Introduce a test for the i128 calling convention on Windows

Currently we both pass and return `i128` indirectly on Windows for MSVC
and MinGW, but this will be adjusted. Introduce a test verifying the
current state.

tests/codegen/i128-x86-callconv.rs

@@ -0,0 +1,79 @@
+//! Verify that Rust implements the expected calling convention for `i128`/`u128`.
+
+//@ revisions: MSVC MINGW
+//@ add-core-stubs
+//@ [MSVC] needs-llvm-components: x86
+//@ [MINGW] needs-llvm-components: x86
+//@ [MSVC] compile-flags: --target x86_64-pc-windows-msvc
+//@ [MINGW] compile-flags: --target x86_64-pc-windows-gnu
+//@ [MSVC] filecheck-flags: --check-prefix=WIN
+//@ [MINGW] filecheck-flags: --check-prefix=WIN
+
+#![crate_type = "lib"]
+#![no_std]
+#![no_core]
+#![feature(no_core, lang_items)]
+
+extern crate minicore;
+
+extern "C" {
+    fn extern_call(arg0: i128);
+    fn extern_ret() -> i128;
+}
+
+#[no_mangle]
+pub extern "C" fn pass(_arg0: u32, arg1: i128) {
+    // CHECK-LABEL: @pass(
+    // i128 is passed indirectly on Windows. It should load the pointer to the stack and pass
+    // a pointer to that allocation.
+    // WIN-SAME: %_arg0, ptr{{.*}} %arg1)
+    // WIN: [[PASS:%[_0-9]+]] = alloca [16 x i8], align 16
+    // WIN: [[LOADED:%[_0-9]+]] = load i128, ptr %arg1
+    // WIN: store i128 [[LOADED]], ptr [[PASS]]
+    // WIN: call void @extern_call
+    unsafe { extern_call(arg1) };
+}
+
+// Check that we produce the correct return ABI
+#[no_mangle]
+pub extern "C" fn ret(_arg0: u32, arg1: i128) -> i128 {
+    // CHECK-LABEL: @ret(
+    // i128 is returned on the stack on Windows.
+    // FIXME: this ABI does not agree with Clang or MinGW GCC
+    // WIN-SAME: ptr{{.*}} sret([16 x i8]){{.*}} [[RET:%_[0-9]+]], i32{{.*}} %_arg0, ptr{{.*}} %arg1)
+    // WIN: [[LOADED:%[0-9]+]] = load i128, ptr %arg1
+    // WIN: store i128 [[LOADED]], ptr [[RET]]
+    // WIN: ret void
+    arg1
+}
+
+// Check that we consume the correct return ABI
+#[no_mangle]
+pub extern "C" fn forward(dst: *mut i128) {
+    // CHECK-LABEL: @forward
+    // WIN-SAME: ptr{{.*}} %dst)
+    // WIN: [[RETURNED:%[_0-9]+]] = alloca [16 x i8], align 16
+    // WIN: call void @extern_ret({{.*}} [[RETURNED]])
+    // WIN: [[TMP:%[_0-9]+]] = load i128, ptr [[RETURNED]]
+    // WIN: store i128 [[TMP]], ptr %dst
+    // WIN: ret void
+    unsafe { *dst = extern_ret() };
+}
+
+#[repr(C)]
+struct RetAggregate {
+    a: i32,
+    b: i128,
+}
+
+#[no_mangle]
+pub extern "C" fn ret_aggregate(_arg0: u32, arg1: i128) -> RetAggregate {
+    // CHECK-LABEL: @ret_aggregate(
+    // Aggregates should also be returned indirectly
+    // WIN-SAME: ptr{{.*}}sret([32 x i8]){{.*}}[[RET:%[_0-9]+]], i32{{.*}}%_arg0, ptr{{.*}}%arg1)
+    // WIN: [[LOADED:%[_0-9]+]] = load i128, ptr %arg1
+    // WIN: [[GEP:%[_0-9]+]] = getelementptr{{.*}}, ptr [[RET]]
+    // WIN: store i128 [[LOADED]], ptr [[GEP]]
+    // WIN: ret void
+    RetAggregate { a: 1, b: arg1 }
+}