Rollup merge of #144962 - Gelbpunkt:aarch64_be-unknown-none-softfloat, r=davidtwco

Add aarch64_be-unknown-none-softfloat target

This adds a new target for bare-metal big endian ARM64 without FPU. We want to use this in [the Hermit unikernel](https://github.com/hermit-os/kernel) because big endian ARM64 is the most accessible big endian architecture for us and it can be supported with our existing aarch64 code. I have compiled our kernel and bootloader with this target and they work as expected in QEMU.

Regarding the [tier 3 target policy](https://doc.rust-lang.org/rustc/target-tier-policy.html#tier-3-target-policy):

> - A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

The maintainer(s) (currently just me) are listed in the markdown document that documents the target.

> - Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.
>   - Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.
>   - If possible, use only letters, numbers, dashes and underscores for the name. Periods (.) are known to cause issues in Cargo.

The target name is consistent with the existing `aarch64-unknown-none-softfloat` target and the existing big endian aarch64 targets like `aarch64_be-unknown-linux-gnu`.

> - Tier 3 targets may have unusual requirements to build or use, but must not create legal issues or impose onerous legal terms for the Rust project or for Rust developers or users.
>   - The target must not introduce license incompatibilities.
>   - Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
>   - The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.
>   - Compiling, linking, and emitting functional binaries, libraries, or other code for the target (whether hosted on the target itself or cross-compiling from another target) must not depend on proprietary (non-FOSS) libraries. Host tools built for the target itself may depend on the ordinary runtime libraries supplied by the platform and commonly used by other applications built for the target, but those libraries must not be required for code generation for the target; cross-compilation to the target must not require such libraries at all. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.
>   - "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

There are no licensing issues and any toolchain that can compile for `aarch64-unknown-none-softfloat` can also compile for `aarch64_be-unknown-none-softfloat` (well, at least GCC and LLVM). No proprietary components are required.

> - Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.
>   - This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.

Ack.

> - Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.

This target does not implement std and is equivalent to `aarch64-unknown-none-softfloat` in all these regards.

> - The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.

Ack, that is part of the markdown document.

> - Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via ```@)``` to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.
>   - Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.

Ack.

> - Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target.
>   - In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

This doesn't break any existing targets.

> - Tier 3 targets must be able to produce assembly using at least one of rustc's supported backends from any host target. (Having support in a fork of the backend is not sufficient, it must be upstream.)

The LLVM backend works.

> - If a tier 3 target stops meeting these requirements, or the target maintainers no longer have interest or time, or the target shows no signs of activity and has not built for some time, or removing the target would improve the quality of the Rust codebase, we may post a PR to remove it; any such PR will be CCed to the target maintainers (and potentially other people who have previously worked on the target), to check potential interest in improving the situation.

Ack.

7 files changed, 124 insertions, 0 deletions

diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index b9fbff8db05..ee408c76006 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -2147,6 +2147,7 @@ supported_targets! {
 
     ("aarch64-unknown-none", aarch64_unknown_none),
     ("aarch64-unknown-none-softfloat", aarch64_unknown_none_softfloat),
+    ("aarch64_be-unknown-none-softfloat", aarch64_be_unknown_none_softfloat),
     ("aarch64-unknown-nuttx", aarch64_unknown_nuttx),
 
