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[clang][RecordLayoutBuilder] Be stricter about inferring packed-ness in ExternalLayouts #97443

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[clang][RecordLayoutBuilder] Be stricter about inferring packed-ness in ExternalLayouts #97443

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38b7837
[clang][RecordLayoutBuilder] Be stricter about inferring packed-ness …
Michael137 Jul 2, 2024
68d71ab
fixup! ignore empty fields instead of checking overlap
Michael137 Oct 2, 2024
2d9dc34
fixup! clang-format
Michael137 Oct 2, 2024
8ac993d
Merge branch 'main' into clang/lldb/inferalignment-removal
Michael137 Sep 3, 2025
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31 changes: 21 additions & 10 deletions clang/lib/AST/RecordLayoutBuilder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1280,13 +1280,6 @@ ItaniumRecordLayoutBuilder::LayoutBase(const BaseSubobjectInfo *Base) {
bool Allowed = EmptySubobjects->CanPlaceBaseAtOffset(Base, Offset);
(void)Allowed;
assert(Allowed && "Base subobject externally placed at overlapping offset");

if (InferAlignment && Offset < getDataSize().alignTo(AlignTo)) {
// The externally-supplied base offset is before the base offset we
// computed. Assume that the structure is packed.
Alignment = CharUnits::One();
InferAlignment = false;
}
}

if (!Base->Class->isEmpty()) {
Expand Down Expand Up @@ -2243,9 +2236,27 @@ ItaniumRecordLayoutBuilder::updateExternalFieldOffset(const FieldDecl *Field,
uint64_t ComputedOffset) {
uint64_t ExternalFieldOffset = External.getExternalFieldOffset(Field);

if (InferAlignment && ExternalFieldOffset < ComputedOffset) {
// The externally-supplied field offset is before the field offset we
// computed. Assume that the structure is packed.
// DWARF doesn't tell us whether a structure was declared as packed.
// So we try to figure out if the supplied Field is at a packed offset
// (i.e., the externally-supplied offset is less than the layout builder
// expected).
//
// There are cases where fields are placed at overlapping offsets (e.g.,
// as a result of [[no_unique_address]]). In those cases we don't want
// to incorrectly deduce that they are placed at packed offsets. Hence,
// ignore empty fields (which are the only fields that can overlap).
//
// FIXME: emit enough information in DWARF to get rid of InferAlignment.
//
CXXRecordDecl *CXX = nullptr;
if (auto *RT = dyn_cast<RecordType>(Field->getType()))
CXX = RT->getAsCXXRecordDecl();
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This is all just auto CXX = Field->getType()->getAsCXXRecordDecl();


const bool assume_packed = ExternalFieldOffset > 0 &&
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@rjmccall rjmccall Sep 5, 2025 •
edited
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We don't generally use snake_case in LLVM projects; this should be named AssumePacked.

Why the > 0 condition?

I'm not sure I understand why we need to update anything here. When we have an external layout, do we not eventually just take the alignment from the external layout anyway? If so, what's the point of the extra bookkeeping?

ExternalFieldOffset < ComputedOffset &&
!(CXX && CXX->isEmpty());

if (InferAlignment && assume_packed) {
Alignment = CharUnits::One();
PreferredAlignment = CharUnits::One();
InferAlignment = false;
Expand Down
23 changes: 23 additions & 0 deletions lldb/test/Shell/SymbolFile/DWARF/no_unique_address-packed-alignment.cpp
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@@ -0,0 +1,23 @@
// RUN: %clangxx_host -gdwarf -o %t %s
// RUN: %lldb %t \
// RUN: -o "expr alignof(PackedNUA)" \
// RUN: -o "expr sizeof(PackedNUA)" \
// RUN: -o exit | FileCheck %s

struct Empty {};
struct __attribute((packed)) PackedNUA {
[[no_unique_address]] Empty a, b, c, d;
char x;
int y;
};
static_assert(alignof(PackedNUA) == 1);
static_assert(sizeof(PackedNUA) == 5);

PackedNUA packed;

// CHECK: (lldb) expr alignof(PackedNUA)
// CHECK-NEXT: ${{.*}} = 1
// CHECK: (lldb) expr sizeof(PackedNUA)
// CHECK-NEXT: ${{.*}} = 5

int main() { PackedNUA d; }
2 changes: 0 additions & 2 deletions lldb/test/Shell/SymbolFile/DWARF/x86/no_unique_address-alignment.cpp
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@@ -1,5 +1,3 @@
// XFAIL: *

// RUN: %clang --target=x86_64-apple-macosx -c -gdwarf -o %t %s
// RUN: %lldb %t \
// RUN: -o "expr alignof(OverlappingFields)" \
Expand Down
2 changes: 0 additions & 2 deletions lldb/test/Shell/SymbolFile/DWARF/x86/no_unique_address-base-alignment.cpp
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@@ -1,5 +1,3 @@
// XFAIL: *

// RUN: %clang --target=x86_64-apple-macosx -c -gdwarf -o %t %s
// RUN: %lldb %t \
// RUN: -o "expr alignof(OverlappingDerived)" \
Expand Down