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

[Tobias Hieta via cfe-commits]

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

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

>From 22f627cac7b2ea08a5569bb25d9c1d054b3a7cae Mon Sep 17 00:00:00 2001
From: Tobias Hieta <tobias.hieta at ubisoft.com>
Date: Tue, 16 Apr 2024 09:38:53 +0200
Subject: [PATCH] [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
---
 clang/docs/ReleaseNotes.rst                |  4 ++++
 clang/lib/CodeGen/MicrosoftCXXABI.cpp      |  5 +++++
 clang/test/CodeGenCXX/trivial_abi_msvc.cpp | 21 +++++++++++++++++++++
 3 files changed, 30 insertions(+)
 create mode 100644 clang/test/CodeGenCXX/trivial_abi_msvc.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 76701dc723b6c3..c5241403711726 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ----------------------------------------
 
diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index d38a26940a3cb6..b930913badcd3f 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/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().)
diff --git a/clang/test/CodeGenCXX/trivial_abi_msvc.cpp b/clang/test/CodeGenCXX/trivial_abi_msvc.cpp
new file mode 100644
index 00000000000000..8826a8224c6671
--- /dev/null
+++ b/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 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;
+}
+