     ("x86_64-fortanix-unknown-sgx", x86_64_fortanix_unknown_sgx),
diff --git a/compiler/rustc_target/src/spec/targets/aarch64_be_unknown_none_softfloat.rs b/compiler/rustc_target/src/spec/targets/aarch64_be_unknown_none_softfloat.rs
new file mode 100644
index 00000000000..7f918e85080
--- /dev/null
+++ b/compiler/rustc_target/src/spec/targets/aarch64_be_unknown_none_softfloat.rs
@@ -0,0 +1,43 @@
+// Generic big-endian AArch64 target for bare-metal code - Floating point disabled
+//
+// Can be used in conjunction with the `target-feature` and
+// `target-cpu` compiler flags to opt-in more hardware-specific
+// features.
+//
+// For example, `-C target-cpu=cortex-a53`.
+use rustc_abi::Endian;
+
+use crate::spec::{
+    Cc, LinkerFlavor, Lld, PanicStrategy, RelocModel, SanitizerSet, StackProbeType, Target,
+    TargetMetadata, TargetOptions,
+};
+
+pub(crate) fn target() -> Target {
+    let opts = TargetOptions {
+        abi: "softfloat".into(),
+        linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
+        linker: Some("rust-lld".into()),
+        features: "+v8a,+strict-align,-neon,-fp-armv8".into(),
+        relocation_model: RelocModel::Static,
+        disable_redzone: true,
+        max_atomic_width: Some(128),
+        supported_sanitizers: SanitizerSet::KCFI | SanitizerSet::KERNELADDRESS,
+        stack_probes: StackProbeType::Inline,
+        panic_strategy: PanicStrategy::Abort,
+        endian: Endian::Big,
+        ..Default::default()
+    };
+    Target {
+        llvm_target: "aarch64_be-unknown-none".into(),
+        metadata: TargetMetadata {
+            description: Some("Bare ARM64 (big-endian), softfloat".into()),
+            tier: Some(3),
+            host_tools: Some(false),
+            std: Some(false),
+        },
+        pointer_width: 64,
+        data_layout: "E-m:e-p270:32:32-p271:32:32-p272:64:64-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128-Fn32".into(),
+        arch: "aarch64".into(),
+        options: opts,
+    }
+}
diff --git a/src/bootstrap/src/core/sanity.rs b/src/bootstrap/src/core/sanity.rs
index 3080e641b5b..bd02131b7fe 100644
--- a/src/bootstrap/src/core/sanity.rs
+++ b/src/bootstrap/src/core/sanity.rs
@@ -35,6 +35,7 @@ pub struct Finder {
 const STAGE0_MISSING_TARGETS: &[&str] = &[
     "armv7a-vex-v5",
     // just a dummy comment so the list doesn't get onelined
+    "aarch64_be-unknown-none-softfloat",
 ];
 
