[clang] [CIR] Introduce data_member_offset attribute (PR #208777)
Erich Keane via cfe-commits
cfe-commits at lists.llvm.org
Wed Jul 15 07:20:07 PDT 2026
https://github.com/erichkeane updated https://github.com/llvm/llvm-project/pull/208777
>From c08da135b07fd247ccad372cec9e38278fc27699 Mon Sep 17 00:00:00 2001
From: erichkeane <ekeane at nvidia.com>
Date: Thu, 9 Jul 2026 08:45:11 -0700
Subject: [PATCH 1/3] [CIR] Introduce data_member_offset attribute
Some fields, like no_unique_address marked ones, aren't present in the
CIR struct type. This attribute represents an offset into a struct
instead, since get_member isn't possible without a member.
I also considered representing these types in the struct, but it ends up
causing a lot of awful looking structs, and forces the LLVMIR to contain
empty fields for each of these, but not in lexical order, which is particularly
awful looking.
I considered making this part of data-member, but it seems like not a
particularly good idea, since this isn't really getting a 'member' in
the traditional sense.
---
.../include/clang/CIR/Dialect/IR/CIRAttrs.td | 39 +++++++++++++
clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp | 12 ++--
clang/lib/CIR/CodeGen/CIRGenModule.cpp | 48 ++++++++++++---
clang/lib/CIR/CodeGen/CIRGenModule.h | 9 +++
clang/lib/CIR/Dialect/IR/CIRDialect.cpp | 3 +-
.../CIR/Dialect/Transforms/CXXABILowering.cpp | 9 ++-
.../Transforms/TargetLowering/CIRCXXABI.h | 7 +++
.../TargetLowering/LowerItaniumCXXABI.cpp | 15 +++++
.../CodeGen/pointer-to-empty-data-member.cpp | 58 ++++++++++++++-----
9 files changed, 170 insertions(+), 30 deletions(-)
diff --git a/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td b/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td
index ec99899355b17..4316c639e0bbd 100644
--- a/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td
+++ b/clang/include/clang/CIR/Dialect/IR/CIRAttrs.td
@@ -598,6 +598,45 @@ def CIR_DataMemberAttr : CIR_ValueLikeAttr<"DataMember", "data_member"> {
}];
}
+//===----------------------------------------------------------------------===//
+// DataMemberOffsetAttr
+//===----------------------------------------------------------------------===//
+
+def CIR_DataMemberOffsetAttr
+ : CIR_ValueLikeAttr<"DataMemberOffset", "data_member_offset"> {
+ let summary = "Constant pointer-to-data-member given by a byte offset";
+ let parameters = (ins AttributeSelfTypeParameter<
+ "", "cir::DataMemberType">:$type,
+ "uint64_t":$offset);
+ let description = [{
+ Like `#cir.data_member`, this is a literal attribute representing a constant
+ (non-null) pointer-to-data-member value. It is used for members that are
+ not laid out in the CIR record and therefore have no field-index path -- in
+ particular `[[no_unique_address]]` empty fields. The concrete byte offset
+ of the member within the class is stored directly.
+
+ Example:
+ ```
+ // Empty S::*p = &S::e (e is a no_unique_address empty field at byte 1)
+ #cir.data_member_offset<1> : !cir.data_member<!rec_Empty in !rec_S>
+ ```
+ }];
+
+ let builders = [
+ AttrBuilderWithInferredContext<(ins "cir::DataMemberType":$type,
+ "uint64_t":$offset), [{
+ return $_get(type.getContext(), type, offset);
+ }]>,
+ ];
+
+ // This attribute gets lowered during CXXABILowering.
+ let hasAttrToValueLowering = 0;
+
+ let assemblyFormat = [{
+ `<` $offset `>`
+ }];
+}
+
//===----------------------------------------------------------------------===//
// MethodAttr
//===----------------------------------------------------------------------===//
diff --git a/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp b/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp
index 4e16351c9a951..2a90cdaac92f5 100644
--- a/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp
@@ -1415,11 +1415,13 @@ mlir::Attribute ConstantEmitter::tryEmitPrivate(const APValue &value,
const auto *fieldDecl = cast<FieldDecl>(memberDecl);
const auto *mpt = destType->castAs<MemberPointerType>();
const auto *destClass = mpt->getMostRecentCXXRecordDecl();
- if (fieldDecl->hasAttr<NoUniqueAddressAttr>()) {
- assert(!cir::MissingFeatures::noUniqueAddressLayout());
- cgm.errorNYI("ConstExprEmitter::tryEmitPrivate: no_unique_address field");
- return {};
- }
+
+ // Empty [[no_unique_address]] fields have no CIR field index; represent the
+ // pointer-to-data-member by its concrete byte offset within the class.
