add llvm_floatabi field to target spec that controls FloatABIType

compiler/rustc_target/src/spec/mod.rs

@@ -1085,6 +1085,35 @@ impl ToJson for CodeModel {
     }
 }
 
+/// The float ABI setting to be configured in the LLVM target machine.
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum FloatAbi {
+    Soft,
+    Hard,
+}
+
+impl FromStr for FloatAbi {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<FloatAbi, ()> {
+        Ok(match s {
+            "soft" => FloatAbi::Soft,
+            "hard" => FloatAbi::Hard,
+            _ => return Err(()),
+        })
+    }
+}
+
+impl ToJson for FloatAbi {
+    fn to_json(&self) -> Json {
+        match *self {
+            FloatAbi::Soft => "soft",
+            FloatAbi::Hard => "hard",
+        }
+        .to_json()
+    }
+}
+
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
 pub enum TlsModel {
     GeneralDynamic,
@@ -2150,6 +2179,8 @@ pub struct TargetOptions {
     pub env: StaticCow<str>,
     /// ABI name to distinguish multiple ABIs on the same OS and architecture. For instance, `"eabi"`
     /// or `"eabihf"`. Defaults to "".
+    /// This field is *not* forwarded directly to LLVM; its primary purpose is `cfg(target_abi)`.
+    /// However, parts of the backend do check this field for specific values to enable special behavior.
     pub abi: StaticCow<str>,
     /// Vendor name to use for conditional compilation (`target_vendor`). Defaults to "unknown".
     pub vendor: StaticCow<str>,
@@ -2446,8 +2477,17 @@ pub struct TargetOptions {
     pub llvm_mcount_intrinsic: Option<StaticCow<str>>,
 
     /// LLVM ABI name, corresponds to the '-mabi' parameter available in multilib C compilers
+    /// and the `-target-abi` flag in llc. In the LLVM API this is `MCOptions.ABIName`.
     pub llvm_abiname: StaticCow<str>,
 
+    /// Control the float ABI to use, for architectures that support it. The only architecture we
+    /// currently use this for is ARM. Corresponds to the `-float-abi` flag in llc. In the LLVM API
+    /// this is `FloatABIType`. (clang's `-mfloat-abi` is similar but more complicated since it
+    /// can also affect the `soft-float` target feature.)
+    ///
+    /// If not provided, LLVM will infer the float ABI from the target triple (`llvm_target`).
+    pub llvm_floatabi: Option<FloatAbi>,
+
     /// Whether or not RelaxElfRelocation flag will be passed to the linker
     pub relax_elf_relocations: bool,
 
@@ -2719,6 +2759,7 @@ impl Default for TargetOptions {
             mcount: "mcount".into(),
             llvm_mcount_intrinsic: None,
             llvm_abiname: "".into(),
+            llvm_floatabi: None,
             relax_elf_relocations: false,
             llvm_args: cvs![],
             use_ctors_section: false,