[c++20] For P0732R2 / P1907R1: Basic code generation and name mangling supportfor non-type template parameters of class type and template parameter objects.

[rsmith]

Summary

The Itanium side of this follows the approach I proposed in
https://github.com/itanium-cxx-abi/cxx-abi/issues/47 on 2020-09-06.

The MSVC side of this was determined empirically by observing MSVC's output. I've asked Jon Caves at Microsoft for clarifications around the FIXMEs.

[rsmith]

• Fix incorrect mangling for zero-but-not-null template arguments.

[rsmith]

1. • Ignore cv-qualifiers in member types for Itanium mangling.
   • Add MSVC-compatible mangling support.
   • Fix bug where template argument value mangling would be used when passing a template parameter object as a pointer or reference template argument.
2. Updating D89998: [c++20] For P0732R2 / P1907R1: Basic code generation and name mangling support

for non-type template parameters of class type and template parameter objects.

[rsmith]

clang/lib/AST/MicrosoftMangle.cpp

657–660 ↗

(On Diff #301070)

For what it's worth, I'm fairly convinced the $ is not actually part of the mangling of the member function pointer, and is instead part of the mangling of a non-type template argument (or more generally of a value).

[rjmccall]

clang/lib/AST/APValue.cpp

1047

I'm not sure what ABIs you're thinking about, but the *symbol* linkage/visibility of the type info isn't important, just its formal linkage/visibility.

1052

Objective-C object literals are interesting here; arguably, we could allow you to use one as a template argument, and it wouldn't naturally limit linkage.

clang/lib/AST/ASTContext.cpp

2490

What should we do on targets that allow virtual bases in member pointer conversions?

clang/lib/AST/ItaniumMangle.cpp

4923

Probably worth reemphasizing the general principle that this is not equivalent to asking whether it has a zero bit-pattern.

5131

Surely we have this as a subroutine already, or could.

5135

Can we extract a function to do this mangling that's called from both here and FixedPointLiteral, and then have that assert, rather than leaving a bomb for a corner case of a corner case?

5154

These can use the float-mangling subroutine that you should extract.

The condition here needs to be !isPosZero(), I think.

5180

This could also be extracted and shared with the template-argument mangler. In fact, isn't most of this case redundant with the template-argument mangler?

[rnk]

clang/lib/AST/ASTContext.cpp

2490

It looks like the MS ABI codepaths don't use this routine. If we could put it somewhere Itanium-specific that would be accessible to the mangler and CGCXXABI implementation, that would be ideal, but ASTContext doesn't seem too bad.

clang/lib/AST/MicrosoftMangle.cpp

657–660 ↗

(On Diff #301070)

That makes sense to me. I'll see if I can simplify in a follow-up.

751–752 ↗

(On Diff #301070)

IIUC, this means we can mangle __int128 non-type template parms now? Let's add a test for that.

1746 ↗

(On Diff #301070)

Why is this even an APValue kind... O_o

clang/test/CodeGenCXX/mangle-class-nttp.cpp

28

These FIXMEs represent a lot of future work. :(

Thanks for adding test coverage on both sides, though.

[rsmith]

• Address review comments.

[rsmith]

clang/lib/AST/APValue.cpp

1052

That makes sense to me; FIXME added.

clang/lib/AST/ASTContext.cpp

2490

While we do support such things in general, we do not support constant-evaluation of a conversion between pointer-to-derived and pointer-to-virtual-base, and there is no APValue representation for such a thing. Because we take an APValue as input, this function should work correctly even under the MS ABI.

clang/lib/AST/ItaniumMangle.cpp

5135

Done. They're somewhat different cases because the other case is actually reachable (via __attribute__((overloadable))) but I think it makes sense to assume that we'll eventually support fixed-point in C++ mode. (I think it's a bit strange we didn't support it from the start.)

5180

Theoretically yes, but while there's a lot of overlap between TemplateArguments and APValues, they're different representations and can represent a somewhat different set of values.

I think we could convert all non-type TemplateArguments to APValues before mangling them, and that might reduce duplication a little, if you'd like?

clang/lib/AST/MicrosoftMangle.cpp

1746 ↗

(On Diff #301070)

I think the idea is that we want to be able to include these in function-local static dispatch arrays with constant initialization. But yeah, this is by far the weirdest kind of APValue.

clang/test/CodeGenCXX/mangle-class-nttp.cpp

28

I've asked Jon Caves about this, maybe we'll just be told what the rule is =)

[rsmith]

• Factor out duplicated mangling of null pointers.

[rsmith]

clang/lib/AST/ItaniumMangle.cpp

5180

I've factored out the duplicated mangling of null pointers. I'm not sure if there's much more that's worth pulling out.

[rjmccall]

LGTM

clang/lib/AST/ItaniumMangle.cpp

5135

Agreed.

clang/include/clang/AST/ASTContext.h

public:
   /// Get the offset of a FieldDecl or IndirectFieldDecl, in bits.
   uint64_t getFieldOffset(const ValueDecl *FD) const;
 
   /// Get the offset of an ObjCIvarDecl in bits.
   uint64_t lookupFieldBitOffset(const ObjCInterfaceDecl *OID,
                                 const ObjCImplementationDecl *ID,
                                 const ObjCIvarDecl *Ivar) const;
 
+  /// Find the 'this' offset for the member path in a pointer-to-member
+  /// APValue.
+  CharUnits getMemberPointerPathAdjustment(const APValue &MP) const;
+
   bool isNearlyEmpty(const CXXRecordDecl *RD) const;
 
   VTableContextBase *getVTableContext();
 
   /// If \p T is null pointer, assume the target in ASTContext.
   MangleContext *createMangleContext(const TargetInfo *T = nullptr);
 
   void DeepCollectObjCIvars(const ObjCInterfaceDecl *OI, bool leafClass,

clang/lib/AST/APValue.cpp

 //===--- APValue.cpp - Union class for APFloat/APSInt/Complex -------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file implements the APValue class.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/APValue.h"
+#include "Linkage.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/AST/Type.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace clang;
 
