[clang] 037f856 - Itanium ABI: Pack non-pod members of packed types

[David Blaikie via cfe-commits]

Author: David Blaikie
Date: 2022-10-14T19:32:57Z
New Revision: 037f856681268c793c660389b4d6407367e68190

URL: https://github.com/llvm/llvm-project/commit/037f856681268c793c660389b4d6407367e68190
DIFF: https://github.com/llvm/llvm-project/commit/037f856681268c793c660389b4d6407367e68190.diff

LOG: Itanium ABI: Pack non-pod members of packed types

Seems there's a narrow case - where a packed type doesn't pack its base
subobjects (only fields), but when packing a field of the derived type,
GCC does pack the resulting total object - effectively packing the base
subobject.

So ensure that this non-pod type (owing to it having a base class) that
is packed, gets packed when placed in /another/ type that is also
packed.

This is a (smallish?) ABI fix to a regression introduced by D117616 -
but that regression/ABI break hasn't been released in LLVM as-yet (it's
been reverted on the release branch from the last two LLVM releases - I
probably should've just reverted the whole patch while we hashed out
this and other issues) so this change isn't itself an ABI break, as far
as LLVM releases are concerned (for folks releasing their own copies of
LLVM from ToT/without the LLVM release branch, and didn't opt into the
clang-abi-compat 14 or below (soon to be 15 or below, I guess I should
say) then this would be an ABI break against clang from the last 9
months or so)

Differential Revision: https://reviews.llvm.org/D135916

Added: 
    

Modified: 
    clang/lib/AST/RecordLayoutBuilder.cpp
    clang/test/SemaCXX/class-layout.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index dea7450613cc3..a897d4c832300 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -1891,6 +1891,7 @@ void ItaniumRecordLayoutBuilder::LayoutField(const FieldDecl *D,
 
   llvm::Triple Target = Context.getTargetInfo().getTriple();
   bool FieldPacked = (Packed && (!FieldClass || FieldClass->isPOD() ||
+                                 FieldClass->hasAttr<PackedAttr>() ||
                                  Context.getLangOpts().getClangABICompat() <=
                                      LangOptions::ClangABI::Ver15 ||
                                  Target.isPS() || Target.isOSDarwin())) ||

diff  --git a/clang/test/SemaCXX/class-layout.cpp b/clang/test/SemaCXX/class-layout.cpp
index df63141fc36dd..f3106ec422dff 100644
--- a/clang/test/SemaCXX/class-layout.cpp
+++ b/clang/test/SemaCXX/class-layout.cpp
@@ -642,3 +642,17 @@ struct t2 {
 _Static_assert(_Alignof(t1) == 1, "");
 _Static_assert(_Alignof(t2) == 1, "");
 } // namespace non_pod_packed
+
+namespace non_pod_packed_packed {
+struct B {
+  int b;
+};
+struct  FromB : B {
+} __attribute__((packed));
+struct C {
+  char a[3];
+  FromB b;
+} __attribute__((packed));
+_Static_assert(__builtin_offsetof(C, b) == 3, "");
+}
+