Auto merge of #88952 - skrap:add-armv7-uclibc, r=nagisa

Add new tier-3 target: armv7-unknown-linux-uclibceabihf

This change adds a new tier-3 target: armv7-unknown-linux-uclibceabihf

This target is primarily used in embedded linux devices where system resources are slim and glibc is deemed too heavyweight.  Cross compilation C toolchains are available [here](https://toolchains.bootlin.com/) or via [buildroot](https://buildroot.org).

The change is based largely on a previous PR #79380 with a few minor modifications.  The author of that PR was unable to push the PR forward, and graciously allowed me to take it over.

Per the [target tier 3 policy](https://github.com/rust-lang/rfcs/blob/master/text/2803-target-tier-policy.md), I volunteer to be the "target maintainer".

This is my first PR to Rust itself, so I apologize if I've missed things!

9 files changed, 103 insertions, 6 deletions

diff --git a/compiler/rustc_codegen_llvm/src/callee.rs b/compiler/rustc_codegen_llvm/src/callee.rs
index 5d68d2b77d4..1bc924d3b90 100644
--- a/compiler/rustc_codegen_llvm/src/callee.rs
+++ b/compiler/rustc_codegen_llvm/src/callee.rs
@@ -175,7 +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"
+                && !matches!(tcx.sess.target.env.as_ref(), "gnu" | "uclibc")
             {
                 llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
             }
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 0bdf70b3ec4..d0c7379c2d9 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -3012,7 +3012,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
         };
 
         let target = &self.tcx.sess.target;
-        let target_env_gnu_like = matches!(&target.env[..], "gnu" | "musl");
+        let target_env_gnu_like = matches!(&target.env[..], "gnu" | "musl" | "uclibc");
         let win_x64_gnu = target.os == "windows" && target.arch == "x86_64" && target.env == "gnu";
         let linux_s390x_gnu_like =
             target.os == "linux" && target.arch == "s390x" && target_env_gnu_like;
@@ -3110,7 +3110,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
             if arg.layout.is_zst() {
                 // For some forsaken reason, x86_64-pc-windows-gnu
                 // doesn't ignore zero-sized struct arguments.
-                // The same is true for {s390x,sparc64,powerpc}-unknown-linux-{gnu,musl}.
+                // The same is true for {s390x,sparc64,powerpc}-unknown-linux-{gnu,musl,uclibc}.
                 if is_return
                     || rust_abi
                     || (!win_x64_gnu
diff --git a/compiler/rustc_target/src/spec/armv7_unknown_linux_uclibceabihf.rs b/compiler/rustc_target/src/spec/armv7_unknown_linux_uclibceabihf.rs
new file mode 100644
index 00000000000..d230f77bde2
--- /dev/null
+++ b/compiler/rustc_target/src/spec/armv7_unknown_linux_uclibceabihf.rs
@@ -0,0 +1,24 @@
+use crate::spec::{Target, TargetOptions};
+
+// This target is for uclibc Linux on ARMv7 without NEON or
+// thumb-mode. See the thumbv7neon variant for enabling both.
+
+pub fn target() -> Target {
+    let base = super::linux_uclibc_base::opts();
+    Target {
+        llvm_target: "armv7-unknown-linux-gnueabihf".to_string(),
+        pointer_width: 32,
+        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(),
+
+        options: TargetOptions {
+            // Info about features at https://wiki.debian.org/ArmHardFloatPort
+            features: "+v7,+vfp3,-d32,+thumb2,-neon".to_string(),
+            cpu: "generic".to_string(),
+            max_atomic_width: Some(64),
+            mcount: "_mcount".to_string(),
+            abi: "eabihf".to_string(),
+            ..base
+        },
+    }
+}
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 5276da1ba5a..ff5dfa3f746 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -952,6 +952,8 @@ supported_targets! {
     ("bpfel-unknown-none", bpfel_unknown_none),
 
     ("armv6k-nintendo-3ds", armv6k_nintendo_3ds),
+
+    ("armv7-unknown-linux-uclibceabihf", armv7_unknown_linux_uclibceabihf),
 }
 
 /// Warnings encountered when parsing the target `json`.
diff --git a/library/std/src/sys/unix/mod.rs b/library/std/src/sys/unix/mod.rs
index 1c37f4ee498..2ba6c8d830e 100644
--- a/library/std/src/sys/unix/mod.rs
+++ b/library/std/src/sys/unix/mod.rs
@@ -307,6 +307,9 @@ cfg_if::cfg_if! {
         #[link(name = "zircon")]
         #[link(name = "fdio")]
         extern "C" {}
+    } else if #[cfg(all(target_os = "linux", target_env = "uclibc"))] {
+        #[link(name = "dl")]
+        extern "C" {}
     }
 }
 
diff --git a/library/std/src/sys/unix/thread.rs b/library/std/src/sys/unix/thread.rs
index 891f3413e91..6f4863057ab 100644
--- a/library/std/src/sys/unix/thread.rs
+++ b/library/std/src/sys/unix/thread.rs
@@ -594,7 +594,8 @@ pub mod guard {
                 Some(stackaddr - guardsize..stackaddr)
             } else if cfg!(all(target_os = "linux", target_env = "musl")) {
                 Some(stackaddr - guardsize..stackaddr)
-            } else if cfg!(all(target_os = "linux", target_env = "gnu")) {
+            } else if cfg!(all(target_os = "linux", any(target_env = "gnu", target_env = "uclibc")))
+            {
                 // glibc used to include the guard area within the stack, as noted in the BUGS
                 // section of `man pthread_attr_getguardsize`.  This has been corrected starting
                 // with glibc 2.27, and in some distro backports, so the guard is now placed at the
diff --git a/library/unwind/src/lib.rs b/library/unwind/src/lib.rs
index 25be9e7cc6c..e263780bf38 100644
--- a/library/unwind/src/lib.rs
+++ b/library/unwind/src/lib.rs
@@ -63,7 +63,7 @@ cfg_if::cfg_if! {
 // don't want to duplicate it here.
 #[cfg(all(
     target_os = "linux",
-    target_env = "gnu",
+    any(target_env = "gnu", target_env = "uclibc"),
     not(feature = "llvm-libunwind"),
     not(feature = "system-llvm-libunwind")
 ))]
@@ -72,7 +72,7 @@ extern "C" {}
 
 #[cfg(all(
     target_os = "linux",
-    target_env = "gnu",
+    any(target_env = "gnu", target_env = "uclibc"),
     not(feature = "llvm-libunwind"),
     feature = "system-llvm-libunwind"
 ))]
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 26fef889462..bbeab598f22 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -220,6 +220,7 @@ target | std | host | notes
 `armv6-unknown-netbsd-eabihf` | ? |  |
 `armv6k-nintendo-3ds` | * |  | ARMv6K Nintendo 3DS, Horizon (Requires devkitARM toolchain)
 `armv7-apple-ios` | ✓ |  | ARMv7 iOS, Cortex-a8
+`armv7-unknown-linux-uclibceabihf` | ✓ | ? | ARMv7 Linux uClibc
 `armv7-unknown-freebsd` | ✓ | ✓ | ARMv7 FreeBSD
 `armv7-unknown-netbsd-eabihf` | ✓ | ✓ |
 `armv7-wrs-vxworks-eabihf` | ? |  |
diff --git a/src/doc/rustc/src/platform-support/armv7-unknown-linux-uclibceabihf.md b/src/doc/rustc/src/platform-support/armv7-unknown-linux-uclibceabihf.md
new file mode 100644
index 00000000000..b3a4275c6ee
--- /dev/null
+++ b/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
+```