Create a generic AVR target: avr-unknown-unknown

This commit removes the `avr-unknown-gnu-atmega328` target and replaces
it with a more generic `avr-unknown-unknown` variant that can be
specialized using `-C target-cpu` (e.g. `-C target-cpu=atmega328p`).

I've decided to go with the `-unknown` tag (i.e. `avr-unknown-unknown`
instead of `avr-unknown-gnu`), because that's the name LLVM already uses
- I see no reason to diverge here.

Seizing the day, I'm adding myself as the maintainer of this target -
I've been already fixing the bugs anyway, might as well make it
official.

Related discussion:
#131171

Author: Patryk27

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1920,10 +1920,20 @@ fn add_post_link_objects(
 
 /// Add arbitrary "pre-link" args defined by the target spec or from command line.
 /// FIXME: Determine where exactly these args need to be inserted.
-fn add_pre_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavor) {
+fn add_pre_link_args(
+    cmd: &mut dyn Linker,
+    sess: &Session,
+    flavor: LinkerFlavor,
+    codegen_results: &CodegenResults,
+) {
     if let Some(args) = sess.target.pre_link_args.get(&flavor) {
         cmd.verbatim_args(args.iter().map(Deref::deref));
     }
+
+    if sess.target.arch == "avr" {
+        cmd.verbatim_arg(format!("-mmcu={}", codegen_results.crate_info.target_cpu));
+    }
+
     cmd.verbatim_args(&sess.opts.unstable_opts.pre_link_args);
 }
 
@@ -2215,7 +2225,7 @@ fn linker_with_args(
     // FIXME: In practice built-in target specs use this for arbitrary order-independent options,
     // introduce a target spec option for order-independent linker options and migrate built-in
     // specs to it.
-    add_pre_link_args(cmd, sess, flavor);
+    add_pre_link_args(cmd, sess, flavor, codegen_results);
 
     // ------------ Object code and libraries, order-dependent ------------
 

compiler/rustc_target/src/spec/base/avr_gnu.rs renamed to compiler/rustc_target/src/spec/base/avr.rs

@@ -1,44 +1,5 @@
 use object::elf;
 
-use crate::spec::{Cc, LinkerFlavor, Lld, RelocModel, Target, TargetOptions};
-
-/// A base target for AVR devices using the GNU toolchain.
-///
-/// Requires GNU avr-gcc and avr-binutils on the host system.
-/// FIXME: Remove the second parameter when const string concatenation is possible.
-pub(crate) fn target(target_cpu: &'static str, mmcu: &'static str) -> Target {
-    Target {
-        arch: "avr".into(),
-        metadata: crate::spec::TargetMetadata {
-            description: None,
-            tier: None,
-            host_tools: None,
-            std: None,
-        },
-        data_layout: "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8".into(),
-        llvm_target: "avr-unknown-unknown".into(),
-        pointer_width: 16,
-        options: TargetOptions {
-            env: "gnu".into(),
-
-            c_int_width: "16".into(),
-            cpu: target_cpu.into(),
-            exe_suffix: ".elf".into(),
-
-            linker: Some("avr-gcc".into()),
-            eh_frame_header: false,
-            pre_link_args: TargetOptions::link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &[mmcu]),
-            late_link_args: TargetOptions::link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &[
-                "-lgcc",
-            ]),
-            max_atomic_width: Some(16),
-            atomic_cas: false,
-            relocation_model: RelocModel::Static,
-            ..TargetOptions::default()
-        },
-    }
-}
-
 /// Resolve the value of the EF_AVR_ARCH field for AVR ELF files, given the
 /// name of the target CPU / MCU.
 ///

compiler/rustc_target/src/spec/base/mod.rs

@@ -1,7 +1,7 @@
 pub(crate) mod aix;
 pub(crate) mod android;
 pub(crate) mod apple;
-pub(crate) mod avr_gnu;
+pub(crate) mod avr;
 pub(crate) mod bpf;
 pub(crate) mod dragonfly;
 pub(crate) mod freebsd;

compiler/rustc_target/src/spec/mod.rs

@@ -63,7 +63,7 @@ pub use base::apple::{
     deployment_target_for_target as current_apple_deployment_target,
     platform as current_apple_platform,
 };
-pub use base::avr_gnu::ef_avr_arch;
+pub use base::avr::ef_avr_arch;
 
 /// Linker is called through a C/C++ compiler.
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
@@ -1734,7 +1734,7 @@ supported_targets! {
     ("riscv64gc-unknown-fuchsia", riscv64gc_unknown_fuchsia),
     ("x86_64-unknown-fuchsia", x86_64_unknown_fuchsia),
 
-    ("avr-unknown-gnu-atmega328", avr_unknown_gnu_atmega328),
+    ("avr-unknown-unknown", avr_unknown_unknown),
 
