModel::Pic,
+            position_independent_executables: true,
+            panic_strategy: PanicStrategy::Abort,
+            features: "-small-data,+hvx-length128b".into(),
+            has_thread_local: true,
+            max_atomic_width: Some(32),
+            emit_debug_gdb_scripts: false,
+            c_enum_min_bits: Some(8),
+            ..Default::default()
+        },
+    }
+}

compiler/rustc_target/src/spec/mod.rs

@@ -1528,6 +1528,7 @@ supported_targets! {
     ("mips64-unknown-linux-muslabi64", mips64_unknown_linux_muslabi64),
     ("mips64el-unknown-linux-muslabi64", mips64el_unknown_linux_muslabi64),
     ("hexagon-unknown-linux-musl", hexagon_unknown_linux_musl),
+    ("hexagon-unknown-none-elf", hexagon_unknown_none_elf),
 
     ("mips-unknown-linux-uclibc", mips_unknown_linux_uclibc),
     ("mipsel-unknown-linux-uclibc", mipsel_unknown_linux_uclibc),

src/doc/rustc/src/SUMMARY.md

@@ -34,6 +34,7 @@
     - [\*-unknown-fuchsia](platform-support/fuchsia.md)
     - [\*-kmc-solid_\*](platform-support/kmc-solid.md)
     - [csky-unknown-linux-gnuabiv2\*](platform-support/csky-unknown-linux-gnuabiv2.md)
+    - [hexagon-unknown-none-elf](platform-support/hexagon-unknown-none-elf.md)
     - [loongarch\*-unknown-linux-\*](platform-support/loongarch-linux.md)
     - [loongarch\*-unknown-none\*](platform-support/loongarch-none.md)
     - [m68k-unknown-linux-gnu](platform-support/m68k-unknown-linux-gnu.md)

src/doc/rustc/src/platform-support/hexagon-unknown-none-elf.md

@@ -0,0 +1,83 @@
+# `hexagon-unknown-none-elf`
+
+**Tier: 3**
+
+Rust for baremetal Hexagon DSPs.
+
+| Target | Descriptions |
+| ------------------------ | ----------------------------------------- |
+| hexagon-unknown-none-elf | Hexagon v60 or later, HVX  |
+
+## Target maintainers
+
+- Brian Cain, `bcain@quicinc.com`, https://github.com/androm3da
+
+## Requirements
+
+This target is cross-compiled.  There is no support for `std`. There is no
+default allocator, but it's possible to use `alloc` by supplying an allocator.
+
+By default, code generated with this target should run on Hexagon DSP
+hardware, v60 or later.
+
+- `-Ctarget-cpu=hexagonv73` adds support for instructions defined up to Hexagon V73.
+
+Functions marked `extern "C"` use the
+[Hexagon architecture calling convention](https://lists.llvm.org/pipermail/llvm-dev/attachments/20190916/21516a52/attachment-0001.pdf).
+
+This target generates PIC ELF binaries.
+
+## Building the target
+
+You can build Rust with support for the target by adding it to the `target`
+list in `config.toml`:
+
+```toml
+[build]
+build-stage = 1
+host = ["<target for your host>"]
+target = ["<target for your host>", "hexagon-unknown-none-elf"]
+
+[target.hexagon-unknown-none-elf]
+
+cc = "hexagon-unknown-none-elf-clang"
+cxx = "hexagon-unknown-none-elf-clang++"
+linker = "hexagon-unknown-none-elf-clang"
+llvm-libunwind = 'in-tree'
+```
+
+Replace `<target for your host>` with `x86_64-unknown-linux-gnu` or whatever
+else is appropriate for your host machine.
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will either need to build Rust with the target enabled (see
+"Building the target" above), or build your own copy of `core` by using
+`build-std` or similar.
+
+## Testing
+
+Since `hexagon-unknown-none-elf` supports a variety of different environments
+and does not support `std`, this target does not support running the Rust
+test suite.
+
+## Cross-compilation toolchains and C code
+
+This target has been tested using `hexagon-sim` from the Hexagon SDK.
+
+`.cargo/config.toml`:
+```toml
+[target.hexagon-unknown-none-elf]
+
+cc = "hexagon-unknown-none-elf-clang"
+cxx = "hexagon-unknown-none-elf-clang++"
+linker = "hexagon-unknown-none-elf-clang"
+runner = "hexagon-sim FIXME TBD details"
+
+[build]
+target = ["hexagon-unknown-none-elf"]
+rustflags = "-Ctarget-cpu=hexagonv73"
+```
+
+FIXME TBD details