add platform support details file for armv7-unknown-linux-uclibc

Author: skrap

compiler/rustc_codegen_llvm/src/callee.rs

@@ -175,8 +175,7 @@ pub fn get_fn(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) -> &'ll Value
             // should use dllimport for functions.
             if cx.use_dll_storage_attrs
                 && tcx.is_dllimport_foreign_item(instance_def_id)
-                && tcx.sess.target.env != "gnu"
-                && tcx.sess.target.env != "uclibc"
+                && !matches!(tcx.sess.target.env.as_ref(), "gnu" | "uclibc")
             {
                 llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
             }

compiler/rustc_target/src/spec/armv7_unknown_linux_uclibceabihf.rs

@@ -17,6 +17,7 @@ pub fn target() -> Target {
             cpu: "generic".to_string(),
             max_atomic_width: Some(64),
             mcount: "_mcount".to_string(),
+            abi: "eabihf".to_string(),
             ..base
         },
     }

library/std/src/sys/unix/process/process_unix.rs

@@ -419,7 +419,7 @@ impl Command {
         }
 
         // Only glibc 2.24+ posix_spawn() supports returning ENOENT directly.
-        #[cfg(all(target_os = "linux", any(target_env = "gnu", target_env = "uclibc")))]
+        #[cfg(all(target_os = "linux", target_env = "gnu"))]
         {
             if let Some(version) = sys::os::glibc_version() {
                 if version < (2, 24) {

src/doc/rustc/src/platform-support/armv7-unknown-linux-uclibceabihf.md

@@ -0,0 +1,66 @@
+# armv7-unknown-linux-uclibceabihf
+
+**Tier: 3**
+
+This tier supports the ARMv7 processor running a Linux kernel and uClibc-ng standard library.  It provides full support for rust and the rust standard library.
+
+## Designated Developers
+
+* [@skrap](https://github.com/skrap)
+
+## Requirements
+
+This target is cross compiled, and requires a cross toolchain.  You can find suitable pre-built toolchains at [bootlin](https://toolchains.bootlin.com/) or build one yourself via [buildroot](https://buildroot.org).
+
+## Building
+
+### Get a C toolchain
+
+Compiling rust for this target has been tested on `x86_64` linux hosts.  Other host types have not been tested, but may work, if you can find a suitable cross compilation toolchain for them.
+
+If you don't already have a suitable toolchain, download one [here](https://toolchains.bootlin.com/downloads/releases/toolchains/armv7-eabihf/tarballs/armv7-eabihf--uclibc--bleeding-edge-2020.08-1.tar.bz2), and unpack it into a directory.
+
+### Configure rust
+
+The target can be built by enabling it for a `rustc` build, by placing the following in `config.toml`:
+
+```toml
+[build]
+target = ["armv7-unknown-linux-uclibceabihf"]
+stage = 2
+
+[target.armv7-unknown-linux-uclibceabihf]
+# ADJUST THIS PATH TO POINT AT YOUR TOOLCHAIN
+cc = "/TOOLCHAIN_PATH/bin/arm-buildroot-linux-uclibcgnueabihf-gcc"
+```
+
+### Build
+
+```sh
+# in rust dir
+./x.py build --stage 2
+```
+
+## Building and Running Rust Programs
+
+To test cross-compiled binaries on a `x86_64` system, you can use the `qemu-arm` [userspace emulation](https://qemu-project.gitlab.io/qemu/user/main.html) program.  This avoids having a full emulated ARM system by doing dynamic binary translation and dynamic system call translation.  It lets you run ARM programs directly on your `x86_64` kernel.  It's very convenient!
+
+To use:
+
+* Install `qemu-arm` according to your distro.
+* Link your built toolchain via:
+  * `rustup toolchain link stage2 ${RUST}/build/x86_64-unknown-linux-gnu/stage2`
+* Create a test program
+
+```sh
+cargo new hello_world
+cd hello_world
+```
+
+* Build and run
+
+```sh
+CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABIHF_RUNNER="qemu-arm -L ${TOOLCHAIN}/arm-buildroot-linux-uclibcgnueabihf/sysroot/" \
+CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABIHF_LINKER=${TOOLCHAIN}/bin/arm-buildroot-linux-uclibcgnueabihf-gcc \
+cargo +stage2 run --target armv7-unknown-linux-uclibceabihf
+```