Add Armv8-M Mainline targets

This commit enables the Armv8-M Mainline architecture profile.
It adds two targets:
  - thumbv8m.main-none-eabi
  - thumbv8m.main-none-eabihf

The second one uses the Floating Point Unit for floating point
operations. It mainly targets the Cortex-M33 processor, which
can have the optional Floating Point Unit extension.

src/librustc_target/spec/mod.rs

@@ -400,6 +400,8 @@ supported_targets! {
     ("thumbv7em-none-eabi", thumbv7em_none_eabi),
     ("thumbv7em-none-eabihf", thumbv7em_none_eabihf),
     ("thumbv8m.base-none-eabi", thumbv8m_base_none_eabi),
+    ("thumbv8m.main-none-eabi", thumbv8m_main_none_eabi),
+    ("thumbv8m.main-none-eabihf", thumbv8m_main_none_eabihf),
 
     ("msp430-none-elf", msp430_none_elf),
 

src/librustc_target/spec/thumb_base.rs

@@ -18,6 +18,7 @@
 // - Cortex-M4(F)
 // - Cortex-M7(F)
 // - Cortex-M23
+// - Cortex-M33
 //
 // We have opted for these instead of one target per processor (e.g. `cortex-m0`, `cortex-m3`,
 // etc) because the differences between some processors like the cortex-m0 and cortex-m1 are almost

src/librustc_target/spec/thumbv8m_main_none_eabi.rs

@@ -0,0 +1,34 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Targets the Cortex-M33 processor (Armv8-M Mainline architecture profile),
+// without the Floating Point extension.
+
+use spec::{LinkerFlavor, LldFlavor, Target, TargetOptions, TargetResult};
+
+pub fn target() -> TargetResult {
+    Ok(Target {
+        llvm_target: "thumbv8m.main-none-eabi".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_c_int_width: "32".to_string(),
+        data_layout: "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64".to_string(),
+        arch: "arm".to_string(),
+        target_os: "none".to_string(),
+        target_env: String::new(),
+        target_vendor: String::new(),
+        linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
+
+        options: TargetOptions {
+            max_atomic_width: Some(32),
+            .. super::thumb_base::opts()
+        },
+    })
+}

src/librustc_target/spec/thumbv8m_main_none_eabihf.rs

@@ -0,0 +1,40 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Targets the Cortex-M33 processor (Armv8-M Mainline architecture profile),
+// with the Floating Point extension.
+
+use spec::{LinkerFlavor, LldFlavor, Target, TargetOptions, TargetResult};
+
+pub fn target() -> TargetResult {
+    Ok(Target {
+        llvm_target: "thumbv8m.main-none-eabihf".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_c_int_width: "32".to_string(),
+        data_layout: "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64".to_string(),
+        arch: "arm".to_string(),
+        target_os: "none".to_string(),
+        target_env: String::new(),
+        target_vendor: String::new(),
+        linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
+
+        options: TargetOptions {
+            // If the Floating Point extension is implemented in the Cortex-M33
+            // processor, the Cortex-M33 Technical Reference Manual states that
+            // the FPU uses the FPv5 architecture, single-precision instructions
+            // and 16 D registers.
+            // These parameters map to the following LLVM features.
+            features: "+fp-armv8,+fp-only-sp,+d16".to_string(),
+            max_atomic_width: Some(32),
+            .. super::thumb_base::opts()
+        },
+    })
+}