[AIX] "aligned" attribute should not decrease type alignment returned by __alignof__

[stevewan]

Summary

__alignof__(x) always returns ABIAlign if the "x" is marked __attribute__((aligned())). However, the "aligned" attribute should only increase the alignment of a struct, or struct member, unless it's used together with the "packed" attribute, or used as a part of a typedef, in which case, the "aligned" attribute can both increase and decrease alignment.

[stevewan]

This is a split of the query portion from https://reviews.llvm.org/D107394.

[sfertile]

LGTM, other then 1 small test update.

clang/test/Layout/aix-alignof-align-and-pack-attr.cpp

21

Minor nit: add

namespace test3 {
struct __attribute__((__aligned__(16))) C {
    double x;
} c;

as a third test.

[stevewan]

Add third test case as suggested.

clang/lib/AST/ASTContext.cpp

unsigned ASTContext::getPreferredTypeAlign(const Type *T) const {
   // The preferred alignment of member pointers is that of a pointer.
   if (T->isMemberPointerType())
     return getPreferredTypeAlign(getPointerDiffType().getTypePtr());
 
   if (!Target->allowsLargerPreferedTypeAlignment())
     return ABIAlign;
 
   if (const auto *RT = T->getAs<RecordType>()) {
-    if (TI.AlignIsRequired || RT->getDecl()->isInvalidDecl())
+    const RecordDecl *RD = RT->getDecl();
+
+    // When used as part of a typedef, or together with a 'packed' attribute,
+    // the 'aligned' attribute can be used to decrease alignment.
+    if ((TI.AlignIsRequired && T->getAs<TypedefType>() != nullptr) ||
+        RD->isInvalidDecl())
       return ABIAlign;
 
     unsigned PreferredAlign = static_cast<unsigned>(
-        toBits(getASTRecordLayout(RT->getDecl()).PreferredAlignment));
+        toBits(getASTRecordLayout(RD).PreferredAlignment));
     assert(PreferredAlign >= ABIAlign &&
            "PreferredAlign should be at least as large as ABIAlign.");
     return PreferredAlign;
   }
 
   // Double (and, for targets supporting AIX `power` alignment, long double) and
   // long long should be naturally aligned (despite requiring less alignment) if
   // possible.

clang/test/Layout/aix-alignof-align-and-pack-attr.cpp

+// RUN: %clang_cc1 -triple powerpc-ibm-aix-xcoff -S -emit-llvm -x c++ < %s | \
+// RUN:   FileCheck %s
+
+// RUN: %clang_cc1 -triple powerpc64-ibm-aix-xcoff -S -emit-llvm -x c++ < %s | \
+// RUN:   FileCheck %s
+
+namespace test1 {
+struct __attribute__((__aligned__(2))) C {
+  double x;
+} c;
+
+// CHECK: @{{.*}}test1{{.*}}c{{.*}} = global %"struct.test1::C" zeroinitializer, align 8
+} // namespace test1
+
+namespace test2 {
+struct __attribute__((__aligned__(2), packed)) C {
+  double x;
+} c;
+
+// CHECK: @{{.*}}test2{{.*}}c{{.*}} = global %"struct.test2::C" zeroinitializer, align 2
+} // namespace test2