Rollup merge of rust-lang#144541 - folkertdev:c-variadic-same-program-multiple-abis-arm, r=RalfJung,davidtwco

c-variadic: multiple ABIs in the same program for arm

similar to rust-lang#144379, but for arm, requested in rust-lang#144066.

Quoting rust-lang/reference#1946 (comment)

> `"aapcs"` specifically refers to the soft-float ABI where floating-point values are passed in integer registers.

However for c-variadic functions, `aapcs` behaves the same as `C`:

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#65parameter-passing

> A variadic function is always marshaled as for the base standard.

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#7the-standard-variants

> This section applies only to non-variadic functions. For a variadic function the base standard is always used both for argument passing and result return.

---

I also noticed that rustc currently emit more instructions than clang for c-variadic functions on arm, see https://godbolt.org/z/hMce9rnTh. I'll fix that separately. (edit: rust-lang#144549)

try-job: armhf-gnu
r? `@RalfJung`

Author: jhpratt

src/tools/compiletest/src/directives/directive_names.rs

@@ -194,6 +194,7 @@ pub(crate) const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "only-bpf",
     "only-cdb",
     "only-dist",
+    "only-eabihf",
     "only-elf",
     "only-emscripten",
     "only-gnu",

tests/ui/c-variadic/same-program-multiple-abis-arm.rs

@@ -0,0 +1,77 @@
+#![feature(extended_varargs_abi_support)]
+//@ run-pass
+//@ only-arm
+//@ ignore-thumb (this test uses arm assembly)
+//@ only-eabihf (the assembly below requires float hardware support)
+
+// Check that multiple c-variadic calling conventions can be used in the same program.
+//
+// Clang and gcc reject defining functions with a non-default calling convention and a variable
+// argument list, so C programs that use multiple c-variadic calling conventions are unlikely
+// to come up. Here we validate that our codegen backends do in fact generate correct code.
+
+extern "C" {
+    fn variadic_c(_: f64, _: ...) -> f64;
+}
+
+extern "aapcs" {
+    fn variadic_aapcs(_: f64, _: ...) -> f64;
+}
+
+fn main() {
+    unsafe {
+        assert_eq!(variadic_c(1.0, 2.0, 3.0), 1.0 + 2.0 + 3.0);
+        assert_eq!(variadic_aapcs(1.0, 2.0, 3.0), 1.0 + 2.0 + 3.0);
+    }
+}
+
+// This assembly was generated using https://godbolt.org/z/xcW6a1Tj5, and corresponds to the
+// following code compiled for the `armv7-unknown-linux-gnueabihf` target:
+//
+// ```rust
+// #![feature(c_variadic)]
+//
+// #[unsafe(no_mangle)]
+// unsafe extern "C" fn variadic(a: f64, mut args: ...) -> f64 {
+//     let b = args.arg::<f64>();
+//     let c = args.arg::<f64>();
+//
+//     a + b + c
+// }
+// ```
+//
+// This function uses floats (and passes one normal float argument) because the aapcs and C calling
+// conventions differ in how floats are passed, e.g. https://godbolt.org/z/sz799f51x. However, for
+// c-variadic functions, both ABIs actually behave the same, based on:
+//
+// https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#65parameter-passing
+//
+// > A variadic function is always marshaled as for the base standard.
+//
+// https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#7the-standard-variants
+//
+// > This section applies only to non-variadic functions. For a variadic function the base standard
+// > is always used both for argument passing and result return.
+core::arch::global_asm!(
+    r#"
+{variadic_c}:
+{variadic_aapcs}:
+        sub     sp, sp, #12
+        stmib   sp, {{r2, r3}}
+        vmov    d0, r0, r1
+        add     r0, sp, #4
+        vldr    d1, [sp, #4]
+        add     r0, r0, #15
+        bic     r0, r0, #7
+        vadd.f64        d0, d0, d1
+        add     r1, r0, #8
+        str     r1, [sp]
+        vldr    d1, [r0]
+        vadd.f64        d0, d0, d1
+        vmov    r0, r1, d0
+        add     sp, sp, #12
+        bx      lr
+    "#,
+    variadic_c = sym variadic_c,
+    variadic_aapcs = sym variadic_aapcs,
+);