add rustc_abi to control ABI decisions LLVM does not have flags for, and use it for x86 softfloat

Author: RalfJung

compiler/rustc_target/src/spec/json.rs

@@ -128,6 +128,19 @@ impl Target {
                     Some(Ok(()))
                 })).unwrap_or(Ok(()))
             } );
+            ($key_name:ident, RustcAbi) => ( {
+                let name = (stringify!($key_name)).replace("_", "-");
+                obj.remove(&name).and_then(|o| o.as_str().and_then(|s| {
+                    match s.parse::<super::RustcAbi>() {
+                        Ok(rustc_abi) => base.$key_name = Some(rustc_abi),
+                        _ => return Some(Err(format!(
+                            "'{s}' is not a valid value for rustc-abi. \
+                            Use 'x86-softfloat' or leave the field unset."
+                        ))),
+                    }
+                    Some(Ok(()))
+                })).unwrap_or(Ok(()))
+            } );
             ($key_name:ident, RelocModel) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
                 obj.remove(&name).and_then(|o| o.as_str().and_then(|s| {
@@ -611,6 +624,7 @@ impl Target {
         key!(llvm_mcount_intrinsic, optional);
         key!(llvm_abiname);
         key!(llvm_floatabi, FloatAbi)?;
+        key!(rustc_abi, RustcAbi)?;
         key!(relax_elf_relocations, bool);
         key!(llvm_args, list);
         key!(use_ctors_section, bool);
@@ -786,6 +800,7 @@ impl ToJson for Target {
         target_option_val!(llvm_mcount_intrinsic);
         target_option_val!(llvm_abiname);
         target_option_val!(llvm_floatabi);
+        target_option_val!(rustc_abi);
         target_option_val!(relax_elf_relocations);
         target_option_val!(llvm_args);
         target_option_val!(use_ctors_section);

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,
@@ -2502,6 +2529,12 @@ 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,
 
@@ -2661,10 +2694,6 @@ 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 {
@@ -2770,6 +2799,7 @@ 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,
@@ -3236,6 +3266,17 @@ 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();

compiler/rustc_target/src/spec/targets/i686_unknown_uefi.rs

@@ -5,7 +5,7 @@
 // The cdecl ABI is used. It differs from the stdcall or fastcall ABI.
 // "i686-unknown-windows" is used to get the minimal subset of windows-specific features.
 
-use crate::spec::{Target, base};
+use crate::spec::{RustcAbi, Target, base};
 
 pub(crate) fn target() -> Target {
     let mut base = base::uefi_msvc::opts();
@@ -22,6 +22,7 @@ pub(crate) fn target() -> Target {
     // If you initialize FP units yourself, you can override these flags with custom linker
     // arguments, thus giving you access to full MMX/SSE acceleration.
     base.features = "-mmx,-sse,+soft-float".into();
+    base.rustc_abi = Some(RustcAbi::X86Softfloat);
 
     // Use -GNU here, because of the reason below:
     // Background and Problem:

compiler/rustc_target/src/spec/targets/x86_64_unknown_none.rs

@@ -5,8 +5,8 @@
 // features.
 
 use crate::spec::{
-    Cc, CodeModel, LinkerFlavor, Lld, PanicStrategy, RelroLevel, SanitizerSet, StackProbeType,
-    Target, TargetOptions,
+    Cc, CodeModel, LinkerFlavor, Lld, PanicStrategy, RelroLevel, RustcAbi, SanitizerSet,
+    StackProbeType, Target, TargetOptions,
 };
 
 pub(crate) fn target() -> Target {
@@ -20,6 +20,7 @@ pub(crate) fn target() -> Target {
         relro_level: RelroLevel::Full,
         linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
         linker: Some("rust-lld".into()),
+        rustc_abi: Some(RustcAbi::X86Softfloat),
         features: "-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2,+soft-float".into(),
         supported_sanitizers: SanitizerSet::KCFI | SanitizerSet::KERNELADDRESS,
         disable_redzone: true,

compiler/rustc_target/src/spec/targets/x86_64_unknown_uefi.rs

@@ -6,7 +6,7 @@
 // LLVM. "x86_64-unknown-windows" is used to get the minimal subset of windows-specific features.
 
 use crate::abi::call::Conv;
-use crate::spec::{Target, base};
+use crate::spec::{RustcAbi, Target, base};
 
 pub(crate) fn target() -> Target {
     let mut base = base::uefi_msvc::opts();
@@ -26,6 +26,7 @@ pub(crate) fn target() -> Target {
     // If you initialize FP units yourself, you can override these flags with custom linker
     // arguments, thus giving you access to full MMX/SSE acceleration.
     base.features = "-mmx,-sse,+soft-float".into();
+    base.rustc_abi = Some(RustcAbi::X86Softfloat);
 
     Target {
         llvm_target: "x86_64-unknown-windows".into(),

compiler/rustc_target/src/target_features.rs

@@ -5,7 +5,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_span::{Symbol, sym};
 
