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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
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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 @@ -1296,13 +1296,6 @@ 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 @@ -2255,9 +2248,27 @@ 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();

const bool assume_packed = ExternalFieldOffset > 0 &&
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
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