[clang] Handle trivial_abi attribute for Microsoft ABI.

Previously the trivial_abi was ignored for records when targetting
the microsoft abi, the MSVC rules where always enforced to ensure
compatibility with MSVC. This commit changes it to be closer to
the itanium abi when a record is marked with the trivial_abi attribute.

Fixes #87993

Author: tru

clang/docs/ReleaseNotes.rst

@@ -63,6 +63,10 @@ ABI Changes in This Version
   MSVC uses a different mangling for these objects, compatibility is not affected.
   (#GH85423).
 
+- The attribute ``trivial_abi`` now works when targetting the Microsoft ABI. Marking
+  a struct with this attribute will now cause clang to emit ABI similar to the Itanium
+  ABI. This is not compatible with the Microsoft ABI. (#GH87993).
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -1105,6 +1105,11 @@ bool MicrosoftCXXABI::hasMostDerivedReturn(GlobalDecl GD) const {
 
 static bool isTrivialForMSVC(const CXXRecordDecl *RD, QualType Ty,
                              CodeGenModule &CGM) {
+  // If the record is marked with the trivial_abi attribute, we don't
+  // have to conform to the standard MSVC ABI.
+  if (RD->hasAttr<TrivialABIAttr>())
+    return true;
+
   // On AArch64, HVAs that can be passed in registers can also be returned
   // in registers. (Note this is using the MSVC definition of an HVA; see
   // isPermittedToBeHomogeneousAggregate().)

clang/test/CodeGenCXX/trivial_abi_msvc.cpp

@@ -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@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;
+}
+