Auto merge of #144379 - folkertdev:c-variadic-same-program-multiple-abis, r=<try>

test using multiple c-variadic ABIs in the same program

try-job: `x86_64-gnu`
try-job: `x86_64-msvc-*`
try-job: `x86_64-apple-2`

Author: rust-bors[bot]

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

@@ -0,0 +1,112 @@
+#![feature(extended_varargs_abi_support)]
+//@ run-pass
+//@ only-x86_64
+
+// 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 "sysv64" {
+    fn variadic_sysv64(_: u32, _: ...) -> u32;
+}
+
+extern "win64" {
+    fn variadic_win64(_: u32, _: ...) -> u32;
+}
+
+fn main() {
+    unsafe {
+        assert_eq!(variadic_win64(1, 2, 3), 1 + 2 + 3);
+        assert_eq!(variadic_sysv64(1, 2, 3), 1 + 2 + 3);
+    }
+}
+
+// This assembly was generated using https://godbolt.org/z/dbTGanoh6, and corresponds to the
+// following code compiled for the `x86_64-unknown-linux-gnu` and `x86_64-pc-windows-gnu`
+// targets, respectively:
+//
+// ```rust
+// #![feature(c_variadic)]
+//
+// #[unsafe(no_mangle)]
+// unsafe extern "C" fn variadic(a: u32, mut args: ...) -> u32 {
+//     let b = args.arg::<u32>();
+//     let c = args.arg::<u32>();
+//
+//     a + b + c
+// }
+// ```
+core::arch::global_asm!(
+    r#"
+{variadic_sysv64}:
+        sub     rsp, 88
+        test    al, al
+        je      .LBB0_7
+        movaps  xmmword ptr [rsp - 48], xmm0
+        movaps  xmmword ptr [rsp - 32], xmm1
+        movaps  xmmword ptr [rsp - 16], xmm2
+        movaps  xmmword ptr [rsp], xmm3
+        movaps  xmmword ptr [rsp + 16], xmm4
+        movaps  xmmword ptr [rsp + 32], xmm5
+        movaps  xmmword ptr [rsp + 48], xmm6
+        movaps  xmmword ptr [rsp + 64], xmm7
+.LBB0_7:
+        mov     qword ptr [rsp - 88], rsi
+        mov     qword ptr [rsp - 80], rdx
+        mov     qword ptr [rsp - 72], rcx
+        mov     qword ptr [rsp - 64], r8
+        mov     qword ptr [rsp - 56], r9
+        movabs  rax, 206158430216
+        mov     qword ptr [rsp - 120], rax
+        lea     rax, [rsp + 96]
+        mov     qword ptr [rsp - 112], rax
+        lea     rax, [rsp - 96]
+        mov     qword ptr [rsp - 104], rax
+        mov     edx, 8
+        cmp     rdx, 41
+        jae     .LBB0_1
+        mov     rax, qword ptr [rsp - 104]
+        mov     ecx, 8
+        add     rcx, 8
+        mov     dword ptr [rsp - 120], ecx
+        mov     eax, dword ptr [rax + rdx]
+        cmp     edx, 32
+        ja      .LBB0_2
+        add     rcx, qword ptr [rsp - 104]
+        add     edx, 16
+        mov     dword ptr [rsp - 120], edx
+        add     eax, edi
+        add     eax, dword ptr [rcx]
+        add     rsp, 88
+        ret
+.LBB0_1:
+        mov     rax, qword ptr [rsp - 112]
+        lea     rcx, [rax + 8]
+        mov     qword ptr [rsp - 112], rcx
+        mov     eax, dword ptr [rax]
+.LBB0_2:
+        mov     rcx, qword ptr [rsp - 112]
+        lea     rdx, [rcx + 8]
+        mov     qword ptr [rsp - 112], rdx
+        add     eax, edi
+        add     eax, dword ptr [rcx]
+        add     rsp, 88
+        ret
+
+{variadic_win64}:
+        push    rax
+        mov     qword ptr [rsp + 40], r9
+        mov     qword ptr [rsp + 24], rdx
+        mov     qword ptr [rsp + 32], r8
+        lea     rax, [rsp + 40]
+        mov     qword ptr [rsp], rax
+        lea     eax, [rdx + rcx]
+        add     eax, r8d
+        pop     rcx
+        ret
+    "#,
+    variadic_win64 = sym variadic_win64,
+    variadic_sysv64 = sym variadic_sysv64,
+);