 /// The identity of a type_info object depends on the canonical unqualified
 /// type only.
 TypeInfoLValue::TypeInfoLValue(const Type *T)

 
 void APValue::MakeMemberPointer(const ValueDecl *Member, bool IsDerivedMember,
                                 ArrayRef<const CXXRecordDecl *> Path) {
   MutableArrayRef<const CXXRecordDecl *> InternalPath =
       setMemberPointerUninit(Member, IsDerivedMember, Path.size());
   for (unsigned I = 0; I != Path.size(); ++I)
     InternalPath[I] = Path[I]->getCanonicalDecl();
 }
+
+LinkageInfo LinkageComputer::getLVForValue(const APValue &V,
+                                           LVComputationKind computation) {
+  LinkageInfo LV = LinkageInfo::external();
+
+  auto MergeLV = [&](LinkageInfo MergeLV) {
+    LV.merge(MergeLV);
+    return LV.getLinkage() == InternalLinkage;
+  };
+  auto Merge = [&](const APValue &V) {
+    return MergeLV(getLVForValue(V, computation));
+  };
+
+  switch (V.getKind()) {
+  case APValue::None:
+  case APValue::Indeterminate:
+  case APValue::Int:
+  case APValue::Float:
+  case APValue::FixedPoint:
+  case APValue::ComplexInt:
+  case APValue::ComplexFloat:
+  case APValue::Vector:
+    break;
+
+  case APValue::AddrLabelDiff:
+    // Even for an inline function, it's not reasonable to treat a difference
+    // between the addresses of labels as an external value.
+    return LinkageInfo::internal();
+
+  case APValue::Struct: {
+    for (unsigned I = 0, N = V.getStructNumBases(); I != N; ++I)
+      if (Merge(V.getStructBase(I)))
+        break;
+    for (unsigned I = 0, N = V.getStructNumFields(); I != N; ++I)
+      if (Merge(V.getStructField(I)))
+        break;
+    break;
+  }
+
+  case APValue::Union:
+    if (const auto *FD = V.getUnionField())
+      Merge(V.getUnionValue());
+    break;
+
+  case APValue::Array: {
+    for (unsigned I = 0, N = V.getArrayInitializedElts(); I != N; ++I)
+      if (Merge(V.getArrayInitializedElt(I)))
+        break;
+    if (V.hasArrayFiller())
+      Merge(V.getArrayFiller());
+    break;
+  }
+
+  case APValue::LValue: {
+    if (!V.getLValueBase()) {
+      // Null or absolute address: this is external.
+    } else if (const auto *VD =
+                   V.getLValueBase().dyn_cast<const ValueDecl *>()) {
+      if (VD && MergeLV(getLVForDecl(VD, computation)))
+        break;
+    } else if (const auto TI = V.getLValueBase().dyn_cast<TypeInfoLValue>()) {
+      // FIXME: Claim that typeinfo(T) has the same linkage as T. This isn't
+      // correct across all ABIs, but for now it shouldn't matter as such
+      // values aren't permitted within template arguments.
+      if (MergeLV(getLVForType(*TI.getType(), computation)))
+        break;
+    } else if (const Expr *E = V.getLValueBase().dyn_cast<const Expr *>()) {
+      // Almost all expression bases are internal. The exception is
+      // lifetime-extended temporaries.
+      // FIXME: These should be modeled as having the
+      // LifetimeExtendedTemporaryDecl itself as the base.
+      auto *MTE = dyn_cast<MaterializeTemporaryExpr>(E);
+      if (!MTE || MTE->getStorageDuration() == SD_FullExpression)
+        return LinkageInfo::internal();
+      if (MergeLV(getLVForDecl(MTE->getExtendingDecl(), computation)))
+        break;
+    } else {
+      assert(V.getLValueBase().is<DynamicAllocLValue>() &&
+             "unexpected LValueBase kind");
+      return LinkageInfo::internal();
+    }
+    // The lvalue path doesn't matter: pointers to all subobjects always have
+    // the same visibility as pointers to the complete object.
+    break;
+  }
+
+  case APValue::MemberPointer:
+    if (const NamedDecl *D = V.getMemberPointerDecl())
+      MergeLV(getLVForDecl(D, computation));
+    // Note that we could have a base-to-derived conversion here to a member of
+    // a derived class with less linkage/visibility. That's covered by the
+    // linkage and visibility of the value's type.
+    break;
+  }
+
+  return LV;
+}

clang/lib/AST/ASTContext.cpp

CharUnits ASTContext::getOffsetOfBaseWithVBPtr(const CXXRecordDecl *RD) const {
   const ASTRecordLayout *Layout = &getASTRecordLayout(RD);
   while (const CXXRecordDecl *Base = Layout->getBaseSharingVBPtr()) {
     Offset += Layout->getBaseClassOffset(Base);
     Layout = &getASTRecordLayout(Base);
   }
   return Offset;
 }
 
+CharUnits ASTContext::getMemberPointerPathAdjustment(const APValue &MP) const {
+  const ValueDecl *MPD = MP.getMemberPointerDecl();
+  CharUnits ThisAdjustment = CharUnits::Zero();
+  ArrayRef<const CXXRecordDecl*> Path = MP.getMemberPointerPath();
+  bool DerivedMember = MP.isMemberPointerToDerivedMember();
+  const CXXRecordDecl *RD = cast<CXXRecordDecl>(MPD->getDeclContext());
+  for (unsigned I = 0, N = Path.size(); I != N; ++I) {
+    const CXXRecordDecl *Base = RD;
+    const CXXRecordDecl *Derived = Path[I];
+    if (DerivedMember)
+      std::swap(Base, Derived);
+    ThisAdjustment += getASTRecordLayout(Derived).getBaseClassOffset(Base);
+    RD = Path[I];
+  }
+  if (DerivedMember)
+    ThisAdjustment = -ThisAdjustment;
+  return ThisAdjustment;
+}
+
 /// DeepCollectObjCIvars -
 /// This routine first collects all declared, but not synthesized, ivars in
 /// super class and then collects all ivars, including those synthesized for
 /// current class. This routine is used for implementation of current class
 /// when all ivars, declared and synthesized are known.
 void ASTContext::DeepCollectObjCIvars(const ObjCInterfaceDecl *OI,
                                       bool leafClass,
                             SmallVectorImpl<const ObjCIvarDecl*> &Ivars) const {

clang/lib/AST/Decl.cpp

case Decl::CXXRecord: {
 
         return getLVForClosure(
                   Record->getDeclContext()->getRedeclContext(),
                   Record->getLambdaContextDecl(), computation);
       }
 
       break;
     }
+
+    case Decl::TemplateParamObject: {
+      // The template parameter object can be referenced from anywhere its type
+      // and value can be referenced.
+      auto *TPO = cast<TemplateParamObjectDecl>(D);
+      LinkageInfo LV = getLVForType(*TPO->getType(), computation);
+      LV.merge(getLVForValue(TPO->getValue(), computation));
+      return LV;
+    }
   }
 
