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[CIR] Handle explicit instantiation declaration in getVTableLinkage #193809

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[CIR] Handle explicit instantiation declaration in getVTableLinkage #193809

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[CIR] Handle explicit instantiation declaration in getVTableLinkage 
Replace errorNYI for TSK_ExplicitInstantiationDeclaration with the
correct linkage logic: use discardable ODR linkage for MSVC, and
for Itanium choose between AvailableExternallyLinkage (when the
vtable can be speculatively emitted) or ExternalLinkage.

Fixes point 3 of #192330.
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@AbdallahRashed
AbdallahRashed committed Apr 23, 2026
commit cc061f77e0fd1daf4e87f972fc7fb0a75ee10d67
16 changes: 11 additions & 5 deletions clang/lib/CIR/CodeGen/CIRGenVTables.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -326,6 +326,9 @@ cir::GlobalOp CIRGenVTables::generateConstructionVTable(
return vtable;
}

static bool shouldEmitAvailableExternallyVTable(const CIRGenModule &cgm,
const CXXRecordDecl *rd);

/// Compute the required linkage of the vtable for the given class.
///
/// Note that we only call this at the end of the translation unit.
Expand Down Expand Up @@ -395,11 +398,14 @@ cir::GlobalLinkageKind CIRGenModule::getVTableLinkage(const CXXRecordDecl *rd) {
case TSK_ImplicitInstantiation:
return discardableODRLinkage;

case TSK_ExplicitInstantiationDeclaration: {
errorNYI(rd->getSourceRange(),
"getVTableLinkage: explicit instantiation declaration");
return cir::GlobalLinkageKind::ExternalLinkage;
}
case TSK_ExplicitInstantiationDeclaration:
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@andykaylor andykaylor Apr 23, 2026

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The llvm_unreachable on line 375 relates to this same kind. It shouldn't be unreachable. That isn't implemented in the incubator, but it is in classic codegen (doing basically what you have here). Can you add that in this PR also. It will require an additional test case to reach that code.

// Explicit instantiations in MSVC do not provide vtables, so we must emit
// our own.
if (getTarget().getCXXABI().isMicrosoft())
return discardableODRLinkage;
return shouldEmitAvailableExternallyVTable(*this, rd)
? cir::GlobalLinkageKind::AvailableExternallyLinkage
: cir::GlobalLinkageKind::ExternalLinkage;

case TSK_ExplicitInstantiationDefinition:
return nonDiscardableODRLinkage;
Expand Down
27 changes: 27 additions & 0 deletions clang/test/CIR/CodeGen/vtable-linkage-explicit-instantiation.cpp
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@@ -0,0 +1,27 @@
// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
// RUN: FileCheck --input-file=%t.cir %s
// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t-cir.ll
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@andykaylor andykaylor Apr 23, 2026

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Can you also add RUN lines to compile without -fclangir? This is useful for verifying that we do the same thing as classic codegen.

// RUN: FileCheck --check-prefix=LLVM --input-file=%t-cir.ll %s

// Test that explicit instantiation declarations don't trigger
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This comment isn't useful. I'd delete it.

// "getVTableLinkage: explicit instantiation declaration" errorNYI.

template <typename T>
struct Base {
virtual ~Base() {}
virtual void foo() {}
T val;
};

extern template class Base<int>;

void use(Base<int> *p) {
p->foo();
}

// Verify the virtual call goes through the vtable.
// CHECK: cir.func {{.*}} @_Z3useP4BaseIiE
// CHECK: cir.vtable.get_vptr
// CHECK: cir.vtable.get_virtual_fn_addr

// LLVM: define {{.*}} @_Z3useP4BaseIiE