+ if (std::optional<uint64_t> offset =
+ cgm.getEmptyFieldMemberPointerOffset(destClass, fieldDecl))
+ return cir::DataMemberOffsetAttr::get(cirTy, *offset);
+
std::optional<llvm::SmallVector<int32_t>> path =
cgm.buildMemberPath(destClass, fieldDecl);
if (!path)
diff --git a/clang/lib/CIR/CodeGen/CIRGenModule.cpp b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
index 6b7d44b09118b..94478052221fc 100644
--- a/clang/lib/CIR/CodeGen/CIRGenModule.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
@@ -2306,12 +2306,12 @@ mlir::Value CIRGenModule::emitMemberPointerConstant(const UnaryOperator *e) {
const auto *mpt = e->getType()->castAs<MemberPointerType>();
const auto *destClass = mpt->getMostRecentCXXRecordDecl();
- if (fieldDecl->hasAttr<NoUniqueAddressAttr>()) {
- assert(!cir::MissingFeatures::noUniqueAddressLayout());
- errorNYI(e->getExprLoc(),
- "emitMemberPointerConstant: no_unique_address field");
- return {};
- }
+ // Empty [[no_unique_address]] fields have no CIR field index; represent the
+ // pointer-to-data-member by its concrete byte offset within the class.
+ if (std::optional<uint64_t> offset =
+ getEmptyFieldMemberPointerOffset(destClass, fieldDecl))
+ return cir::ConstantOp::create(builder, loc,
+ cir::DataMemberOffsetAttr::get(ty, *offset));
std::optional<llvm::SmallVector<int32_t>> path =
buildMemberPath(destClass, fieldDecl);
@@ -2324,8 +2324,6 @@ mlir::Value CIRGenModule::emitMemberPointerConstant(const UnaryOperator *e) {
std::optional<llvm::SmallVector<int32_t>>
CIRGenModule::buildMemberPath(const CXXRecordDecl *destClass,
const FieldDecl *field) {
- assert(!field->hasAttr<NoUniqueAddressAttr>());
-
llvm::SmallVector<int32_t> path;
if (!findFieldMemberPath(destClass, field, path))
return std::nullopt;
@@ -2395,6 +2393,40 @@ bool CIRGenModule::findFieldMemberPath(const CXXRecordDecl *currentClass,
return false;
}
+/// Computes the byte offset of \p field within \p cls, descending through base
+/// subobjects when the field is inherited.
+static uint64_t getFieldByteOffsetInClass(const ASTContext &ctx,
+ const CXXRecordDecl *cls,
+ const FieldDecl *field) {
+ const ASTRecordLayout &layout = ctx.getASTRecordLayout(cls);
+ const auto *fieldParent = cast<CXXRecordDecl>(field->getParent());
+ if (fieldParent == cls)
+ return ctx
+ .toCharUnitsFromBits(layout.getFieldOffset(field->getFieldIndex()))
+ .getQuantity();
+
+ for (const CXXBaseSpecifier &base : cls->bases()) {
+ const auto *baseDecl = base.getType()->getAsCXXRecordDecl();
+ if (baseDecl == fieldParent || baseDecl->isDerivedFrom(fieldParent))
+ return layout.getBaseClassOffset(baseDecl).getQuantity() +
+ getFieldByteOffsetInClass(ctx, baseDecl, field);
+ }
+
+ llvm_unreachable("field is not a member of the given class");
+}
+
+std::optional<uint64_t>
+CIRGenModule::getEmptyFieldMemberPointerOffset(const CXXRecordDecl *destClass,
+ const FieldDecl *field) {
+ // Only empty, potentially-overlapping fields are omitted from the CIR record
+ // (see CIRRecordLowering::accumulateFields).