   // Handle linkage for namespace-scope names.
   if (D->getDeclContext()->getRedeclContext()->isFileContext())
     return getLVForNamespaceScopeDecl(D, computation, IgnoreVarTypeLinkage);
 
   // C++ [basic.link]p5:
   //   In addition, a member function, static data member, a named

clang/lib/AST/ItaniumMangle.cpp

#include "clang/AST/TypeNodes.inc"
   void mangleCXXDtorType(CXXDtorType T);
 
   void mangleTemplateArgs(const TemplateArgumentLoc *TemplateArgs,
                           unsigned NumTemplateArgs);
   void mangleTemplateArgs(const TemplateArgument *TemplateArgs,
                           unsigned NumTemplateArgs);
   void mangleTemplateArgs(const TemplateArgumentList &AL);
   void mangleTemplateArg(TemplateArgument A);
+  void mangleValueInTemplateArg(QualType T, const APValue &V);
 
   void mangleTemplateParameter(unsigned Depth, unsigned Index);
 
   void mangleFunctionParam(const ParmVarDecl *parm);
 
   void writeAbiTags(const NamedDecl *ND,
                     const AbiTagList *AdditionalAbiTags);
 

 
 void CXXNameMangler::mangle(GlobalDecl GD) {
   // <mangled-name> ::= _Z <encoding>
   //            ::= <data name>
   //            ::= <special-name>
   Out << "_Z";
   if (isa<FunctionDecl>(GD.getDecl()))
     mangleFunctionEncoding(GD);
-  else if (const VarDecl *VD = dyn_cast<VarDecl>(GD.getDecl()))
-    mangleName(VD);
+  else if (isa<VarDecl, FieldDecl, MSGuidDecl, TemplateParamObjectDecl,
+               BindingDecl>(GD.getDecl()))
+    mangleName(GD);
   else if (const IndirectFieldDecl *IFD =
                dyn_cast<IndirectFieldDecl>(GD.getDecl()))
     mangleName(IFD->getAnonField());
-  else if (const FieldDecl *FD = dyn_cast<FieldDecl>(GD.getDecl()))
-    mangleName(FD);
-  else if (const MSGuidDecl *GuidD = dyn_cast<MSGuidDecl>(GD.getDecl()))
-    mangleName(GuidD);
-  else if (const BindingDecl *BD = dyn_cast<BindingDecl>(GD.getDecl()))
-    mangleName(BD);
-  else if (isa<TemplateParamObjectDecl>(GD.getDecl())) {
-    DiagnosticsEngine &Diags = Context.getDiags();
-    unsigned DiagID = Diags.getCustomDiagID(DiagnosticsEngine::Error,
-      "cannot mangle template parameter objects yet");
-    Diags.Report(SourceLocation(), DiagID);
-  } else
+  else
     llvm_unreachable("unexpected kind of global decl");
 }
 
 void CXXNameMangler::mangleFunctionEncoding(GlobalDecl GD) {
   const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
   // <encoding> ::= <function name> <bare-function-type>
 
   // Don't mangle in the type if this isn't a decl we should typically mangle.
if (auto *GD = dyn_cast<MSGuidDecl>(ND)) {
       // with a particular reserved name. Continue the pretense here.
       SmallString<sizeof("_GUID_12345678_1234_1234_1234_1234567890ab")> GUID;
       llvm::raw_svector_ostream GUIDOS(GUID);
       Context.mangleMSGuidDecl(GD, GUIDOS);
       Out << GUID.size() << GUID;
       break;
     }
 
+    if (auto *TPO = dyn_cast<TemplateParamObjectDecl>(ND)) {
+      // Proposed in https://github.com/itanium-cxx-abi/cxx-abi/issues/63.
+      Out << "TAX";
+      mangleValueInTemplateArg(TPO->getType().getUnqualifiedType(),
+                               TPO->getValue());
+      Out << "E";
+      break;
+    }
+
     if (II) {
       // Match GCC's naming convention for internal linkage symbols, for
       // symbols that are not actually visible outside of this TU. GCC
       // distinguishes between internal and external linkage symbols in
       // its mangling, to support cases like this that were valid C++ prior
       // to DR426:
       //
       //   void test() { extern void foo(); }
case TemplateArgument::Expression: {
     Out << 'E';
     break;
   }
   case TemplateArgument::Integral:
     mangleIntegerLiteral(A.getIntegralType(), A.getAsIntegral());
     break;
   case TemplateArgument::Declaration: {
     //  <expr-primary> ::= L <mangled-name> E # external name
+    ValueDecl *D = A.getAsDecl();
+
+    // Template parameter objects are modeled by reproducing a source form
+    // produced as if by aggregate initialization.
+    if (auto *TPO = dyn_cast<TemplateParamObjectDecl>(D)) {
+      Out << 'X';
+      mangleValueInTemplateArg(TPO->getType().getUnqualifiedType(),
+                               TPO->getValue());
+      Out << 'E';
+      break;
+    }
+
     // Clang produces AST's where pointer-to-member-function expressions
     // and pointer-to-function expressions are represented as a declaration not
     // an expression. We compensate for it here to produce the correct mangling.
-    ValueDecl *D = A.getAsDecl();
     bool compensateMangling = !A.getParamTypeForDecl()->isReferenceType();
     if (compensateMangling) {
       Out << 'X';
       mangleOperatorName(OO_Amp, 1);
     }
 
     Out << 'L';
     // References to external entities use the mangled name; if the name would
case TemplateArgument::Pack: {
     Out << 'J';
     for (const auto &P : A.pack_elements())
       mangleTemplateArg(P);
     Out << 'E';
   }
   }
 }
 
