[clang] Return larger CXX records in memory (#120670)

We incorrectly return CXX records in AVX registers when they should be
returned in memory. This is violation of x86-64 psABI.

Detailed discussion is here:
https://groups.google.com/g/x86-64-abi/c/BjOOyihHuqg/m/KurXdUcWAgAJ

clang/docs/ReleaseNotes.rst

@@ -45,6 +45,8 @@ C++ Specific Potentially Breaking Changes
 ABI Changes in This Version
 ---------------------------
 
+- Return larger CXX records in memory instead of using AVX registers. Code compiled with older clang will be incompatible with newer version of the clang unless -fclang-abi-compat=20 is provided. (#GH120670)
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 

clang/include/clang/Basic/LangOptions.h

@@ -250,6 +250,11 @@ public:
     ///   passing them as if they had a size of 1 byte.
     Ver19,
 
+    /// Attempt to be ABI-compatible with code generated by Clang 20.0.x.
+    /// This causes clang to:
+    ///   - Incorrectly return C++ records in AVX registers on x86_64.
+    Ver20,
+
     /// Conform to the underlying platform's C and C++ ABIs as closely
     /// as we can.
     Latest

clang/lib/CodeGen/Targets/X86.cpp

@@ -1334,6 +1334,15 @@ class X86_64ABIInfo : public ABIInfo {
     return T.isOSLinux() || T.isOSNetBSD();
   }
 
+  bool returnCXXRecordGreaterThan128InMem() const {
+    // Clang <= 20.0 did not do this.
+    if (getContext().getLangOpts().getClangABICompat() <=
+        LangOptions::ClangABI::Ver20)
+      return false;
+
+    return true;
+  }
+
   X86AVXABILevel AVXLevel;
   // Some ABIs (e.g. X32 ABI and Native Client OS) use 32 bit pointers on
   // 64-bit hardware.
@@ -2067,6 +2076,13 @@ void X86_64ABIInfo::classify(QualType Ty, uint64_t OffsetBase, Class &Lo,
         classify(I.getType(), Offset, FieldLo, FieldHi, isNamedArg);
         Lo = merge(Lo, FieldLo);
         Hi = merge(Hi, FieldHi);
+        if (returnCXXRecordGreaterThan128InMem() &&
+            (Size > 128 && (Size != getContext().getTypeSize(I.getType()) ||
+                            Size > getNativeVectorSizeForAVXABI(AVXLevel)))) {
+          // The only case a 256(or 512)-bit wide vector could be used to return
+          // is when CXX record contains a single 256(or 512)-bit element.
+          Lo = Memory;
+        }
         if (Lo == Memory || Hi == Memory) {
           postMerge(Size, Lo, Hi);
           return;

clang/test/CodeGen/X86/avx-cxx-record.cpp

@@ -0,0 +1,23 @@
+// RUN: %clang %s -S --target=x86_64-unknown-linux-gnu -emit-llvm -O2 -march=x86-64-v3 -o - | FileCheck %s
+
+using UInt64x2 = unsigned long long __attribute__((__vector_size__(16), may_alias));
+
+template<int id>
+struct XMM1 {
+    UInt64x2 x;
+};
+
+struct XMM2 : XMM1<0>, XMM1<1> {
+};
+
+// CHECK: define{{.*}} @_Z3foov({{.*}} [[ARG:%.*]]){{.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: store {{.*}}, ptr [[ARG]]{{.*}}
+// CHECK-NEXT: [[TMP1:%.*]] = getelementptr {{.*}}, ptr [[ARG]]{{.*}}
+// CHECK-NEXT: store {{.*}}, ptr [[TMP1]]{{.*}}
+XMM2 foo() {
+  XMM2 result;
+  ((XMM1<0>*)&result)->x = UInt64x2{1, 2};
+  ((XMM1<1>*)&result)->x = UInt64x2{3, 4};
+  return result;
+}