+ if (!isEmptyFieldForLayout(astContext, field) ||
+ !field->isPotentiallyOverlapping())
+ return std::nullopt;
+
+ return getFieldByteOffsetInClass(astContext, destClass, field);
+}
+
void CIRGenModule::emitDeclContext(const DeclContext *dc) {
for (Decl *decl : dc->decls()) {
// Unlike other DeclContexts, the contents of an ObjCImplDecl at TU scope
diff --git a/clang/lib/CIR/CodeGen/CIRGenModule.h b/clang/lib/CIR/CodeGen/CIRGenModule.h
index 144f8c7b9f3e7..66008d240b4e0 100644
--- a/clang/lib/CIR/CodeGen/CIRGenModule.h
+++ b/clang/lib/CIR/CodeGen/CIRGenModule.h
@@ -726,6 +726,15 @@ class CIRGenModule : public CIRGenTypeCache {
std::optional<llvm::SmallVector<int32_t>>
buildMemberPath(const CXXRecordDecl *destClass, const FieldDecl *field);
+ /// If \p field is an empty field that isn't laid out in the CIR record
+ /// (e.g. a [[no_unique_address]] empty member), returns its concrete byte
+ /// offset within \p destClass. Such fields have no CIR field index, so a
+ /// pointer-to-data-member to them is represented by an explicit offset
+ /// rather than a field-index path. Returns std::nullopt otherwise.
+ std::optional<uint64_t>
+ getEmptyFieldMemberPointerOffset(const CXXRecordDecl *destClass,
+ const FieldDecl *field);
+
llvm::StringRef getMangledName(clang::GlobalDecl gd);
// This function is to support the OpenACC 'bind' clause, which names an
// alternate name for the function to be called by. This function mangles
diff --git a/clang/lib/CIR/Dialect/IR/CIRDialect.cpp b/clang/lib/CIR/Dialect/IR/CIRDialect.cpp
index 67cc5e09f26d0..24883fc3a2adb 100644
--- a/clang/lib/CIR/Dialect/IR/CIRDialect.cpp
+++ b/clang/lib/CIR/Dialect/IR/CIRDialect.cpp
@@ -546,7 +546,8 @@ static LogicalResult checkConstantTypes(mlir::Operation *op, mlir::Type opType,
return success();
}
- if (isa<cir::DataMemberAttr, cir::MethodAttr>(attrType)) {
+ if (isa<cir::DataMemberAttr, cir::DataMemberOffsetAttr, cir::MethodAttr>(
+ attrType)) {
// More detailed type verifications are already done in
// DataMemberAttr::verify or MethodAttr::verify. Don't need to repeat here.
return success();
diff --git a/clang/lib/CIR/Dialect/Transforms/CXXABILowering.cpp b/clang/lib/CIR/Dialect/Transforms/CXXABILowering.cpp
index 704ebbeb1ecd9..50a51d9618118 100644
--- a/clang/lib/CIR/Dialect/Transforms/CXXABILowering.cpp
+++ b/clang/lib/CIR/Dialect/Transforms/CXXABILowering.cpp
@@ -67,7 +67,8 @@ bool isCXXABIAttributeLegal(const mlir::TypeConverter &tc,
return true;
// Data Member and method are ALWAYS illegal.
- if (isa<cir::DataMemberAttr, cir::MethodAttr>(attr))
+ if (isa<cir::DataMemberAttr, cir::DataMemberOffsetAttr, cir::MethodAttr>(
+ attr))
return false;
return llvm::TypeSwitch<mlir::Attribute, bool>(attr)
@@ -394,6 +395,12 @@ static mlir::TypedAttr lowerInitialValue(const LowerModule *lowerModule,
mlir::Type ty,
mlir::Attribute initVal) {
if (mlir::isa<cir::DataMemberType>(ty)) {
+ // Members without a CIR field index (e.g. no_unique_address empty fields)
+ // are represented by an explicit byte offset instead of a field path.