     ("x86_64-unknown-l4re-uclibc", x86_64_unknown_l4re_uclibc),
 

compiler/rustc_target/src/spec/targets/avr_unknown_gnu_atmega328.rs

@@ -0,0 +1,31 @@
+use crate::spec::{Cc, LinkerFlavor, Lld, RelocModel, Target, TargetOptions};
+
+pub(crate) fn target() -> Target {
+    Target {
+        arch: "avr".into(),
+        metadata: crate::spec::TargetMetadata {
+            description: None,
+            tier: None,
+            host_tools: None,
+            std: None,
+        },
+        data_layout: "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8".into(),
+        llvm_target: "avr-unknown-unknown".into(),
+        pointer_width: 16,
+        options: TargetOptions {
+            c_int_width: "16".into(),
+            exe_suffix: ".elf".into(),
+            linker: Some("avr-gcc".into()),
+            eh_frame_header: false,
+            pre_link_args: TargetOptions::link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &[]),
+            late_link_args: TargetOptions::link_args(
+                LinkerFlavor::Gnu(Cc::Yes, Lld::No),
+                &["-lgcc"],
+            ),
+            max_atomic_width: Some(16),
+            atomic_cas: false,
+            relocation_model: RelocModel::Static,
+            ..TargetOptions::default()
+        },
+    }
+}

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

@@ -293,7 +293,7 @@ target | std | host | notes
 [`armv7k-apple-watchos`](platform-support/apple-watchos.md) | ✓ |  | Armv7-A Apple WatchOS
 [`armv7s-apple-ios`](platform-support/apple-ios.md) | ✓ |  | Armv7-A Apple-A6 Apple iOS
 [`armv8r-none-eabihf`](platform-support/armv8r-none-eabihf.md) | * |  | Bare Armv8-R, hardfloat
-`avr-unknown-gnu-atmega328` | * |  | AVR. Requires `-Z build-std=core`
+`avr-unknown-unknown` | * |  | AVR
 `bpfeb-unknown-none` | * |  | BPF (big endian)
 `bpfel-unknown-none` | * |  | BPF (little endian)
 `csky-unknown-linux-gnuabiv2` | ✓ |  | C-SKY abiv2 Linux (little endian)

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

@@ -0,0 +1,75 @@
+# `avr-unknown-unknown`
+
+Series of microcontrollers from Atmel: ATmega8, ATmega328p etc.
+
+**Tier: 3**
+
+## Target maintainers
+
+-[Patryk Wychowaniec](https://github.com/Patryk27) <[email protected]>
+
+## Requirements
+
+This target is only cross-compiled - x86_64 / aarch64 x Lixux / MacOS hosts are
+confirmed to work, but in principle any machine able to run rustc and avr-gcc
+should be good.
+
+Compiling for this target requires `avr-gcc`, because a couple of intrinsics
+(like 32-bit multiplication) rely on [`libgcc`](https://github.com/gcc-mirror/gcc/blob/3269a722b7a03613e9c4e2862bc5088c4a17cc11/libgcc/config/avr/lib1funcs.S)
+and can't be provided through `compiler-builtins` yet; this is a limitation that
+we hope to lift in the future.
+
+## Building the target
+
+Rust comes with AVR support enabled, you don't have to rebuild the compiler
+itself.
+
+## Building Rust programs
+
+Install `avr-gcc`:
+
+```console
+# Ubuntu:
+$ sudo apt-get install gcc-avr
+
+# Mac:
+$ brew tap osx-cross/avr && brew install avr-gcc
+
+# NixOS (takes a couple of minutes, since Hydra doesn't build it):
+$ nix shell nixpkgs#pkgsCross.avr.buildPackages.gcc11
+```
+
+... setup `.cargo/config` for your project:
+
+```toml
+[build]
+target = "avr-unknown-unknown"
+rustflags = ["-C", "target-cpu=atmega328p"]
+
+[unstable]
+build-std = ["core"]
+```
+
+... and then simply run:
+
+```console
+$ cargo build --release
+```
+
+The final binary will be placed into
+`./target/avr-unknown-unknown/release/your-project.elf`.
+
+Note that since AVRs have rather small amounts of registers, ROM and RAM, it's
+recommended to always use `--release` to avoid running out of space.
+
+## Testing
+
+You can use [`simavr`](https://github.com/buserror/simavr) to emulate the
+resulting firmware on your machine:
+
+```
+simavr -m atmega328p ./target/avr-unknown-unknown/release/your-project.elf
+```
+
+Alternatively, if you want to write a couple of actual `#[test]`s, you can use
+[`avr-tester`](https://github.com/Patryk27/avr-tester).

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

@@ -380,7 +380,7 @@ fn profiler_runtime() {
 #[test]
 fn asm_support() {
     let asms = [
-        ("avr-unknown-gnu-atmega328", false),
+        ("avr-unknown-unknown", false),
         ("i686-unknown-netbsd", true),
         ("riscv32gc-unknown-linux-gnu", true),
         ("riscv64imac-unknown-none-elf", true),

src/tools/compiletest/src/header/tests.rs

@@ -1,5 +1,5 @@
 //@ assembly-output: emit-asm
-//@ compile-flags: --target avr-unknown-gnu-atmega328
+//@ compile-flags: --target avr-unknown-unknown -C target-cpu=atmega328p
 //@ needs-llvm-components: avr
 
 #![feature(no_core, lang_items, rustc_attrs, asm_experimental_arch)]