Auto merge of #135085 - knickish:m68k_unknown_none, r=workingjubilee

add m68k-unknown-none-elf target

r? `@workingjubilee`

The existing `m68k-unknown-linux-gnu` target builds `std` by default, requires atomics, and has a base cpu with an fpu. A smaller/more embedded target is desirable both to have a baseline target for the ISA, as well to make debugging easier for working on the llvm backend. Currently this target is using the `M68010` as the minimum CPU due, but as missing features are merged into the `M68k` llvm backend I am hoping to lower this further.

I have been able to build very small crates using a toolchain built against this target (together with a later version of `object`) using the configuration described in the target platform-support documentation, although getting anything of substantial complexity to build quickly hits errors in the llvm backend

compiler/rustc_target/src/spec/mod.rs

@@ -1656,6 +1656,7 @@ supported_targets! {
     ("loongarch64-unknown-linux-gnu", loongarch64_unknown_linux_gnu),
     ("loongarch64-unknown-linux-musl", loongarch64_unknown_linux_musl),
     ("m68k-unknown-linux-gnu", m68k_unknown_linux_gnu),
+    ("m68k-unknown-none-elf", m68k_unknown_none_elf),
     ("csky-unknown-linux-gnuabiv2", csky_unknown_linux_gnuabiv2),
     ("csky-unknown-linux-gnuabiv2hf", csky_unknown_linux_gnuabiv2hf),
     ("mips-unknown-linux-gnu", mips_unknown_linux_gnu),

compiler/rustc_target/src/spec/targets/m68k_unknown_none_elf.rs

@@ -0,0 +1,33 @@
+use crate::abi::Endian;
+use crate::spec::{CodeModel, PanicStrategy, RelocModel, Target, TargetOptions};
+
+pub(crate) fn target() -> Target {
+    let options = TargetOptions {
+        cpu: "M68010".into(),
+        max_atomic_width: None,
+        endian: Endian::Big,
+        // LLD currently does not have support for M68k
+        linker: Some("m68k-linux-gnu-ld".into()),
+        panic_strategy: PanicStrategy::Abort,
+        code_model: Some(CodeModel::Medium),
+        has_rpath: false,
+        // should be soft-float
+        llvm_floatabi: None,
+        relocation_model: RelocModel::Static,
+        ..Default::default()
+    };
+
+    Target {
+        llvm_target: "m68k".into(),
+        metadata: crate::spec::TargetMetadata {
+            description: Some("Motorola 680x0".into()),
+            tier: Some(3),
+            host_tools: Some(false),
+            std: Some(false),
+        },
+        pointer_width: 32,
+        data_layout: "E-m:e-p:32:16:32-i8:8:8-i16:16:16-i32:16:32-n8:16:32-a:0:16-S16".into(),
+        arch: "m68k".into(),
+        options,
+    }
+}

src/doc/rustc/src/SUMMARY.md

@@ -60,6 +60,7 @@
     - [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)
+    - [m68k-unknown-none-elf](platform-support/m68k-unknown-none-elf.md)
     - [mips64-openwrt-linux-musl](platform-support/mips64-openwrt-linux-musl.md)
     - [mipsel-sony-psx](platform-support/mipsel-sony-psx.md)
     - [mips\*-mti-none-elf](platform-support/mips-mti-none-elf.md)

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

@@ -317,6 +317,7 @@ target | std | host | notes
 [`i686-wrs-vxworks`](platform-support/vxworks.md) | ✓ |  | [^x86_32-floats-return-ABI]
 [`loongarch64-unknown-linux-ohos`](platform-support/openharmony.md) | ✓ |   | LoongArch64 OpenHarmony
 [`m68k-unknown-linux-gnu`](platform-support/m68k-unknown-linux-gnu.md) | ? |  | Motorola 680x0 Linux
+[`m68k-unknown-none-elf`](platform-support/m68k-unknown-none-elf.md) |  |  | Motorola 680x0
 `mips-unknown-linux-gnu` | ✓ | ✓ | MIPS Linux (kernel 4.4, glibc 2.23)
 `mips-unknown-linux-musl` | ✓ |  | MIPS Linux with musl 1.2.3
 `mips-unknown-linux-uclibc` | ✓ |  | MIPS Linux with uClibc

