Rollup merge of rust-lang#68253 - japaric:bare-metal-cortex-a, r=alexcrichton

add bare metal ARM Cortex-A targets to rustc

-> `rustc --target armv7a-none-eabi` will work

also build rust-std (rustup) components for them

-> `rustup target add armv7a-none-eabi` will work

this completes our bare-metal support of ARMv7 cores on stable Rust (by 1.42 or 1.43)

(these target specifications have been tested on a real (no emulation / QEMU) [Cortex-A7 core](https://github.com/iqlusioninc/usbarmory.rs/))

Author: tmandry

src/ci/docker/dist-various-1/Dockerfile

@@ -160,6 +160,7 @@ ENV TARGETS=$TARGETS,armebv7r-none-eabihf
 ENV TARGETS=$TARGETS,armv7r-none-eabi
 ENV TARGETS=$TARGETS,armv7r-none-eabihf
 ENV TARGETS=$TARGETS,thumbv7neon-unknown-linux-gnueabihf
+ENV TARGETS=$TARGETS,armv7a-none-eabi
 
 # riscv targets currently do not need a C compiler, as compiler_builtins
 # doesn't currently have it enabled, and the riscv gcc compiler is not
@@ -173,6 +174,10 @@ ENV CC_mipsel_unknown_linux_musl=mipsel-openwrt-linux-gcc \
     CC_thumbv7neon_unknown_linux_gnueabihf=arm-linux-gnueabihf-gcc \
     AR_thumbv7neon_unknown_linux_gnueabihf=arm-linux-gnueabihf-ar \
     CXX_thumbv7neon_unknown_linux_gnueabihf=arm-linux-gnueabihf-g++ \
+    CC_armv7a_none_eabi=arm-none-eabi-gcc \
+    CC_armv7a_none_eabihf=arm-none-eabi-gcc \
+    CFLAGS_armv7a_none_eabi=-march=armv7-a \
+    CFLAGS_armv7a_none_eabihf=-march=armv7-a+vfpv3 \
     CC_riscv64gc_unknown_linux_gnu=riscv64-unknown-linux-gnu-gcc \
     AR_riscv64gc_unknown_linux_gnu=riscv64-unknown-linux-gnu-ar \
     CXX_riscv64gc_unknown_linux_gnu=riscv64-unknown-linux-gnu-g++ \

src/librustc_target/spec/armv7a_none_eabi.rs

@@ -0,0 +1,48 @@
+// Generic ARMv7-A target for bare-metal code - floating point disabled
+//
+// This is basically the `armv7-unknown-linux-gnueabi` target with some changes
+// (listed below) to bring it closer to the bare-metal `thumb` & `aarch64`
+// targets:
+//
+// - `TargetOptions.features`: added `+strict-align`. rationale: unaligned
+// memory access is disabled on boot on these cores
+// - linker changed to LLD. rationale: C is not strictly needed to build
+// bare-metal binaries (the `gcc` linker has the advantage that it knows where C
+// libraries and crt*.o are but it's not much of an advantage here); LLD is also
+// faster
+// - `target_os` set to `none`. rationale: matches `thumb` targets
+// - `target_{env,vendor}` set to an empty string. rationale: matches `thumb`
+// targets
+// - `panic_strategy` set to `abort`. rationale: matches `thumb` targets
+// - `relocation-model` set to `static`; also no PIE, no relro and no dynamic
+// linking. rationale: matches `thumb` targets
+
+use super::{LinkerFlavor, LldFlavor, PanicStrategy, Target, TargetOptions};
+
+pub fn target() -> Result<Target, String> {
+    let opts = TargetOptions {
+        linker: Some("rust-lld".to_owned()),
+        features: "+v7,+thumb2,+soft-float,-neon,+strict-align".to_string(),
+        executables: true,
+        relocation_model: "static".to_string(),
+        disable_redzone: true,
+        max_atomic_width: Some(64),
+        panic_strategy: PanicStrategy::Abort,
+        abi_blacklist: super::arm_base::abi_blacklist(),
+        emit_debug_gdb_scripts: false,
+        ..Default::default()
+    };
+    Ok(Target {
+        llvm_target: "armv7a-none-eabi".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_c_int_width: "32".to_string(),
+        target_os: "none".to_string(),
+        target_env: String::new(),
+        target_vendor: String::new(),
+        data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".to_string(),
+        arch: "arm".to_string(),
+        linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
+        options: opts,
+    })
+}

src/librustc_target/spec/armv7a_none_eabihf.rs

@@ -0,0 +1,36 @@
+// Generic ARMv7-A target for bare-metal code - floating point enabled (assumes
+// FPU is present and emits FPU instructions)
+//
+// This is basically the `armv7-unknown-linux-gnueabihf` target with some
+// changes (list in `armv7a_none_eabi.rs`) to bring it closer to the bare-metal
+// `thumb` & `aarch64` targets.
+
+use super::{LinkerFlavor, LldFlavor, PanicStrategy, Target, TargetOptions};
+
+pub fn target() -> Result<Target, String> {
+    let opts = TargetOptions {
+        linker: Some("rust-lld".to_owned()),
+        features: "+v7,+vfp3,-d32,+thumb2,-neon,+strict-align".to_string(),
+        executables: true,
+        relocation_model: "static".to_string(),
+        disable_redzone: true,
+        max_atomic_width: Some(64),
+        panic_strategy: PanicStrategy::Abort,
+        abi_blacklist: super::arm_base::abi_blacklist(),
+        emit_debug_gdb_scripts: false,
+        ..Default::default()
+    };
+    Ok(Target {
+        llvm_target: "armv7a-none-eabihf".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_c_int_width: "32".to_string(),
+        target_os: "none".to_string(),
+        target_env: String::new(),
+        target_vendor: String::new(),
+        data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".to_string(),
+        arch: "arm".to_string(),
+        linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
+        options: opts,
+    })
+}

src/librustc_target/spec/mod.rs

@@ -472,6 +472,9 @@ supported_targets! {
     ("thumbv8m.main-none-eabi", thumbv8m_main_none_eabi),
     ("thumbv8m.main-none-eabihf", thumbv8m_main_none_eabihf),
 
+    ("armv7a-none-eabi", armv7a_none_eabi),
+    ("armv7a-none-eabihf", armv7a_none_eabihf),
+
     ("msp430-none-elf", msp430_none_elf),
 
     ("aarch64-unknown-cloudabi", aarch64_unknown_cloudabi),

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

@@ -69,6 +69,7 @@ static TARGETS: &[&str] = &[
     "thumbv7neon-linux-androideabi",
     "armv7-unknown-linux-gnueabi",
     "armv7-unknown-linux-gnueabihf",
+    "armv7a-none-eabi",
     "thumbv7neon-unknown-linux-gnueabihf",
     "armv7-unknown-linux-musleabi",
     "armv7-unknown-linux-musleabihf",