Add armv4t-none-eabi take2

compiler/rustc_target/src/spec/armv4t_none_eabi.rs

@@ -0,0 +1,54 @@
+//! Targets the ARMv4T, with code as `a32` code by default.
+//!
+//! Primarily of use for the GBA, but usable with other devices too.
+//!
+//! Please ping @Lokathor if changes are needed.
+//!
+//! This target profile assumes that you have the ARM binutils in your path
+//! (specifically the linker, `arm-none-eabi-ld`). They can be obtained for free
+//! for all major OSes from the ARM developer's website, and they may also be
+//! available in your system's package manager. Unfortunately, the standard
+//! linker that Rust uses (`lld`) only supports as far back as `ARMv5TE`, so we
+//! must use the GNU `ld` linker.
+//!
+//! **Important:** This target profile **does not** specify a linker script. You
+//! just get the default link script when you build a binary for this target.
+//! The default link script is very likely wrong, so you should use
+//! `-Clink-arg=-Tmy_script.ld` to override that with a correct linker script.
+
+use crate::spec::{cvs, LinkerFlavor, PanicStrategy, RelocModel, Target, TargetOptions};
+
+pub fn target() -> Target {
+    Target {
+        llvm_target: "armv4t-none-eabi".into(),
+        pointer_width: 32,
+        arch: "arm".into(),
+        /* Data layout args are '-' separated:
+         * little endian
+         * stack is 64-bit aligned (EABI)
+         * pointers are 32-bit
+         * i64 must be 64-bit aligned (EABI)
+         * mangle names with ELF style
+         * native integers are 32-bit
+         * All other elements are default
+         */
+        data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".into(),
+        options: TargetOptions {
+            abi: "eabi".into(),
+            linker_flavor: LinkerFlavor::Ld,
+            linker: Some("arm-none-eabi-ld".into()),
+            asm_args: cvs!["-mthumb-interwork", "-march=armv4t", "-mlittle-endian",],
+            features: "+soft-float,+strict-align".into(),
+            main_needs_argc_argv: false,
+            atomic_cas: false,
+            has_thumb_interworking: true,
+            relocation_model: RelocModel::Static,
+            panic_strategy: PanicStrategy::Abort,
+            // from thumb_base, rust-lang/rust#44993.
+            emit_debug_gdb_scripts: false,
+            // from thumb_base, apparently gcc/clang give enums a minimum of 8 bits on no-os targets
+            c_enum_min_bits: 8,
+            ..Default::default()
+        },
+    }
+}

src/doc/rustc/src/SUMMARY.md

@@ -18,6 +18,7 @@
     - [aarch64-apple-ios-sim](platform-support/aarch64-apple-ios-sim.md)
     - [\*-apple-watchos\*](platform-support/apple-watchos.md)
     - [aarch64-nintendo-switch-freestanding](platform-support/aarch64-nintendo-switch-freestanding.md)
+    - [armv4t-none-eabi](platform-support/armv4t-none-eabi.md)
     - [armv6k-nintendo-3ds](platform-support/armv6k-nintendo-3ds.md)
     - [armv7-unknown-linux-uclibceabi](platform-support/armv7-unknown-linux-uclibceabi.md)
     - [armv7-unknown-linux-uclibceabihf](platform-support/armv7-unknown-linux-uclibceabihf.md)

src/doc/rustc/src/platform-support/armv4t_none_eabi.md

@@ -0,0 +1,70 @@
+# armv4t-none-eabi
+
+Tier 3
+
+Bare-metal target for any cpu in the ARMv4T architecture family, supporting
+ARM/Thumb code interworking (aka `a32`/`t32`), with ARM code as the default code
+generation.
+
+In particular this supports the Gameboy Advance (GBA), but there's nothing GBA
+specific with this target, so any ARMv4T device should work fine.
+
+## Target Maintainers
+
+* [@Lokathor](https://github.com/lokathor)
+
+## Requirements
+
+The target is cross-compiled, and uses static linking.
+
+The linker that comes with rustc cannot link for this platform (the platform is
+too old). You will need the `arm-none-eabi-ld` linker from a GNU Binutils
+targeting ARM. This can be obtained for Windows/Mac/Linux from the [ARM
+Developer Website][arm-dev], or possibly from your OS's package manager.
+
+[arm-dev]: https://developer.arm.com/Tools%20and%20Software/GNU%20Toolchain
+
+This target doesn't provide a linker script, you'll need to bring your own
+according to the specific device you want to target. Pass
+`-Clink-arg=-Tyour_script.ld` as a rustc argument to make the linker use
+`your_script.ld` during linking.
+
+## Building Rust Programs
+
+Because it is Tier 3, rust does not yet ship pre-compiled artifacts for this target.
+
+Just use the `build-std` nightly cargo feature to build the `core` library. You
+can pass this as a command line argument to cargo, or your `.cargo/config.toml`
+file might include the following lines:
+
+```toml
+[unstable]
+build-std = ["core"]
+```
+
+Most of `core` should work as expected, with the following notes:
+* the target is "soft float", so `f32` and `f64` operations are emulated in
+  software.
+* integer division is also emulated in software.
+* the target is old enough that it doesn't have atomic instructions.
+
+Rust programs are output as ELF files.
+
+For running on hardware, you'll generally need to extract the "raw" program code
+out of the ELF and into a file of its own. The `objcopy` program provided as
+part of the GNU Binutils can do this:
+
+```shell
+arm-none-eabi-objcopy --output-target binary [in_file] [out_file]
+```
+
+## Testing
+
+This is a cross-compiled target that you will need to emulate during testing.
+
+Because this is a device-agnostic target, and the exact emulator that you'll
+need depends on the specific device you want to run your code on.
+
+For example, when programming for the Gameboy Advance, the
+[mgba-test-runner](https://github.com/agbrs/agb) program could be used to make a
+normal set of rust tests be run within the `mgba` emulator.