[clang] [clang] Handle trivial_abi attribute for Microsoft ABI. (PR #88857)

[Tobias Hieta via cfe-commits]

================
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -triple x86_64-pc-windows-msvc -std=c++11 -fcxx-exceptions -fexceptions -emit-llvm -o - %s | FileCheck %s
+
+// CHECK: %[[STRUCT_TRIVIAL:.*]] = type { ptr }
+struct __attribute__((trivial_abi)) Trivial {
+  int *p;
+  Trivial() : p(0) {}
+  Trivial(const Trivial &) noexcept = default;
+};
+
+// CHECK-LABEL: define{{.*}} i64 @"?retTrivial@@YA?AUTrivial@@XZ"(
+// CHECK: %retval = alloca %[[STRUCT_TRIVIAL]], align 8
+// CHECK: %call = call noundef ptr @"??0Trivial@@QEAA at XZ"(ptr noundef nonnull align 8 dereferenceable(8) %retval)
+// CHECK: %coerce.dive = getelementptr inbounds %[[STRUCT_TRIVIAL]], ptr %retval, i32 0, i32 0
+// CHECK: %0 = load ptr, ptr %coerce.dive, align 8
+// CHECK: %coerce.val.pi = ptrtoint ptr %0 to i64
+// CHECK: ret i64 %coerce.val.pi
+Trivial retTrivial() {
+  Trivial s;
+  return s;
+}
+
----------------
tru wrote:

I checked your example there and the instanceMethod is no problem, but I noticed that the staticMethod doesn't contain a `sret` return value with this patch. Since we are not passing `this` to it - I think this is expected, but just want to make sure.

```
; Function Attrs: mustprogress noinline optnone uwtable
define dso_local i64 @"?staticMethod at TrivialInstance@@SA?AUTrivial@@XZ"() #0 align 2 {
entry:
  %retval = alloca %struct.Trivial, align 8
  %call = call noundef ptr @"??0Trivial@@QEAA at XZ"(ptr noundef nonnull align 8 dereferenceable(8) %retval)
  %coerce.dive = getelementptr inbounds %struct.Trivial, ptr %retval, i32 0, i32 0
  %0 = load ptr, ptr %coerce.dive, align 8
  %coerce.val.pi = ptrtoint ptr %0 to i64
  ret i64 %coerce.val.pi
}
```

https://github.com/llvm/llvm-project/pull/88857