+ if (auto offsetVal =
+ mlir::dyn_cast_if_present<cir::DataMemberOffsetAttr>(initVal))
+ return lowerModule->getCXXABI().lowerDataMemberOffsetConstant(offsetVal,
+ layout, tc);
auto dataMemberVal = mlir::cast_if_present<cir::DataMemberAttr>(initVal);
return lowerModule->getCXXABI().lowerDataMemberConstant(dataMemberVal,
layout, tc);
diff --git a/clang/lib/CIR/Dialect/Transforms/TargetLowering/CIRCXXABI.h b/clang/lib/CIR/Dialect/Transforms/TargetLowering/CIRCXXABI.h
index 55607e24ce836..5c5d6421aea71 100644
--- a/clang/lib/CIR/Dialect/Transforms/TargetLowering/CIRCXXABI.h
+++ b/clang/lib/CIR/Dialect/Transforms/TargetLowering/CIRCXXABI.h
@@ -57,6 +57,13 @@ class CIRCXXABI {
const mlir::DataLayout &layout,
const mlir::TypeConverter &typeConverter) const = 0;
+ /// Lower the given by-offset data member pointer constant (used for members
+ /// with no CIR field index, e.g. no_unique_address empty fields) to a
+ /// constant of the ABI type.
+ virtual mlir::TypedAttr lowerDataMemberOffsetConstant(
+ cir::DataMemberOffsetAttr attr, const mlir::DataLayout &layout,
+ const mlir::TypeConverter &typeConverter) const = 0;
+
/// Lower the given member function pointer constant to a constant of the ABI
/// type. The returned constant is represented as an attribute as well.
virtual mlir::TypedAttr
diff --git a/clang/lib/CIR/Dialect/Transforms/TargetLowering/LowerItaniumCXXABI.cpp b/clang/lib/CIR/Dialect/Transforms/TargetLowering/LowerItaniumCXXABI.cpp
index 5218d4979f353..6c276a83f18cf 100644
--- a/clang/lib/CIR/Dialect/Transforms/TargetLowering/LowerItaniumCXXABI.cpp
+++ b/clang/lib/CIR/Dialect/Transforms/TargetLowering/LowerItaniumCXXABI.cpp
@@ -55,6 +55,10 @@ class LowerItaniumCXXABI : public CIRCXXABI {
cir::DataMemberAttr attr, const mlir::DataLayout &layout,
const mlir::TypeConverter &typeConverter) const override;
+ mlir::TypedAttr lowerDataMemberOffsetConstant(
+ cir::DataMemberOffsetAttr attr, const mlir::DataLayout &layout,
+ const mlir::TypeConverter &typeConverter) const override;
+
mlir::TypedAttr
lowerMethodConstant(cir::MethodAttr attr, const mlir::DataLayout &layout,
const mlir::TypeConverter &typeConverter) const override;
@@ -217,6 +221,17 @@ mlir::TypedAttr LowerItaniumCXXABI::lowerDataMemberConstant(
return cir::IntAttr::get(abiTy, memberOffset);
}
+mlir::TypedAttr LowerItaniumCXXABI::lowerDataMemberOffsetConstant(
+ cir::DataMemberOffsetAttr attr, const mlir::DataLayout &layout,
+ const mlir::TypeConverter &typeConverter) const {
+ // Itanium C++ ABI 2.3:
+ // A pointer to data member is an offset from the base address of the class
+ // object containing it, represented as a ptrdiff_t.
+ // The offset is already known (the member has no CIR field index).
+ mlir::Type abiTy = lowerDataMemberType(attr.getType(), typeConverter);
+ return cir::IntAttr::get(abiTy, static_cast<int64_t>(attr.getOffset()));
+}
+
mlir::TypedAttr LowerItaniumCXXABI::lowerMethodConstant(
cir::MethodAttr attr, const mlir::DataLayout &layout,
const mlir::TypeConverter &typeConverter) const {
diff --git a/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp b/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
index 5d368f27033f5..4251c659565bd 100644
--- a/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
+++ b/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
@@ -87,7 +87,6 @@ struct hasNUA {
[[no_unique_address]] EmptyBase eb6;
int i;
};
-// FIXME(cir): We should represent eb1-6 somehow
// CIR-DAG: !rec_hasNUA = !cir.struct<"hasNUA" padded {!s32i, !cir.array<!u8i x 4>}>
// LLVM-DAG: %struct.hasNUA = type { i32, [4 x i8] }
@@ -96,14 +95,33 @@ const hasNUA nua = {{},{},{},{},{},{}, 1};
// LLVMCIR-DAG: @_ZL3nua = internal constant %struct.hasNUA { i32 1, [4 x i8] zeroinitializer }
// OGCG-DAG: @_ZL3nua = internal constant %struct.hasNUA { i32 1, [4 x i8] undef }
-// FIXME(cir): These are still an NYI, and we should enable tests here once we
-// figure out how to represent these in IR.
