Rollup merge of #133102 - RalfJung:aarch64-softfloat, r=davidtwco,wesleywiser

aarch64 softfloat target: always pass floats in int registers

This is a part of https://github.com/rust-lang/rust/issues/131058: on softfloat aarch64 targets, the float registers may be unavailable. And yet, LLVM will happily use them to pass float types if the corresponding target features are enabled. That's a problem as it means enabling/disabling `neon` instructions can change the ABI.

Other targets have a `soft-float` target feature that forces the use of the soft-float ABI no matter whether float registers are enabled or not; aarch64 has nothing like that.

So we follow the aarch64 [softfloat ABI](https://github.com/rust-lang/rust/issues/131058#issuecomment-2385027423) and treat floats like integers for `extern "C"` functions. For the "Rust" ABI, we do the same for scalars, and then just do something reasonable for ScalarPair that avoids the pointer indirection.

Cc ```@workingjubilee```

1 files changed, 48 insertions, 0 deletions

diff --git a/tests/codegen/aarch64-softfloat.rs b/tests/codegen/aarch64-softfloat.rs
new file mode 100644
index 00000000000..14d0054f80c
--- /dev/null
+++ b/tests/codegen/aarch64-softfloat.rs
@@ -0,0 +1,48 @@
+//@ compile-flags: --target aarch64-unknown-none-softfloat -Zmerge-functions=disabled
+//@ 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 f32 {}
+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_f32_pair_C(i64 {{[^,]*}})
+#[no_mangle]
+extern "C" fn pass_f32_pair_C(x: (f32, f32)) -> (f32, f32) {
+    x
+}
+
+// CHECK: [2 x i64] @pass_f64_pair_C([2 x i64] {{[^,]*}})
+#[no_mangle]
+extern "C" fn pass_f64_pair_C(x: (f64, f64)) -> (f64, f64) {
+    x
+}
+
+// CHECK: i64 @pass_f64_Rust(i64 {{[^,]*}})
+#[no_mangle]
+fn pass_f64_Rust(x: f64) -> f64 {
+    x
+}
+
+// CHECK: i64 @pass_f32_pair_Rust(i64 {{[^,]*}})
+#[no_mangle]
+fn pass_f32_pair_Rust(x: (f32, f32)) -> (f32, f32) {
+    x
+}
+
+// CHECK: void @pass_f64_pair_Rust(ptr {{[^,]*}}, ptr {{[^,]*}})
+#[no_mangle]
+fn pass_f64_pair_Rust(x: (f64, f64)) -> (f64, f64) {
+    x
+}