Auto merge of #36874 - japaric:thumbs, r=alexcrichton

add Thumbs to the compiler

this commit adds 4 new target definitions to the compiler for easier
cross compilation to ARM Cortex-M devices.

- `thumbv6m-none-eabi`
  - For the Cortex-M0, Cortex-M0+ and Cortex-M1
  - This architecture doesn't have hardware support (instructions) for
    atomics. Hence, the `Atomic*` structs are not available for this
    target.
- `thumbv7m-none-eabi`
  - For the Cortex-M3
- `thumbv7em-none-eabi`
  - For the FPU-less variants of the Cortex-M4 and Cortex-M7
  - On this target, all the floating point operations will be lowered
    software routines (intrinsics)
- `thumbv7em-none-eabihf`
  - For the variants of the Cortex-M4 and Cortex-M7 that do have a FPU.
  - On this target, all the floating point operations will be lowered
    to hardware instructions

No binary releases of standard crates, like `core`, are planned for
these targets because Cargo, in the future, will compile e.g. the `core`
crate on the fly as part of the `cargo build` process. In the meantime,
you'll have to compile the `core` crate yourself. [Xargo] is the easiest
way to do that as in handles the compilation of `core` automatically and
can be used just like Cargo: `xargo build --target thumbv6m-none-eabi`
is all that's needed.

[Xargo]: https://crates.io/crates/xargo

---

cc @brson @alexcrichton

src/librustc/session/config.rs

@@ -933,7 +933,7 @@ pub fn default_configuration(sess: &Session) -> ast::CrateConfig {
     let os = &sess.target.target.target_os;
     let env = &sess.target.target.target_env;
     let vendor = &sess.target.target.target_vendor;
-    let max_atomic_width = sess.target.target.options.max_atomic_width;
+    let max_atomic_width = sess.target.target.max_atomic_width();
 
     let fam = if let Some(ref fam) = sess.target.target.options.target_family {
         intern(fam)

src/librustc_back/target/aarch64_apple_ios.rs

@@ -25,7 +25,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             features: "+neon,+fp-armv8,+cyclone".to_string(),
             eliminate_frame_pointer: false,
-            max_atomic_width: 128,
+            max_atomic_width: Some(128),
             .. base
         },
     })

src/librustc_back/target/aarch64_linux_android.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
-    base.max_atomic_width = 128;
+    base.max_atomic_width = Some(128);
     // As documented in http://developer.android.com/ndk/guides/cpu-features.html
     // the neon (ASIMD) and FP must exist on all android aarch64 targets.
     base.features = "+neon,+fp-armv8".to_string();

src/librustc_back/target/aarch64_unknown_linux_gnu.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 128;
+    base.max_atomic_width = Some(128);
     Ok(Target {
         llvm_target: "aarch64-unknown-linux-gnu".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/arm_linux_androideabi.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+vfp3,+d16".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "arm-linux-androideabi".to_string(),

src/librustc_back/target/arm_unknown_linux_gnueabi.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         llvm_target: "arm-unknown-linux-gnueabi".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/arm_unknown_linux_gnueabihf.rs

@@ -12,7 +12,7 @@ use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         llvm_target: "arm-unknown-linux-gnueabihf".to_string(),
         target_endian: "little".to_string(),

src/librustc_back/target/arm_unknown_linux_musleabi.rs

@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     // Most of these settings are copied from the arm_unknown_linux_gnueabi
     // target.
     base.features = "+v6".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabi" and not "musleabi" here. LLVM uses it
         // to determine the calling convention and float ABI, and it doesn't

src/librustc_back/target/arm_unknown_linux_musleabihf.rs

@@ -16,7 +16,7 @@ pub fn target() -> TargetResult {
     // Most of these settings are copied from the arm_unknown_linux_gnueabihf
     // target.
     base.features = "+v6,+vfp2".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and it

src/librustc_back/target/armv7_apple_ios.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "apple".to_string(),
         options: TargetOptions {
             features: "+v7,+vfp3,+neon".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/armv7_linux_androideabi.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+thumb2,+vfp3,+d16".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     Ok(Target {
         llvm_target: "armv7-none-linux-android".to_string(),

src/librustc_back/target/armv7_unknown_linux_gnueabihf.rs

@@ -26,7 +26,7 @@ pub fn target() -> TargetResult {
             // Info about features at https://wiki.debian.org/ArmHardFloatPort
             features: "+v7,+vfp3,+d16,+thumb2".to_string(),
             cpu: "generic".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/armv7_unknown_linux_musleabihf.rs