-//EmptyBase hasNUA::* eb1 = &hasNUA::eb1;
-//EmptyBase hasNUA::* eb2 = &hasNUA::eb2;
-//EmptyBase hasNUA::* eb3 = &hasNUA::eb3;
-//EmptyBase hasNUA::* eb4 = &hasNUA::eb4;
-//EmptyBase hasNUA::* eb5 = &hasNUA::eb5;
-//EmptyBase hasNUA::* eb6 = &hasNUA::eb6;
+// A pointer-to-data-member for a no_unique_address empty field has no CIR
+// field index (the field isn't laid out), so it is represented by its concrete
+// byte offset via #cir.data_member_offset.
+EmptyBase hasNUA::* eb1 = &hasNUA::eb1;
+// CIR-BEFORE-DAG: cir.global external @eb1 = #cir.data_member_offset<0> : !cir.data_member<!rec_EmptyBase in !rec_hasNUA>
+// CIR-AFTER-DAG: cir.global external @eb1 = #cir.int<0> : !s64i
+// LLVM-DAG: @eb1 = global i64 0
+EmptyBase hasNUA::* eb2 = &hasNUA::eb2;
+// CIR-BEFORE-DAG: cir.global external @eb2 = #cir.data_member_offset<1> : !cir.data_member<!rec_EmptyBase in !rec_hasNUA>
+// CIR-AFTER-DAG: cir.global external @eb2 = #cir.int<1> : !s64i
+// LLVM-DAG: @eb2 = global i64 1
+EmptyBase hasNUA::* eb3 = &hasNUA::eb3;
+// CIR-BEFORE-DAG: cir.global external @eb3 = #cir.data_member_offset<2> : !cir.data_member<!rec_EmptyBase in !rec_hasNUA>
+// CIR-AFTER-DAG: cir.global external @eb3 = #cir.int<2> : !s64i
+// LLVM-DAG: @eb3 = global i64 2
+EmptyBase hasNUA::* eb4 = &hasNUA::eb4;
+// CIR-BEFORE-DAG: cir.global external @eb4 = #cir.data_member_offset<3> : !cir.data_member<!rec_EmptyBase in !rec_hasNUA>
+// CIR-AFTER-DAG: cir.global external @eb4 = #cir.int<3> : !s64i
+// LLVM-DAG: @eb4 = global i64 3
+EmptyBase hasNUA::* eb5 = &hasNUA::eb5;
+// CIR-BEFORE-DAG: cir.global external @eb5 = #cir.data_member_offset<4> : !cir.data_member<!rec_EmptyBase in !rec_hasNUA>
+// CIR-AFTER-DAG: cir.global external @eb5 = #cir.int<4> : !s64i
+// LLVM-DAG: @eb5 = global i64 4
+EmptyBase hasNUA::* eb6 = &hasNUA::eb6;
+// CIR-BEFORE-DAG: cir.global external @eb6 = #cir.data_member_offset<5> : !cir.data_member<!rec_EmptyBase in !rec_hasNUA>
+// CIR-AFTER-DAG: cir.global external @eb6 = #cir.int<5> : !s64i
+// LLVM-DAG: @eb6 = global i64 5
int hasNUA::* nua_i = &hasNUA::i;
// CIR-BEFORE-DAG: cir.global external @nua_i = #cir.data_member<[0]> : !cir.data_member<!s32i in !rec_hasNUA>
// CIR-AFTER-DAG: cir.global external @nua_i = #cir.int<0> : !s64i
@@ -205,9 +223,14 @@ int U6::* u6i = &U6::i;
// CIR-AFTER-DAG: cir.global external @u6i = #cir.int<0> : !s64i
// LLVM-DAG: @u6i = global i64 0
-// FIXME(cir): See above.
