Auto merge of #136146 - RalfJung:x86-abi, r=workingjubilee

Explicitly choose x86 softfloat/hardfloat ABI

Part of https://github.com/rust-lang/rust/pull/135408:
Instead of choosing this based on the target features listed in the target spec, make that choice explicit.
All built-in targets are being updated here; custom (JSON-defined) x86 (32bit and 64bit) softfloat targets need to explicitly set `rustc-abi` to `x86-softfloat`.

compiler/rustc_target/src/spec/mod.rs

@@ -1114,6 +1114,33 @@ impl ToJson for FloatAbi {
     }
 }
 
+/// The Rustc-specific variant of the ABI used for this target.
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum RustcAbi {
+    /// On x86-32/64 only: do not use any FPU or SIMD registers for the ABI.
+    X86Softfloat,
+}
+
+impl FromStr for RustcAbi {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<RustcAbi, ()> {
+        Ok(match s {
+            "x86-softfloat" => RustcAbi::X86Softfloat,
+            _ => return Err(()),
+        })
+    }
+}
+
+impl ToJson for RustcAbi {
+    fn to_json(&self) -> Json {
+        match *self {
+            RustcAbi::X86Softfloat => "x86-softfloat",
+        }
+        .to_json()
+    }
+}
+
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
 pub enum TlsModel {
     GeneralDynamic,
@@ -2505,6 +2532,12 @@ pub struct TargetOptions {
     /// If not provided, LLVM will infer the float ABI from the target triple (`llvm_target`).
     pub llvm_floatabi: Option<FloatAbi>,
 
+    /// Picks a specific ABI for this target. This is *not* just for "Rust" ABI functions,
+    /// it can also affect "C" ABI functions; the point is that this flag is interpreted by
+    /// rustc and not forwarded to LLVM.
+    /// So far, this is only used on x86.
+    pub rustc_abi: Option<RustcAbi>,
+
     /// Whether or not RelaxElfRelocation flag will be passed to the linker
     pub relax_elf_relocations: bool,
 
@@ -2664,10 +2697,6 @@ impl TargetOptions {
                 .collect();
         }
     }
-
-    pub(crate) fn has_feature(&self, search_feature: &str) -> bool {
-        self.features.split(',').any(|f| f.strip_prefix('+').is_some_and(|f| f == search_feature))
-    }
 }
 
 impl Default for TargetOptions {
@@ -2774,6 +2803,7 @@ impl Default for TargetOptions {
             llvm_mcount_intrinsic: None,
             llvm_abiname: "".into(),
             llvm_floatabi: None,
+            rustc_abi: None,
             relax_elf_relocations: false,
             llvm_args: cvs![],
             use_ctors_section: false,
@@ -3240,6 +3270,17 @@ impl Target {
             _ => {}
         }
 
+        // Check consistency of Rust ABI declaration.
+        if let Some(rust_abi) = self.rustc_abi {
+            match rust_abi {
+                RustcAbi::X86Softfloat => check_matches!(
+                    &*self.arch,
+                    "x86" | "x86_64",
+                    "`x86-softfloat` ABI is only valid for x86 targets"
+                ),
+            }
+        }
+
         // Check that the given target-features string makes some basic sense.
         if !self.features.is_empty() {
             let mut features_enabled = FxHashSet::default();