 /// Minimum version threshold for libstdc++ required when using prebuilt LLVM
diff --git a/src/doc/rustc/src/SUMMARY.md b/src/doc/rustc/src/SUMMARY.md
index 25f154f1180..b942e4bfa61 100644
--- a/src/doc/rustc/src/SUMMARY.md
+++ b/src/doc/rustc/src/SUMMARY.md
@@ -47,6 +47,7 @@
     - [\*-apple-visionos](platform-support/apple-visionos.md)
     - [aarch64-nintendo-switch-freestanding](platform-support/aarch64-nintendo-switch-freestanding.md)
     - [aarch64-unknown-linux-musl](platform-support/aarch64-unknown-linux-musl.md)
+    - [aarch64_be-unknown-none-softfloat](platform-support/aarch64_be-unknown-none-softfloat.md)
     - [amdgcn-amd-amdhsa](platform-support/amdgcn-amd-amdhsa.md)
     - [armeb-unknown-linux-gnueabi](platform-support/armeb-unknown-linux-gnueabi.md)
     - [arm-none-eabi](platform-support/arm-none-eabi.md)
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 8ebaa8dd874..89b43cda9b9 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -273,6 +273,7 @@ target | std | host | notes
 `aarch64_be-unknown-linux-gnu` | ✓ | ✓ | ARM64 Linux (big-endian)
 `aarch64_be-unknown-linux-gnu_ilp32` | ✓ | ✓ | ARM64 Linux (big-endian, ILP32 ABI)
 [`aarch64_be-unknown-netbsd`](platform-support/netbsd.md) | ✓ | ✓ | ARM64 NetBSD (big-endian)
+[`aarch64_be-unknown-none-softfloat`](platform-support/aarch64_be-unknown-none-softfloat.md) | * |  | Bare big-endian ARM64, softfloat
 [`amdgcn-amd-amdhsa`](platform-support/amdgcn-amd-amdhsa.md) | * |  | `-Ctarget-cpu=gfx...` to specify [the AMD GPU] to compile for
 [`arm64_32-apple-watchos`](platform-support/apple-watchos.md) | ✓ |  | Arm Apple WatchOS 64-bit with 32-bit pointers
 [`arm64e-apple-darwin`](platform-support/arm64e-apple-darwin.md)  | ✓ | ✓ | ARM64e Apple Darwin
diff --git a/src/doc/rustc/src/platform-support/aarch64_be-unknown-none-softfloat.md b/src/doc/rustc/src/platform-support/aarch64_be-unknown-none-softfloat.md
new file mode 100644
index 00000000000..a28ddcdf7f2
--- /dev/null
+++ b/src/doc/rustc/src/platform-support/aarch64_be-unknown-none-softfloat.md
@@ -0,0 +1,74 @@
+# aarch64_be-unknown-none-softfloat
+
+**Tier: 3**
+
+Target for freestanding/bare-metal big-endian ARM64 binaries in ELF format:
+firmware, kernels, etc.
+
+## Target maintainers
+
+[@Gelbpunkt](https://github.com/Gelbpunkt)
+
+## 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.
+
+The target does not assume existence of a FPU and does not make use of any
+non-GPR register. This allows the generated code to run in environments, such
+as kernels, which may need to avoid the use of such registers or which may have
+special considerations about the use of such registers (e.g. saving and
+restoring them to avoid breaking userspace code using the same registers). You
+can change code generation to use additional CPU features via the
+`-C target-feature=` codegen options to rustc, or via the `#[target_feature]`
+mechanism within Rust code.
+
+By default, code generated with the soft-float target should run on any
+big-endian ARM64 hardware, enabling additional target features may raise this
+baseline.
+
+`extern "C"` uses the [architecture's standard calling convention][aapcs64].
+
+[aapcs64]: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst
+
+The targets generate binaries in the ELF format. Any alternate formats or
+special considerations for binary layout will require linker options or linker
+scripts.
+
+## Building the target
+
+You can build Rust with support for the target by adding it to the `target`
+list in `bootstrap.toml`:
+
+```toml
+[build]
+target = ["aarch64_be-unknown-none-softfloat"]
+```
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will first need to build Rust with the target enabled (see
+"Building the target" above).
+
+## Cross-compilation
+
+For cross builds, you will need an appropriate ARM64 C/C++ toolchain for
+linking, or if you want to compile C code along with Rust (such as for Rust
+crates with C dependencies).
+
+Rust *may* be able to use an `aarch64_be-unknown-linux-{gnu,musl}-` toolchain
+with appropriate standalone flags to build for this target (depending on the
+assumptions of that toolchain, see below), or you may wish to use a separate
+`aarch64_be-unknown-none-softfloat` toolchain.
+
+On some ARM64 hosts that use ELF binaries, you *may* be able to use the host C
+toolchain, if it does not introduce assumptions about the host environment that
+don't match the expectations of a standalone environment. Otherwise, you may
+need a separate toolchain for standalone/freestanding development, just as when
+cross-compiling from a non-ARM64 platform.
+
+## Testing
+
+As the target supports a variety of different environments and does not support
+`std`, it does not support running the Rust test suite.
diff --git a/tests/assembly-llvm/targets/targets-elf.rs b/tests/assembly-llvm/targets/targets-elf.rs
index ee63dffe9ea..a1d759ede2b 100644
--- a/tests/assembly-llvm/targets/targets-elf.rs
+++ b/tests/assembly-llvm/targets/targets-elf.rs
@@ -10,6 +10,9 @@
 //@ revisions: aarch64_be_unknown_netbsd
 //@ [aarch64_be_unknown_netbsd] compile-flags: --target aarch64_be-unknown-netbsd
 //@ [aarch64_be_unknown_netbsd] needs-llvm-components: aarch64
+//@ revisions: aarch64_be_unknown_none_softfloat
+//@ [aarch64_be_unknown_none_softfloat] compile-flags: --target aarch64_be-unknown-none-softfloat
+//@ [aarch64_be_unknown_none_softfloat] needs-llvm-components: aarch64
 //@ revisions: aarch64_kmc_solid_asp3
 //@ [aarch64_kmc_solid_asp3] compile-flags: --target aarch64-kmc-solid_asp3
 //@ [aarch64_kmc_solid_asp3] needs-llvm-components: aarch64