-//EmptyBase U6::* u6eb = &U6::eb;
-//EmptyBase U6::* u6eb2 = &U6::eb2;
+EmptyBase U6::* u6eb = &U6::eb;
+// CIR-BEFORE-DAG: cir.global external @u6eb = #cir.data_member_offset<0> : !cir.data_member<!rec_EmptyBase in !rec_U6>
+// CIR-AFTER-DAG: cir.global external @u6eb = #cir.int<0> : !s64i
+// LLVM-DAG: @u6eb = global i64 0
+EmptyBase U6::* u6eb2 = &U6::eb2;
+// CIR-BEFORE-DAG: cir.global external @u6eb2 = #cir.data_member_offset<0> : !cir.data_member<!rec_EmptyBase in !rec_U6>
+// CIR-AFTER-DAG: cir.global external @u6eb2 = #cir.int<0> : !s64i
+// LLVM-DAG: @u6eb2 = global i64 0
union U7 {
int i;
@@ -222,9 +245,14 @@ int U7::* u7i = &U7::i;
// CIR-AFTER-DAG: cir.global external @u7i = #cir.int<0> : !s64i
// LLVM-DAG: @u7i = global i64 0
-// FIXME(cir): See above.
-//EmptyBase U7::* u7eb = &U7::eb;
-//EmptyBase U7::* u7eb2 = &U7::eb2;
+EmptyBase U7::* u7eb = &U7::eb;
+// CIR-BEFORE-DAG: cir.global external @u7eb = #cir.data_member_offset<0> : !cir.data_member<!rec_EmptyBase in !rec_U7>
+// CIR-AFTER-DAG: cir.global external @u7eb = #cir.int<0> : !s64i
+// LLVM-DAG: @u7eb = global i64 0
+EmptyBase U7::* u7eb2 = &U7::eb2;
+// CIR-BEFORE-DAG: cir.global external @u7eb2 = #cir.data_member_offset<0> : !cir.data_member<!rec_EmptyBase in !rec_U7>
+// CIR-AFTER-DAG: cir.global external @u7eb2 = #cir.int<0> : !s64i
+// LLVM-DAG: @u7eb2 = global i64 0
void uses() {
auto x = &HasEmpty::s;
>From 46db8234ff7752c70b9f04cc05f5b12766cbefc7 Mon Sep 17 00:00:00 2001
From: erichkeane <ekeane at nvidia.com>
Date: Mon, 13 Jul 2026 15:11:34 -0700
Subject: [PATCH 2/3] simplify how I calculate the offset of empty fields.
Self-review revealed that I was probably doing this the 'hard' way, so I
went back to classic codegen and realized I was mis-understanding the
use in emitMemberPointerCast, and could do it just with getFieldOffset.
This reverts the calculation I was doing to look into each base, and
instead replaces it with the way that classic codegen does it.
---
clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp | 12 +++--
clang/lib/CIR/CodeGen/CIRGenModule.cpp | 49 ++++++-------------
clang/lib/CIR/CodeGen/CIRGenModule.h | 14 +++---
.../CodeGen/pointer-to-empty-data-member.cpp | 8 +++
4 files changed, 36 insertions(+), 47 deletions(-)
diff --git a/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp b/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp
index 2a90cdaac92f5..30b2e9a6d6d9f 100644
--- a/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp
@@ -1417,10 +1417,14 @@ mlir::Attribute ConstantEmitter::tryEmitPrivate(const APValue &value,
const auto *destClass = mpt->getMostRecentCXXRecordDecl();
// Empty [[no_unique_address]] fields have no CIR field index; represent the
- // pointer-to-data-member by its concrete byte offset within the class.
- if (std::optional<uint64_t> offset =
- cgm.getEmptyFieldMemberPointerOffset(destClass, fieldDecl))
- return cir::DataMemberOffsetAttr::get(cirTy, *offset);
+ // pointer-to-data-member by its concrete byte offset.