-use crate::spec::{FloatAbi, Target};
+use crate::spec::{FloatAbi, RustcAbi, Target};
 
 /// Features that control behaviour of rustc, rather than the codegen.
 /// These exist globally and are not in the target-specific lists below.
@@ -417,7 +417,9 @@ const X86_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("sha512", Unstable(sym::sha512_sm_x86), &["avx2"]),
     ("sm3", Unstable(sym::sha512_sm_x86), &["avx"]),
     ("sm4", Unstable(sym::sha512_sm_x86), &["avx2"]),
-    ("soft-float", Stability::Forbidden { reason: "unsound because it changes float ABI" }, &[]),
+    // This cannot actually be toggled, the ABI always fixes it, so it'd make little sense to
+    // stabilize. It must be in this list for the ABI check to be able to use it.
+    ("soft-float", Stability::Unstable(sym::x87_target_feature), &[]),
     ("sse", Stable, &[]),
     ("sse2", Stable, &["sse"]),
     ("sse3", Stable, &["sse2"]),
@@ -768,23 +770,41 @@ impl Target {
         // questions "which ABI is used".
         match &*self.arch {
             "x86" => {
-                // We support 2 ABIs, hardfloat (default) and softfloat.
-                // x86 has no sane ABI indicator so we have to use the target feature.
-                if self.has_feature("soft-float") {
-                    NOTHING
-                } else {
-                    // Hardfloat ABI. x87 must be enabled.
-                    FeatureConstraints { required: &["x87"], incompatible: &[] }
+                // We use our own ABI indicator here; LLVM does not have anything native.
+                // Every case should require or forbid `soft-float`!
+                match self.rustc_abi {
+                    None => {
+                        // Default hardfloat ABI.
+                        // x87 must be enabled, soft-float must be disabled.
+                        FeatureConstraints { required: &["x87"], incompatible: &["soft-float"] }
+                    }
+                    Some(RustcAbi::X86Softfloat) => {
+                        // Softfloat ABI, requires corresponding target feature. That feature trumps
+                        // `x87` and all other FPU features so those do not matter.
+                        // Note that this one requirement is the entire implementation of the ABI!
+                        // LLVM handles the rest.
+                        FeatureConstraints { required: &["soft-float"], incompatible: &[] }
+                    }
                 }
             }
             "x86_64" => {
-                // We support 2 ABIs, hardfloat (default) and softfloat.
-                // x86 has no sane ABI indicator so we have to use the target feature.
-                if self.has_feature("soft-float") {
-                    NOTHING
-                } else {
-                    // Hardfloat ABI. x87 and SSE2 must be enabled.
-                    FeatureConstraints { required: &["x87", "sse2"], incompatible: &[] }
+                // We use our own ABI indicator here; LLVM does not have anything native.
+                // Every case should require or forbid `soft-float`!
+                match self.rustc_abi {
+                    None => {
+                        // Default hardfloat ABI. On x86-64, this always includes SSE2.
+                        FeatureConstraints {
+                            required: &["x87", "sse2"],
+                            incompatible: &["soft-float"],
+                        }
+                    }
+                    Some(RustcAbi::X86Softfloat) => {
+                        // Softfloat ABI, requires corresponding target feature. That feature trumps
+                        // `x87` and all other FPU features so those do not matter.
+                        // Note that this one requirement is the entire implementation of the ABI!
+                        // LLVM handles the rest.
+                        FeatureConstraints { required: &["soft-float"], incompatible: &[] }
+                    }
                 }
             }
             "arm" => {

tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.rs

@@ -1,3 +1,4 @@
+//! Ensure ABI-incompatible features cannot be enabled via `#[target_feature]`.
 //@ compile-flags: --target=riscv32e-unknown-none-elf --crate-type=lib
 //@ needs-llvm-components: riscv
 #![feature(no_core, lang_items, riscv_target_feature)]

tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.stderr

@@ -1,5 +1,5 @@
 error: target feature `d` cannot be enabled with `#[target_feature]`: this feature is incompatible with the target ABI
-  --> $DIR/forbidden-hardfloat-target-feature-attribute.rs:9:18
+  --> $DIR/forbidden-hardfloat-target-feature-attribute.rs:10:18
    |
 LL | #[target_feature(enable = "d")]
    |                  ^^^^^^^^^^^^

tests/ui/target-feature/forbidden-hardfloat-target-feature-cfg.rs

@@ -1,8 +1,11 @@
+//! Ensure that if disabling a target feature implies disabling an ABI-required target feature,
+//! we complain.
 //@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
 //@ needs-llvm-components: x86
 //@ compile-flags: -Ctarget-feature=-sse
 // For now this is just a warning.
 //@ build-pass
+//@error-pattern: must be enabled to ensure that the ABI
 #![feature(no_core, lang_items)]
 #![no_core]