@@ -17,7 +17,7 @@ pub fn target() -> TargetResult {
     // target.
     base.features = "+v7,+vfp3,+neon".to_string();
     base.cpu = "cortex-a8".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and LLVM

src/librustc_back/target/armv7s_apple_ios.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         target_vendor: "apple".to_string(),
         options: TargetOptions {
             features: "+v7,+vfp4,+neon".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/asmjs_unknown_emscripten.rs

@@ -21,7 +21,7 @@ pub fn target() -> Result<Target, String> {
         linker_is_gnu: true,
         allow_asm: false,
         obj_is_bitcode: true,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
         .. Default::default()
     };

src/librustc_back/target/i386_apple_ios.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         target_env: "".to_string(),
         target_vendor: "apple".to_string(),
         options: TargetOptions {
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             .. base
         }
     })

src/librustc_back/target/i686_apple_darwin.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::apple_base::opts();
     base.cpu = "yonah".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_linux_android.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
 
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // http://developer.android.com/ndk/guides/abis.html#x86
     base.cpu = "pentiumpro".to_string();

src/librustc_back/target/i686_pc_windows_gnu.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/librustc_back/target/i686_pc_windows_msvc.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::windows_msvc_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address
     // space available to x86 Windows binaries on x86_64.

src/librustc_back/target/i686_unknown_dragonfly.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::dragonfly_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_freebsd.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::freebsd_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_haiku.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::haiku_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_linux_gnu.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
 
     Ok(Target {

src/librustc_back/target/i686_unknown_linux_musl.rs

@@ -13,7 +13,7 @@ use target::{Target, TargetResult};
 pub fn target() -> TargetResult {
     let mut base = super::linux_musl_base::opts();
     base.cpu = "pentium4".to_string();
-    base.max_atomic_width = 64;
+    base.max_atomic_width = Some(64);
     base.pre_link_args.push("-m32".to_string());
     base.pre_link_args.push("-Wl,-melf_i386".to_string());
 

src/librustc_back/target/le32_unknown_nacl.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         exe_suffix: ".pexe".to_string(),
         linker_is_gnu: true,
         allow_asm: false,
-        max_atomic_width: 32,
+        max_atomic_width: Some(32),
         .. Default::default()
     };
     Ok(Target {

src/librustc_back/target/mips64_unknown_linux_gnuabi64.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
             // NOTE(mips64r2) matches C toolchain
             cpu: "mips64r2".to_string(),
             features: "+mips64r2".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mips64el_unknown_linux_gnuabi64.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
             // NOTE(mips64r2) matches C toolchain
             cpu: "mips64r2".to_string(),
             features: "+mips64r2".to_string(),
-            max_atomic_width: 64,
+            max_atomic_width: Some(64),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mips_unknown_linux_gnu.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mips_unknown_linux_musl.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         }
     })

src/librustc_back/target/mips_unknown_linux_uclibc.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32r2".to_string(),
             features: "+mips32r2,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mipsel_unknown_linux_gnu.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })

src/librustc_back/target/mipsel_unknown_linux_musl.rs

@@ -23,7 +23,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         }
     })

src/librustc_back/target/mipsel_unknown_linux_uclibc.rs

@@ -24,7 +24,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             cpu: "mips32".to_string(),
             features: "+mips32,+soft-float".to_string(),
-            max_atomic_width: 32,
+            max_atomic_width: Some(32),
             ..super::linux_base::opts()
         },
     })