+ if (cgm.isEmptyFieldForMemberPointer(fieldDecl)) {
+ const ASTContext &astContext = cgm.getASTContext();
+ CharUnits offset =
+ astContext.getMemberPointerPathAdjustment(value) +
+ astContext.toCharUnitsFromBits(astContext.getFieldOffset(fieldDecl));
+ return cir::DataMemberOffsetAttr::get(cirTy, offset.getQuantity());
+ }
std::optional<llvm::SmallVector<int32_t>> path =
cgm.buildMemberPath(destClass, fieldDecl);
diff --git a/clang/lib/CIR/CodeGen/CIRGenModule.cpp b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
index 94478052221fc..a3764f0844738 100644
--- a/clang/lib/CIR/CodeGen/CIRGenModule.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
@@ -2308,10 +2308,18 @@ mlir::Value CIRGenModule::emitMemberPointerConstant(const UnaryOperator *e) {
// Empty [[no_unique_address]] fields have no CIR field index; represent the
// pointer-to-data-member by its concrete byte offset within the class.
- if (std::optional<uint64_t> offset =
- getEmptyFieldMemberPointerOffset(destClass, fieldDecl))
+ if (isEmptyFieldForMemberPointer(fieldDecl)) {
+ // This function should ONLY be accessed in reference to itself, I don't see
+ // any cases/couldn't find any cases where anything else could get here, and
+ // classic-codegen does the same.
+ assert(fieldDecl->getParent() == destClass &&
+ "scalar member pointer should be relative to the declaring class");
+ uint64_t offset =
+ astContext.toCharUnitsFromBits(astContext.getFieldOffset(fieldDecl))
+ .getQuantity();
return cir::ConstantOp::create(builder, loc,
- cir::DataMemberOffsetAttr::get(ty, *offset));
+ cir::DataMemberOffsetAttr::get(ty, offset));
+ }
std::optional<llvm::SmallVector<int32_t>> path =
buildMemberPath(destClass, fieldDecl);
@@ -2393,38 +2401,9 @@ bool CIRGenModule::findFieldMemberPath(const CXXRecordDecl *currentClass,
return false;
}
-/// Computes the byte offset of \p field within \p cls, descending through base
-/// subobjects when the field is inherited.
-static uint64_t getFieldByteOffsetInClass(const ASTContext &ctx,
- const CXXRecordDecl *cls,
- const FieldDecl *field) {
- const ASTRecordLayout &layout = ctx.getASTRecordLayout(cls);
- const auto *fieldParent = cast<CXXRecordDecl>(field->getParent());
- if (fieldParent == cls)
- return ctx
- .toCharUnitsFromBits(layout.getFieldOffset(field->getFieldIndex()))
- .getQuantity();
-
- for (const CXXBaseSpecifier &base : cls->bases()) {
- const auto *baseDecl = base.getType()->getAsCXXRecordDecl();
- if (baseDecl == fieldParent || baseDecl->isDerivedFrom(fieldParent))
- return layout.getBaseClassOffset(baseDecl).getQuantity() +
- getFieldByteOffsetInClass(ctx, baseDecl, field);
- }
-
- llvm_unreachable("field is not a member of the given class");
-}
-
-std::optional<uint64_t>
-CIRGenModule::getEmptyFieldMemberPointerOffset(const CXXRecordDecl *destClass,
- const FieldDecl *field) {
- // Only empty, potentially-overlapping fields are omitted from the CIR record
- // (see CIRRecordLowering::accumulateFields).
- if (!isEmptyFieldForLayout(astContext, field) ||
- !field->isPotentiallyOverlapping())
- return std::nullopt;
-
- return getFieldByteOffsetInClass(astContext, destClass, field);
+bool CIRGenModule::isEmptyFieldForMemberPointer(const FieldDecl *field) {
+ return isEmptyFieldForLayout(astContext, field) &&
+ field->isPotentiallyOverlapping();
}
void CIRGenModule::emitDeclContext(const DeclContext *dc) {
diff --git a/clang/lib/CIR/CodeGen/CIRGenModule.h b/clang/lib/CIR/CodeGen/CIRGenModule.h
index 66008d240b4e0..82fd0d4405ef3 100644
--- a/clang/lib/CIR/CodeGen/CIRGenModule.h
+++ b/clang/lib/CIR/CodeGen/CIRGenModule.h
@@ -726,14 +726,12 @@ class CIRGenModule : public CIRGenTypeCache {
std::optional<llvm::SmallVector<int32_t>>
buildMemberPath(const CXXRecordDecl *destClass, const FieldDecl *field);
- /// If \p field is an empty field that isn't laid out in the CIR record
- /// (e.g. a [[no_unique_address]] empty member), returns its concrete byte
- /// offset within \p destClass. Such fields have no CIR field index, so a
- /// pointer-to-data-member to them is represented by an explicit offset
- /// rather than a field-index path. Returns std::nullopt otherwise.