+/// Determine whether a given value is equivalent to zero-initialization for
+/// the purpose of discarding a trailing portion of a 'tl' mangling.
+static bool isZeroInitialized(QualType T, const APValue &V) {
+  // FIXME: mangleValueInTemplateArg has quadratic time complexity in
+  // pathological cases due to using this, but it's a little awkward
+  // to do this in linear time in general.
+  switch (V.getKind()) {
+  case APValue::None:
+  case APValue::Indeterminate:
+  case APValue::AddrLabelDiff:
+    return false;
+
+  case APValue::Struct: {
+    const CXXRecordDecl *RD = T->getAsCXXRecordDecl();
+    assert(RD && "unexpected type for record value");
+    unsigned I = 0;
+    for (const CXXBaseSpecifier &BS : RD->bases()) {
+      if (!isZeroInitialized(BS.getType(), V.getStructBase(I)))
+        return false;
+      ++I;
+    }
+    I = 0;
+    for (const FieldDecl *FD : RD->fields()) {
+      if (!FD->isUnnamedBitfield() &&
+          !isZeroInitialized(FD->getType(), V.getStructField(I)))
+        return false;
+      ++I;
+    }
+    return true;
+  }
+
+  case APValue::Union: {
+    const CXXRecordDecl *RD = T->getAsCXXRecordDecl();
+    assert(RD && "unexpected type for union value");
+    // Zero-initialization zeroes the first non-unnamed-bitfield field, if any.
+    for (const FieldDecl *FD : RD->fields()) {
+      if (!FD->isUnnamedBitfield())
+        return V.getUnionField() && declaresSameEntity(FD, V.getUnionField()) &&
+               isZeroInitialized(FD->getType(), V.getUnionValue());
+    }
+    // If there are no fields (other than unnamed bitfields), the value is
+    // necessarily zero-initialized.
+    return true;
+  }
+
+  case APValue::Array: {
+    QualType ElemT(T->getArrayElementTypeNoTypeQual(), 0);
+    for (unsigned I = 0, N = V.getArrayInitializedElts(); I != N; ++I)
+      if (!isZeroInitialized(ElemT, V.getArrayInitializedElt(I)))
+        return false;
+    return !V.hasArrayFiller() || isZeroInitialized(ElemT, V.getArrayFiller());
+  }
+
+  case APValue::Vector: {
+    const VectorType *VT = T->castAs<VectorType>();
+    for (unsigned I = 0, N = V.getVectorLength(); I != N; ++I)
+      if (!isZeroInitialized(VT->getElementType(), V.getVectorElt(I)))
+        return false;
+    return true;
+  }
+
+  case APValue::Int:
+    return !V.getInt();
+
+  case APValue::Float:
+    return V.getFloat().isPosZero();
+
+  case APValue::FixedPoint:
+    return !V.getFixedPoint().getValue();
+
+  case APValue::ComplexFloat:
+    return V.getComplexFloatReal().isPosZero() &&
+           V.getComplexFloatImag().isPosZero();
+
+  case APValue::ComplexInt:
+    return !V.getComplexIntReal() && !V.getComplexIntImag();
+
+  case APValue::LValue:
+    return V.isNullPointer();
+
+  case APValue::MemberPointer:
+    return !V.getMemberPointerDecl();
+  }
+}
+
+void CXXNameMangler::mangleValueInTemplateArg(QualType T, const APValue &V) {
+  // Proposed in https://github.com/itanium-cxx-abi/cxx-abi/issues/63.
+  switch (V.getKind()) {
+  case APValue::None:
+  case APValue::Indeterminate:
+    Out << 'L';
+    mangleType(T);
+    Out << 'E';
+    return;
+
+  case APValue::AddrLabelDiff:
+    llvm_unreachable("unexpected value kind in template argument");
+
+  case APValue::Struct: {
+    const CXXRecordDecl *RD = T->getAsCXXRecordDecl();
+    assert(RD && "unexpected type for record value");
+
+    // Drop trailing zero-initialized elements.
+    llvm::SmallVector<const FieldDecl *, 16> Fields(RD->field_begin(),
+                                                    RD->field_end());
+    while (
+        !Fields.empty() &&
+        (Fields.back()->isUnnamedBitfield() ||
+         isZeroInitialized(Fields.back()->getType(),
+                           V.getStructField(Fields.back()->getFieldIndex())))) {
+      Fields.pop_back();
+    }
+    llvm::ArrayRef<CXXBaseSpecifier> Bases(RD->bases_begin(), RD->bases_end());
+    if (Fields.empty()) {
+      while (!Bases.empty() &&
+             isZeroInitialized(Bases.back().getType(),
+                               V.getStructBase(Bases.size() - 1)))
+        Bases = Bases.drop_back();
+    }
+
+    // <expression> ::= tl <type> <braced-expression>* E
+    Out << "tl";
+    mangleType(T);
+    for (unsigned I = 0, N = Bases.size(); I != N; ++I)
+      mangleValueInTemplateArg(Bases[I].getType(), V.getStructBase(I));
+    for (unsigned I = 0, N = Fields.size(); I != N; ++I) {
+      if (Fields[I]->isUnnamedBitfield())
+        continue;
+      mangleValueInTemplateArg(Fields[I]->getType(),
+                               V.getStructField(Fields[I]->getFieldIndex()));
+    }
+    Out << 'E';
+    return;
+  }
+
+  case APValue::Union: {
+    const CXXRecordDecl *RD = T->getAsCXXRecordDecl();
+    assert(RD && "unexpected type for union value");
+    const FieldDecl *FD = V.getUnionField();
+
+    if (!FD) {
+      Out << 'L';
+      mangleType(T);
+      Out << 'E';
+      return;
+    }
+
+    // <braced-expression> ::= di <field source-name> <braced-expression>
+    Out << "tl";
+    mangleType(T);
+    if (!isZeroInitialized(T, V)) {
+      Out << "di";
+      mangleSourceName(FD->getIdentifier());
+      mangleValueInTemplateArg(FD->getType(), V.getUnionValue());
+    }
+    Out << 'E';
+    return;
+  }
+
+  case APValue::Array: {
+    QualType ElemT(T->getArrayElementTypeNoTypeQual(), 0);
+
+    Out << "tl";
+    mangleType(T);
+
+    // Drop trailing zero-initialized elements.
+    unsigned N = V.getArraySize();
+    if (!V.hasArrayFiller() || isZeroInitialized(ElemT, V.getArrayFiller())) {
+      N = V.getArrayInitializedElts();
+      while (N && isZeroInitialized(ElemT, V.getArrayInitializedElt(N - 1)))
+        --N;
+    }
+
+    for (unsigned I = 0; I != N; ++I) {
+      const APValue &Elem = I < V.getArrayInitializedElts()
+                                ? V.getArrayInitializedElt(I)
+                                : V.getArrayFiller();
+      mangleValueInTemplateArg(ElemT, Elem);
+    }
+    Out << 'E';
+    return;
+  }
+
+  case APValue::Vector: {
+    const VectorType *VT = T->castAs<VectorType>();
+
+    Out << "tl";
+    mangleType(T);
+    unsigned N = V.getVectorLength();
+    while (N && isZeroInitialized(VT->getElementType(), V.getVectorElt(N - 1)))
+      --N;
+    for (unsigned I = 0; I != N; ++I)