src/doc/rustc/src/platform-support/m68k-unknown-none-elf.md

@@ -0,0 +1,108 @@
+# m68k-unknown-none-elf
+
+**Tier: 3**
+
+Bare metal Motorola 680x0
+
+## Designated Developers
+
+* [@knickish](https://github.com/knickish)
+
+## Requirements
+
+This target requires an m68k build environment for cross-compilation which
+is available on Debian, Debian-based systems, openSUSE, and other distributions.
+
+On Debian-based systems, it should be sufficient to install a g++ cross-compiler for the m68k
+architecture which will automatically pull in additional dependencies such as
+the glibc cross development package:
+
+```sh
+apt install g++-m68k-linux-gnu
+```
+
+Binaries can be run using QEMU user emulation. On Debian-based systems, it should be
+sufficient to install the package `qemu-user-static` to be able to run simple static
+binaries:
+
+```text
+# apt install qemu-user-static
+```
+
+To run more complex programs, it will be necessary to set up a Debian/m68k chroot with
+the help of the command `debootstrap`:
+
+```text
+# apt install debootstrap debian-ports-archive-keyring
+# debootstrap --keyring=/usr/share/keyrings/debian-ports-archive-keyring.gpg --arch=m68k unstable debian-68k http://ftp.ports.debian.org/debian-ports
+```
+
+This chroot can then seamlessly entered using the normal `chroot` command thanks to
+QEMU user emulation:
+
+```text
+# chroot /path/to/debian-68k
+```
+
+To get started with native builds, which are currently untested, a native Debian/m68k
+system can be installed either on real hardware such as 68k-based Commodore Amiga or
+Atari systems or emulated environments such as QEMU version 4.2 or newer or ARAnyM.
+
+ISO images for installation are provided by the Debian Ports team and can be obtained
+from the Debian CD image server available at:
+
+[https://cdimage.debian.org/cdimage/ports/current](https://cdimage.debian.org/cdimage/ports/current/)
+
+Documentation for Debian/m68k is available on the Debian Wiki at:
+
+[https://wiki.debian.org/M68k](https://wiki.debian.org/M68k)
+
+Support is available either through the `debian-68k` mailing list:
+
+[https://lists.debian.org/debian-68k/](https://lists.debian.org/debian-68k/)
+
+or the `#debian-68k` IRC channel on OFTC network.
+
+## Building
+
+At least llvm version `19.1.5` is required to build `core` and `alloc` for this target, and currently the gnu linker is required, as `lld` has no support for the `m68k` architecture
+
+## Cross-compilation
+
+This target can be cross-compiled from a standard Debian or Debian-based, openSUSE or any
+other distribution which has a basic m68k cross-toolchain available.
+
+## Testing
+
+Currently there is no support to run the rustc test suite for this target.
+
+## Building Rust programs
+
+Recommended `.cargo/config.toml`:
+```toml
+[unstable]
+build-std = ["panic_abort", "core", "alloc"]
+
+[target.m68k-unknown-none-elf]
+# as we're building for ELF, the m68k-linux linker should be adequate
+linker = "m68k-linux-gnu-ld"
+
+# the mold linker also supports m68k, remove the above line and uncomment the
+# following ones to use that instead
+# linker = "clang"
+# rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold/binary"]
+```
+
+Rust programs can be built for this target using:
+
+```sh
+cargo build --target m68k-unknown-none-elf
+```
+
+Very simple programs can be run using the `qemu-m68k-static` program:
+
+```sh
+qemu-m68k-static your-code
+```
+
+For more complex applications, a chroot or native m68k system is required for testing.

src/tools/build-manifest/src/main.rs

@@ -114,6 +114,7 @@ static TARGETS: &[&str] = &[
     "loongarch64-unknown-none",
     "loongarch64-unknown-none-softfloat",
     "m68k-unknown-linux-gnu",
+    "m68k-unknown-none-elf",
     "csky-unknown-linux-gnuabiv2",
     "csky-unknown-linux-gnuabiv2hf",
     "mips-unknown-linux-gnu",

tests/assembly/targets/targets-elf.rs

@@ -261,6 +261,9 @@
 //@ revisions: m68k_unknown_linux_gnu
 //@ [m68k_unknown_linux_gnu] compile-flags: --target m68k-unknown-linux-gnu
 //@ [m68k_unknown_linux_gnu] needs-llvm-components: m68k
+//@ revisions: m68k_unknown_none_elf
+//@ [m68k_unknown_none_elf] compile-flags: --target m68k-unknown-none-elf
+//@ [m68k_unknown_none_elf] needs-llvm-components: m68k
 //@ revisions: mips64_openwrt_linux_musl
 //@ [mips64_openwrt_linux_musl] compile-flags: --target mips64-openwrt-linux-musl
 //@ [mips64_openwrt_linux_musl] needs-llvm-components: mips