- std::optional<uint64_t>
- getEmptyFieldMemberPointerOffset(const CXXRecordDecl *destClass,
- const FieldDecl *field);
+ /// Returns true if \p field is an empty field that isn't laid out in the CIR
+ /// record (e.g. a [[no_unique_address]] empty member). Such fields have no
+ /// CIR field index, so a pointer-to-data-member to them is represented by an
+ /// explicit byte offset (#cir.data_member_offset) rather than a field-index
+ /// path.
+ bool isEmptyFieldForMemberPointer(const FieldDecl *field);
llvm::StringRef getMangledName(clang::GlobalDecl gd);
// This function is to support the OpenACC 'bind' clause, which names an
diff --git a/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp b/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
index 4251c659565bd..4ce8620a496f1 100644
--- a/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
+++ b/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
@@ -127,6 +127,14 @@ int hasNUA::* nua_i = &hasNUA::i;
// CIR-AFTER-DAG: cir.global external @nua_i = #cir.int<0> : !s64i
// LLVM-DAG: @nua_i = global i64 0
+struct FirstField { int a; };
+struct HoldsNUA { int b; [[no_unique_address]] EmptyBase e; };
+struct DerivesNUA : FirstField, HoldsNUA {};
+EmptyBase DerivesNUA::* nua_in_base = &DerivesNUA::e;
+// CIR-BEFORE-DAG: cir.global external @nua_in_base = #cir.data_member_offset<4> : !cir.data_member<!rec_EmptyBase in !rec_DerivesNUA>
+// CIR-AFTER-DAG: cir.global external @nua_in_base = #cir.int<4> : !s64i
+// LLVM-DAG: @nua_in_base = global i64 4
+
union U1 {
EmptyBase eb;
};
>From 43cbc6b2bd4cb707654f4ba9b90f9a1a5a04181e Mon Sep 17 00:00:00 2001
From: erichkeane <ekeane at nvidia.com>
Date: Wed, 15 Jul 2026 07:19:44 -0700
Subject: [PATCH 3/3] Add an additional field offset test
---
clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp b/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
index 4ce8620a496f1..0b493dc306594 100644
--- a/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
+++ b/clang/test/CIR/CodeGen/pointer-to-empty-data-member.cpp
@@ -128,12 +128,16 @@ int hasNUA::* nua_i = &hasNUA::i;
// LLVM-DAG: @nua_i = global i64 0
struct FirstField { int a; };
-struct HoldsNUA { int b; [[no_unique_address]] EmptyBase e; };
+struct HoldsNUA { int b; [[no_unique_address]] EmptyBase e; [[no_unique_address]] EmptyBase e2; };
struct DerivesNUA : FirstField, HoldsNUA {};
EmptyBase DerivesNUA::* nua_in_base = &DerivesNUA::e;
// CIR-BEFORE-DAG: cir.global external @nua_in_base = #cir.data_member_offset<4> : !cir.data_member<!rec_EmptyBase in !rec_DerivesNUA>
// CIR-AFTER-DAG: cir.global external @nua_in_base = #cir.int<4> : !s64i
// LLVM-DAG: @nua_in_base = global i64 4
+EmptyBase DerivesNUA::* nua_in_base2 = &DerivesNUA::e2;
+// CIR-BEFORE-DAG: cir.global external @nua_in_base2 = #cir.data_member_offset<8> : !cir.data_member<!rec_EmptyBase in !rec_DerivesNUA>
+// CIR-AFTER-DAG: cir.global external @nua_in_base2 = #cir.int<8> : !s64i
+// LLVM-DAG: @nua_in_base2 = global i64 8
union U1 {
EmptyBase eb;
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