+      mangleValueInTemplateArg(VT->getElementType(), V.getVectorElt(I));
+    Out << 'E';
+    return;
+  }
+
+  case APValue::Int:
+    mangleIntegerLiteral(T, V.getInt());
+    return;
+
+  case APValue::Float:
+    Out << 'L';
+    mangleType(T);
+    mangleFloat(V.getFloat());
+    Out << 'E';
+    return;
+
+  case APValue::FixedPoint:
+    llvm_unreachable("Fixed point types are disabled for c++");
+    return;
+
+  case APValue::ComplexFloat: {
+    const ComplexType *CT = T->castAs<ComplexType>();
+    Out << "tl";
+    mangleType(T);
+    if (V.getComplexFloatReal().isNonZero() ||
+        V.getComplexFloatImag().isNonZero()) {
+      Out << 'L';
+      mangleType(CT->getElementType());
+      mangleFloat(V.getComplexFloatReal());
+      Out << 'E';
+    }
+    if (V.getComplexFloatImag().isNonZero()) {
+      Out << 'L';
+      mangleType(CT->getElementType());
+      mangleFloat(V.getComplexFloatImag());
+      Out << 'E';
+    }
+    Out << 'E';
+    return;
+  }
+
+  case APValue::ComplexInt: {
+    const ComplexType *CT = T->castAs<ComplexType>();
+    Out << "tl";
+    mangleType(T);
+    if (V.getComplexIntReal().getBoolValue() ||
+        V.getComplexIntImag().getBoolValue())
+      mangleIntegerLiteral(CT->getElementType(), V.getComplexIntReal());
+    if (V.getComplexIntImag().getBoolValue())
+      mangleIntegerLiteral(CT->getElementType(), V.getComplexIntImag());
+    Out << 'E';
+    return;
+  }
+
+  case APValue::LValue: {
+    // Proposed in https://github.com/itanium-cxx-abi/cxx-abi/issues/47.
+    assert((T->isPointerType() || T->isReferenceType()) &&
+           "unexpected type for LValue template arg");
+
+    if (V.isNullPointer()) {
+      Out << "L";
+      mangleType(T);
+      Out << "0E";
+      return;
+    }
+
+    APValue::LValueBase B = V.getLValueBase();
+    if (!B) {
+      // Non-standard mangling for integer cast to a pointer; this can only
+      // occur as an extension.
+      CharUnits Offset = V.getLValueOffset();
+      if (Offset.isZero()) {
+        // This is reinterpret_cast<T*>(0), not a null pointer. Mangle this as
+        // a cast, because L <type> 0 E means something else.
+        Out << "rc";
+        mangleType(T);
+        Out << "Li0EE";
+      } else {
+        Out << "L";
+        mangleType(T);
+        Out << Offset.getQuantity() << 'E';
+      }
+      return;
+    }
+
+    enum { Base, Offset, Path } Kind;
+    if (!V.hasLValuePath()) {
+      // Mangle as (T*)((char*)&base + N).
+      if (T->isReferenceType()) {
+        Out << "decvP";
+        mangleType(T->getPointeeType());
+      } else {
+        Out << "cv";
+        mangleType(T);
+      }
+      Out << "plcvPcad";
+      Kind = Offset;
+    } else {
+      if (T->isPointerType())
+        Out << "ad";
+      if (!V.getLValuePath().empty() || V.isLValueOnePastTheEnd()) {
+        Out << "so";
+        mangleType(T->getPointeeType());
+        Kind = Path;
+      } else {
+        Kind = Base;
+      }
+    }
+
+    QualType TypeSoFar;
+    if (auto *VD = B.dyn_cast<const ValueDecl*>()) {
+      Out << 'L';
+      mangle(VD);
+      Out << 'E';
+      TypeSoFar = VD->getType();
+    } else if (auto *E = B.dyn_cast<const Expr*>()) {
+      mangleExpression(E);
+      TypeSoFar = E->getType();
+    } else if (auto TI = B.dyn_cast<TypeInfoLValue>()) {
+      Out << "ti";
+      mangleType(QualType(TI.getType(), 0));
+      TypeSoFar = B.getTypeInfoType();
+    } else {
+      // We should never see dynamic allocations here.
+      llvm_unreachable("unexpected lvalue base kind in template argument");
+    }
+
+    switch (Kind) {
+    case Base:
+      break;
+
+    case Offset:
+      Out << 'L';
+      mangleType(Context.getASTContext().getPointerDiffType());
+      mangleNumber(V.getLValueOffset().getQuantity());
+      Out << 'E';
+      break;
+
+    case Path:
+      // <expression> ::= so <referent type> <expr> [<offset number>]
+      //                  <union-selector>* [p] E
+      if (!V.getLValueOffset().isZero())
+        mangleNumber(V.getLValueOffset().getQuantity());
+
+      // We model a past-the-end array pointer as array indexing with index N,
+      // not with the "past the end" flag. Compensate for that.
+      bool OnePastTheEnd = V.isLValueOnePastTheEnd();
+
+      for (APValue::LValuePathEntry E : V.getLValuePath()) {
+        if (auto *AT = TypeSoFar->getAsArrayTypeUnsafe()) {
+          if (auto *CAT = dyn_cast<ConstantArrayType>(AT))
+            OnePastTheEnd |= CAT->getSize() == E.getAsArrayIndex();
+          TypeSoFar = AT->getElementType();
+        } else {
+          const Decl *D = E.getAsBaseOrMember().getPointer();
+          if (auto *FD = dyn_cast<FieldDecl>(D)) {
+            // <union-selector> ::= _ <number>
+            if (FD->getParent()->isUnion()) {
+              Out << '_';
+              if (FD->getFieldIndex())
+                Out << (FD->getFieldIndex() - 1);
+            }
+            TypeSoFar = FD->getType();
+          } else {
+            TypeSoFar =
+                Context.getASTContext().getRecordType(cast<CXXRecordDecl>(D));
+          }
+        }
+      }
+
+      if (OnePastTheEnd)
+        Out << 'p';
+      Out << 'E';
+      break;
+    }
+
+    return;
+  }
+
+  case APValue::MemberPointer:
+    // Proposed in https://github.com/itanium-cxx-abi/cxx-abi/issues/47.
+    if (!V.getMemberPointerDecl()) {
+      Out << 'L';
+      mangleType(T);
+      Out << "0E";
+      return;
+    }
+
+    if (!V.getMemberPointerPath().empty()) {
+      Out << "mc";
+      mangleType(T);
+    }
+    Out << "adL";
+    mangle(V.getMemberPointerDecl());
+    Out << 'E';
+    if (!V.getMemberPointerPath().empty()) {
+      CharUnits Offset =
+          Context.getASTContext().getMemberPointerPathAdjustment(V);
+      if (!Offset.isZero())
+        mangleNumber(Offset.getQuantity());
+      Out << 'E';
+    }
+    return;
+  }
+}
+
 void CXXNameMangler::mangleTemplateParameter(unsigned Depth, unsigned Index) {
   // <template-param> ::= T_    # first template parameter
   //                  ::= T <parameter-2 non-negative number> _
   //                  ::= TL <L-1 non-negative number> __
   //                  ::= TL <L-1 non-negative number> _
   //                         <parameter-2 non-negative number> _
   //
   // The latter two manglings are from a proposal here:

 /// If the declaration D requires a mangled name, this routine will emit that
 /// mangled name to \p os and return true. Otherwise, \p os will be unchanged
 /// and this routine will return false. In this case, the caller should just
 /// emit the identifier of the declaration (\c D->getIdentifier()) as its
 /// name.
 void ItaniumMangleContextImpl::mangleCXXName(GlobalDecl GD,
                                              raw_ostream &Out) {
   const NamedDecl *D = cast<NamedDecl>(GD.getDecl());
-  assert((isa<FunctionDecl>(D) || isa<VarDecl>(D)) &&
-          "Invalid mangleName() call, argument is not a variable or function!");
+  assert((isa<FunctionDecl, VarDecl, TemplateParamObjectDecl>(D)) &&
 
   PrettyStackTraceDecl CrashInfo(D, SourceLocation(),
                                  getASTContext().getSourceManager(),
                                  "Mangling declaration");
 
   if (auto *CD = dyn_cast<CXXConstructorDecl>(D)) {
     auto Type = GD.getCtorType();
     CXXNameMangler Mangler(*this, Out, CD, Type);

clang/lib/AST/Linkage.h

class LinkageComputer {
   LinkageInfo getLVForLocalDecl(const NamedDecl *D,
                                 LVComputationKind computation);
 
   LinkageInfo getLVForType(const Type &T, LVComputationKind computation);
 
   LinkageInfo getLVForTemplateParameterList(const TemplateParameterList *Params,
                                             LVComputationKind computation);
 
+  LinkageInfo getLVForValue(const APValue &V, LVComputationKind computation);
+
 public:
   LinkageInfo computeLVForDecl(const NamedDecl *D,
                                LVComputationKind computation,
                                bool IgnoreVarTypeLinkage = false);
 
   LinkageInfo getLVForDecl(const NamedDecl *D, LVComputationKind computation);
 
   LinkageInfo computeTypeLinkageInfo(const Type *T);

clang/lib/CodeGen/CGCXXABI.h

 protected:
   /// A utility method for computing the offset required for the given
   /// base-to-derived or derived-to-base member-pointer conversion.
   /// Does not handle virtual conversions (in case we ever fully
   /// support an ABI that allows this).  Returns null if no adjustment
   /// is required.
   llvm::Constant *getMemberPointerAdjustment(const CastExpr *E);
 
-  /// Computes the non-virtual adjustment needed for a member pointer
-  /// conversion along an inheritance path stored in an APValue.  Unlike
-  /// getMemberPointerAdjustment(), the adjustment can be negative if the path
-  /// is from a derived type to a base type.
-  CharUnits getMemberPointerPathAdjustment(const APValue &MP);
-
 public:
   virtual void emitVirtualObjectDelete(CodeGenFunction &CGF,
                                        const CXXDeleteExpr *DE,
                                        Address Ptr, QualType ElementType,
                                        const CXXDestructorDecl *Dtor) = 0;
   virtual void emitRethrow(CodeGenFunction &CGF, bool isNoReturn) = 0;
   virtual void emitThrow(CodeGenFunction &CGF, const CXXThrowExpr *E) = 0;
   virtual llvm::GlobalVariable *getThrowInfo(QualType T) { return nullptr; }

clang/lib/CodeGen/CGCXXABI.cpp

llvm::Constant *CGCXXABI::getMemberPointerAdjustment(const CastExpr *E) {
   const CXXRecordDecl *derivedClass =
     derivedType->castAs<MemberPointerType>()->getClass()->getAsCXXRecordDecl();
 
   return CGM.GetNonVirtualBaseClassOffset(derivedClass,
                                           E->path_begin(),
                                           E->path_end());
 }
 
-CharUnits CGCXXABI::getMemberPointerPathAdjustment(const APValue &MP) {
-  // TODO: Store base specifiers in APValue member pointer paths so we can
-  // easily reuse CGM.GetNonVirtualBaseClassOffset().
-  const ValueDecl *MPD = MP.getMemberPointerDecl();
-  CharUnits ThisAdjustment = CharUnits::Zero();
-  ArrayRef<const CXXRecordDecl*> Path = MP.getMemberPointerPath();
-  bool DerivedMember = MP.isMemberPointerToDerivedMember();
-  const CXXRecordDecl *RD = cast<CXXRecordDecl>(MPD->getDeclContext());
-  for (unsigned I = 0, N = Path.size(); I != N; ++I) {
-    const CXXRecordDecl *Base = RD;
-    const CXXRecordDecl *Derived = Path[I];
-    if (DerivedMember)
-      std::swap(Base, Derived);
-    ThisAdjustment +=
-      getContext().getASTRecordLayout(Derived).getBaseClassOffset(Base);
-    RD = Path[I];
-  }
-  if (DerivedMember)
-    ThisAdjustment = -ThisAdjustment;
-  return ThisAdjustment;
-}
-
 llvm::BasicBlock *
 CGCXXABI::EmitCtorCompleteObjectHandler(CodeGenFunction &CGF,
                                         const CXXRecordDecl *RD) {
   if (CGM.getTarget().getCXXABI().hasConstructorVariants())
     llvm_unreachable("shouldn't be called in this ABI");
 
