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

[Reid Kleckner 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;
+}
+
----------------
rnk wrote:

For completeness, please add the instance and static method return cases, since those are indirect for other reasons. For me, it's sufficient to check the function prototype, both of which should have an sret pointer:
https://godbolt.org/z/fn7vrGrKh

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