aarch64 softfloat target: always pass floats in int registers

Author: RalfJung

compiler/rustc_target/src/callconv/aarch64.rs

@@ -1,5 +1,10 @@
+use std::iter;
+
+use rustc_abi::{BackendRepr, Primitive};
+
 use crate::abi::call::{ArgAbi, FnAbi, Reg, RegKind, Uniform};
 use crate::abi::{HasDataLayout, TyAbiInterface};
+use crate::spec::{HasTargetSpec, Target};
 
 /// Indicates the variant of the AArch64 ABI we are compiling for.
 /// Used to accommodate Apple and Microsoft's deviations from the usual AAPCS ABI.
@@ -35,10 +40,25 @@ where
     })
 }
 
+fn softfloat_float_abi<Ty>(target: &Target, arg: &mut ArgAbi<'_, Ty>) {
+    if let BackendRepr::Scalar(s) = arg.layout.backend_repr
+        && let Primitive::Float(f) = s.primitive()
+    {
+        if target.abi == "softfloat" {
+            // Do *not* use the floag registers for passing arguments, as that would make
+            // the ABI depend on whether `neon` instructions are enabled.
+            // Apparently there is no standard ABI here [1], so we can do whatever we want.
+            // We choose to pass floats via equal-sized integer registers.
+            // [1]: https://github.com/rust-lang/rust/issues/131058#issuecomment-2384960972
+            arg.cast_to(Reg { kind: RegKind::Integer, size: f.size() });
+        }
+    }
+}
+
 fn classify_ret<'a, Ty, C>(cx: &C, ret: &mut ArgAbi<'a, Ty>, kind: AbiKind)
 where
     Ty: TyAbiInterface<'a, C> + Copy,
-    C: HasDataLayout,
+    C: HasDataLayout + HasTargetSpec,
 {
     if !ret.layout.is_sized() {
         // Not touching this...
@@ -51,6 +71,7 @@ where
             // See also: <https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Pass-Arguments-to-Functions-Correctly>
             ret.extend_integer_width_to(32)
         }
+        softfloat_float_abi(cx.target_spec(), ret);
         return;
     }
     if let Some(uniform) = is_homogeneous_aggregate(cx, ret) {
@@ -69,7 +90,7 @@ where
 fn classify_arg<'a, Ty, C>(cx: &C, arg: &mut ArgAbi<'a, Ty>, kind: AbiKind)
 where
     Ty: TyAbiInterface<'a, C> + Copy,
-    C: HasDataLayout,
+    C: HasDataLayout + HasTargetSpec,
 {
     if !arg.layout.is_sized() {
         // Not touching this...
@@ -82,6 +103,8 @@ where
             // See also: <https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Pass-Arguments-to-Functions-Correctly>
             arg.extend_integer_width_to(32);
         }
+        softfloat_float_abi(cx.target_spec(), arg);
+
         return;
     }
     if let Some(uniform) = is_homogeneous_aggregate(cx, arg) {
@@ -112,7 +135,7 @@ where
 pub(crate) fn compute_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>, kind: AbiKind)
 where
     Ty: TyAbiInterface<'a, C> + Copy,
-    C: HasDataLayout,
+    C: HasDataLayout + HasTargetSpec,
 {
     if !fn_abi.ret.is_ignore() {
         classify_ret(cx, &mut fn_abi.ret, kind);
@@ -125,3 +148,13 @@ where
         classify_arg(cx, arg, kind);
     }
 }
+
+pub(crate) fn compute_rust_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+    C: HasDataLayout + HasTargetSpec,
+{
+    for arg in fn_abi.args.iter_mut().chain(iter::once(&mut fn_abi.ret)) {
+        softfloat_float_abi(cx.target_spec(), arg);
+    }
+}

compiler/rustc_target/src/callconv/mod.rs

@@ -738,6 +738,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "x86" => x86::compute_rust_abi_info(cx, self, abi),
             "riscv32" | "riscv64" => riscv::compute_rust_abi_info(cx, self, abi),
             "loongarch64" => loongarch::compute_rust_abi_info(cx, self, abi),
+            "aarch64" => aarch64::compute_rust_abi_info(cx, self),
             _ => {}
         };
 

tests/codegen/aarch64-softfloat.rs

@@ -0,0 +1,23 @@
+//@ compile-flags: --target aarch64-unknown-none-softfloat
+//@ needs-llvm-components: aarch64
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+#[lang = "copy"]
+trait Copy {}
+impl Copy for f64 {}
+
+// CHECK: i64 @pass_f64_C(i64
+#[no_mangle]
+extern "C" fn pass_f64_C(x: f64) -> f64 {
+    x
+}
+
+// CHECK: i64 @pass_f64_Rust(i64
+#[no_mangle]
+fn pass_f64_Rust(x: f64, _y: f64) -> f64 {
+    x
+}

tests/ui/abi/aarch64-softfloat.rs

@@ -0,0 +1,5 @@
+//@ compile-flags: --target aarch64-unknown-linux-gnu
+//@ needs-llvm-components: aarch64
+#![feature(no_core, lang_items)]
+#![no_core]
+