   ErrorUnsupportedABI(CGF, "complete object detection in ctor");
   return nullptr;

clang/lib/CodeGen/CodeGenModule.cpp

if (!V.isAbsent()) {
     Addr = llvm::ConstantExpr::getBitCast(
         GV, Ty->getPointerTo(GV->getAddressSpace()));
   }
   return ConstantAddress(Addr, Alignment);
 }
 
 ConstantAddress CodeGenModule::GetAddrOfTemplateParamObject(
     const TemplateParamObjectDecl *TPO) {
+  StringRef Name = getMangledName(TPO);
+  CharUnits Alignment = getNaturalTypeAlignment(TPO->getType());
+
+  if (llvm::GlobalVariable *GV = getModule().getNamedGlobal(Name))
+    return ConstantAddress(GV, Alignment);
+
+  ConstantEmitter Emitter(*this);
+  llvm::Constant *Init = Emitter.emitForInitializer(
+        TPO->getValue(), TPO->getType().getAddressSpace(), TPO->getType());
+
+  if (!Init) {
     ErrorUnsupported(TPO, "template parameter object");
     return ConstantAddress::invalid();
   }
 
+  auto *GV = new llvm::GlobalVariable(
+      getModule(), Init->getType(),
+      /*isConstant=*/true, llvm::GlobalValue::LinkOnceODRLinkage, Init, Name);
+  if (supportsCOMDAT())
+    GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
+  Emitter.finalize(GV);
+
+  return ConstantAddress(GV, Alignment);
+}
+
 ConstantAddress CodeGenModule::GetWeakRefReference(const ValueDecl *VD) {
   const AliasAttr *AA = VD->getAttr<AliasAttr>();
   assert(AA && "No alias?");
 
   CharUnits Alignment = getContext().getDeclAlign(VD);
   llvm::Type *DeclTy = getTypes().ConvertTypeForMem(VD->getType());
 
   // See if there is already something with the target's name in the module.

clang/lib/CodeGen/ItaniumCXXABI.cpp

 
 llvm::Constant *ItaniumCXXABI::EmitMemberPointer(const APValue &MP,
                                                  QualType MPType) {
   const MemberPointerType *MPT = MPType->castAs<MemberPointerType>();
   const ValueDecl *MPD = MP.getMemberPointerDecl();
   if (!MPD)
     return EmitNullMemberPointer(MPT);
 
-  CharUnits ThisAdjustment = getMemberPointerPathAdjustment(MP);
+  CharUnits ThisAdjustment = getContext().getMemberPointerPathAdjustment(MP);
 
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(MPD))
     return BuildMemberPointer(MD, ThisAdjustment);
 
   CharUnits FieldOffset =
     getContext().toCharUnitsFromBits(getContext().getFieldOffset(MPD));
   return EmitMemberDataPointer(MPT, ThisAdjustment + FieldOffset);
 }

