[cfe-dev] Using the Sema Object to Generate Implicit Definitions

[Richard Smith via cfe-dev]

On Thu, 15 Oct 2020 at 14:23, Gregory Malecha via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hello --
>
> I've been trying to use the Sema object to generate implicit definitions,
> e.g. default constructors, destructors, etc, in a tool that I have but I'm
> running into some issues. The code that I have is a DeclVisitor with the
> following for the CXXRecord case.
>
> void VisitCXXRecordDecl(CXXRecordDecl *decl, bool is_specialization) {
>   if (not(decl->isImplicit() or decl->isAnonymousStructOrUnion())) {
>     ci_->getSema().ForceDeclarationOfImplicitMembers(decl);
>   }
>   // ...
> }
>
> This works great, and always works to get me constructor definitions with
> empty bodies but marked as default.
>
> To generate the bodies, I have (roughly) the following code for
> constructors, destructors, and methods:
>
> if (not decl->getBody() && decl->isDefaulted()) {
>   if (decl->isMoveAssignmentOperator()) {
>     ci_->getSema().DefineImplicitMoveAssignment(decl->getLocation(), decl);
>   } else if (decl->isCopyAssignmentOperator()) {
>     ci_->getSema().DefineImplicitCopyAssignment(decl->getLocation(), decl);
>   }
> }
>
> In most cases, this seems to do the trick, but when the implementation of
> the operator uses something like memcpy I get a segfault (stack trace
> below) due (it seems) to the fact that TUScope is null when my code runs.
>
> #0  0x00007ffff5ff733a in clang::Sema::PushOnScopeChains(clang::NamedDecl*, clang::Scope*, bool) () from /usr/lib/libclang-cpp.so.10#1  0x00007ffff601dc18 in clang::Sema::LazilyCreateBuiltin(clang::IdentifierInfo*, unsigned int, clang::Scope*, bool, clang::SourceLocation) () from /usr/lib/libclang-cpp.so.10#2  0x00007ffff63599ac in clang::Sema::LookupBuiltin(clang::LookupResult&) () from /usr/lib/libclang-cpp.so.10#3  0x00007ffff636e713 in clang::Sema::LookupName(clang::LookupResult&, clang::Scope*, bool) () from /usr/lib/libclang-cpp.so.10#4  0x00007ffff61512e4 in ?? () from /usr/lib/libclang-cpp.so.10#5  0x00007ffff6151643 in ?? () from /usr/lib/libclang-cpp.so.10#6  0x00007ffff6159481 in clang::Sema::DefineImplicitCopyAssignment(clang::SourceLocation, clang::CXXMethodDecl*) () from /usr/lib/libclang-cpp.so.10
>
> I can avoid this problem if I put the preprocessor in "incremental
> processing mode" using `enableIncrementalProcessing()` from
> BeginSourceFileAction in my ASTFrontendAction, but doing this causes clang
> to not generate the bodies of template specializations. I assume that this
> is because you can't generate specializations until you have the complete
> module which makes sense.
>
> Is there some way that I can get both of these, i.e. generate all implicit
> declarations *with their bodies* and get template specializations?
> Perhaps by using Sema to generate the bodies and then somehow finishing the
> file to get clang to generate the specializations? Or maybe I can manually
> call something in Sema to generate the bodies of the templates?
>

You can call Sema::ActOnEndOfTranslationUnit() to get it to perform the
pending actions that it's been saving up, such as template instantiations.


> Thanks in advance.
>
> --
> gregory malecha
> gmalecha.github.io
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> cfe-dev at lists.llvm.org
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>
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