clang/test/CodeGenCXX/mangle-class-nttp.cpp

+// RUN: %clang_cc1 -std=c++20 %s -triple x86_64-linux-gnu -emit-llvm -o - | FileCheck %s
+
+#define fold(x) (__builtin_constant_p(x) ? (x) : (x))
+
+struct A { int a, b; };
+template<A> void f() {}
+
+// CHECK: define weak_odr void @_Z1fIXtl1ALi1ELi2EEEEvv(
+template void f<A{1, 2}>();
+
+struct B { int *p; int k; };
+template<B> void f() {}
+
+int n = 0;
+// CHECK: define weak_odr void @_Z1fIXtl1BadL_Z1nEEEEvv(
+template void f<B{&n}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BLPi0ELi1EEEEvv(
+template void f<B{nullptr, 1}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BEEEvv(
+template void f<B{nullptr}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BLPi32EEEEvv(
+template void f<B{fold((int*)32)}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BrcPiLi0EEEEEvv(
+template void f<B{fold(reinterpret_cast<int*>(0))}>();
+
+// Pointers to subobjects.
+struct Nested { union { int k; int arr[2]; }; } nested[2];
+struct Derived : A, Nested {} derived;
+// CHECK: define weak_odr void @_Z1fIXtl1BadsoiL_Z6nestedE_EEEEvv
+template void f<B{&nested[0].k}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BadsoiL_Z6nestedE16_0pEEEEvv
+template void f<B{&nested[1].arr[2]}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BadsoiL_Z7derivedE8pEEEEvv
+template void f<B{&derived.b + 1}>();
+// CHECK: define weak_odr void @_Z1fIXtl1BcvPiplcvPcadL_Z7derivedELl16EEEEvv
+template void f<B{fold(&derived.b + 3)}>();
+
+// References to subobjects.
+struct BR { int &r; };
+template<BR> void f() {}
+// CHECK: define weak_odr void @_Z1fIXtl2BRsoiL_Z6nestedE_EEEEvv
+template void f<BR{nested[0].k}>();
+// CHECK: define weak_odr void @_Z1fIXtl2BRsoiL_Z6nestedE12_0EEEEvv
+template void f<BR{nested[1].arr[1]}>();
+// CHECK: define weak_odr void @_Z1fIXtl2BRsoiL_Z7derivedE4EEEEvv
+template void f<BR{derived.b}>();
+// CHECK: define weak_odr void @_Z1fIXtl2BRdecvPiplcvPcadL_Z7derivedELl16EEEEvv
+template void f<BR{fold(*(&derived.b + 3))}>();
+
+// Qualification conversions.
+struct C { const int *p; };
+template<C> void f() {}
+// CHECK: define weak_odr void @_Z1fIXtl1CadsoKiL_Z7derivedE4EEEEvv
+template void f<C{&derived.b}>();
+
+// Pointers to members.
+struct D { const int Derived::*p; int k; };
+template<D> void f() {}
+// CHECK: define weak_odr void @_Z1fIXtl1DLM7DerivedKi0ELi1EEEEvv
+template void f<D{nullptr, 1}>();
+// CHECK: define weak_odr void @_Z1fIXtl1DEEEvv
+template void f<D{nullptr}>();
+// CHECK: define weak_odr void @_Z1fIXtl1DmcM7DerivedKiadL_ZN1A1aEEEEEEvv
+template void f<D{&A::a}>();
+// CHECK: define weak_odr void @_Z1fIXtl1DmcM7DerivedKiadL_ZN1A1bEEEEEEvv
+template void f<D{&A::b}>();
+// FIXME: Is the Ut_1 mangling here correct?
+// CHECK: define weak_odr void @_Z1fIXtl1DmcM7DerivedKiadL_ZN6NestedUt_1kEE8EEEEvv
+template void f<D{&Nested::k}>();
+struct MoreDerived : A, Derived { int z; };
+// CHECK: define weak_odr void @_Z1fIXtl1DmcM7DerivedKiadL_ZN11MoreDerived1zEEn8EEEEvv
+template void f<D{(int Derived::*)&MoreDerived::z}>();
+
+union E {
+  int n;
+  float f;
+  constexpr E() {}
+  constexpr E(int n) : n(n) {}
+  constexpr E(float f) : f(f) {}
+};
+template<E> void f() {}
+
+// Union members.
+// CHECK: define weak_odr void @_Z1fIXL1EEEEvv(
+template void f<E{}>();
+// CHECK: define weak_odr void @_Z1fIXtl1EEEEvv(
+template void f<E(0)>();
+// CHECK: define weak_odr void @_Z1fIXtl1Edi1nLi42EEEEvv(
+template void f<E(42)>();
+// CHECK: define weak_odr void @_Z1fIXtl1Edi1fLf00000000EEEEvv(
+template void f<E(0.f)>();
+
+// Extensions, and dropping trailing zero-initialized elements of 'tl'
+// manglings.
+typedef int __attribute__((ext_vector_type(3))) VI3;
+struct F { VI3 v; _Complex int ci; _Complex float cf; };
+template<F> void f() {}
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1ELi2ELi3EEtlCiLi4ELi5EEtlCfLf40c00000ELf40e00000EEEEEvv
+template void f<F{{1, 2, 3}, {4, 5}, {6, 7}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1ELi2ELi3EEtlCiLi4ELi5EEtlCfLf40c00000EEEEEvv
+template void f<F{{1, 2, 3}, {4, 5}, {6, 0}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1ELi2ELi3EEtlCiLi4ELi5EEEEEvv
+template void f<F{{1, 2, 3}, {4, 5}, {0, 0}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1ELi2ELi3EEtlCiLi4EEEEEvv
+template void f<F{{1, 2, 3}, {4, 0}, {0, 0}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1ELi2ELi3EEEEEvv
+template void f<F{{1, 2, 3}, {0, 0}, {0, 0}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1ELi2EEEEEvv
+template void f<F{{1, 2, 0}, {0, 0}, {0, 0}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FtlDv3_iLi1EEEEEvv
+template void f<F{{1, 0, 0}, {0, 0}, {0, 0}}>();
+// CHECK: define weak_odr void @_Z1fIXtl1FEEEvv
+template void f<F{{0, 0, 0}, {0, 0}, {0, 0}}>();
+
+// Unnamed bit-fields.
+struct G {
+  int : 3;
+  int a : 4;
+  int : 5;
+  int b : 6;
+  int : 7;
+};
+template<G> void f() {}
+// CHECK: define weak_odr void @_Z1fIXtl1GEEEvv
+template void f<(G())>();
+// CHECK: define weak_odr void @_Z1fIXtl1GLi1EEEEvv
+template void f<G{1}>();
+// CHECK: define weak_odr void @_Z1fIXtl1GLi1ELi2EEEEvv
+template void f<G{1, 2}>();
+
+// Empty and nearly-empty unions.
+union H1 {};
+union H2 { int : 1, : 2, : 3; };
+union H3 { int : 1, a, : 2, b, : 3; };
+struct H4 { H2 h2; };
+template<H1> void f() {}
+template<H2> void f() {}
+template<H3> void f() {}
+template<H4> void f() {}
+// CHECK: define weak_odr void @_Z1fIXL2H1EEEvv
+template void f<H1{}>();
+// CHECK: define weak_odr void @_Z1fIXL2H2EEEvv
+template void f<H2{}>();
+// CHECK: define weak_odr void @_Z1fIXtl2H3EEEvv
+template void f<H3{.a = 0}>();
+// CHECK: define weak_odr void @_Z1fIXtl2H3di1aLi1EEEEvv
+template void f<H3{.a = 1}>();
+// CHECK: define weak_odr void @_Z1fIXtl2H3di1bLi0EEEEvv
+template void f<H3{.b = 0}>();
+// CHECK: define weak_odr void @_Z1fIXtl2H4EEEvv
+template void f<H4{}>();
+
+// Floating-point.
+struct I {
+  float f;
+  double d;
+  long double ld;
+};
+template<I> void f() {}
+// CHECK: define weak_odr void @_Z1fIXtl1IEEEvv
+template void f<I{0.0, 0.0, 0.0}>();
+// CHECK: define weak_odr void @_Z1fIXtl1ILf80000000ELd8000000000000000ELe80000000000000000000EEEEvv
+template void f<I{-0.0, -0.0, -0.0}>();
+// CHECK: define weak_odr void @_Z1fIXtl1ILf3f800000ELd4000000000000000ELe4000c000000000000000EEEEvv
+template void f<I{1.0, 2.0, 3.0}>();

clang/test/CodeGenCXX/template-param-objects.cpp

+// RUN: %clang_cc1 -triple x86_64-linux-gnu -std=c++20 %s -emit-llvm -o - | FileCheck %s
+
+struct S { char buf[32]; };
+template<S s> constexpr const char *begin() { return s.buf; }
+template<S s> constexpr const char *end() { return s.buf + __builtin_strlen(s.buf); }
+
+// CHECK: [[HELLO:@_ZTAXtl1StlA32_cLc104ELc101ELc108ELc108ELc111ELc32ELc119ELc111ELc114ELc108ELc100EEEE]] = linkonce_odr constant { <{ [11 x i8], [21 x i8] }> } { <{ [11 x i8], [21 x i8] }> <{ [11 x i8] c"hello world", [21 x i8] zeroinitializer }> }, comdat
+
+// CHECK: @p = global i8* getelementptr inbounds ({{.*}}* [[HELLO]], i32 0, i32 0, i32 0, i32 0)
+const char *p = begin<S{"hello world"}>();
+// CHECK: @q = global i8* getelementptr (i8, i8* getelementptr inbounds ({{.*}}* [[HELLO]], i32 0, i32 0, i32 0, i32 0), i64 11)
+const char *q = end<S{"hello world"}>();