[llvm-dev] Generating function definition for function that's only called during unwinding

[Keith Wyss via llvm-dev]

Hmmm... It seems like I should check out how the UseList on Value (and its
child BasicBlock) work.

On Tue, May 1, 2018 at 8:34 PM, Keith Wyss <wyssman at gmail.com> wrote:

> Hi,
>
> I'm trying to understand how clang keeps track of which declarations are
> called within a translation unit and decides to codegen their definitions.
>
> DeclBase.h has a markUsed to keep track of ODR use, and I think that the
> decl can be found from the symbol table via ASTContext.h (for example
> looking up a template via GetQualifiedTemplateName -> getAsTemplateDecl ->
> setIsUsed ). This is just a result of fumbling around in the dark, I could
> be totally wrong.
>
> I'd like to know what part of the system is responsible for tracking
> usage, and whether that's the same mechanism that decides whether to
> CodeGen basic blocks with function definitions.
>
> Particularly, I'm curious about how the decision should be made whether to
> emit a definition for a destructor that it is only called in a cleanup
> block that's only invoked during unwinding. I don't actually understand how
> or whether the control flow for unwinding is expressed in the AST at all. I
> can't see anything related to unwinding control flow with -ast-dump for
> instance.
>
> I've been spending some time debugging a particular test program at bug
> 33778 <https://bugs.llvm.org/show_bug.cgi?id=33778> that fails to link
> because a block only referenced from an LSDA contains a call instruction to
> a destructor that has no generated definition. I'm hitting a wall because
> I'm pretty ignorant of the structure of the codebase and any guidance would
> be appreciated.
>
> Any general guidance or documentation to help find my way around clang
> would be awesome too.
>
> Thanks,
> Keith
>
> P.S. The program that doesn't link without -fno-exceptions:
>
> #include <functional>
>
> struct X {
>    std::function<int()> F;
>    std::function<int()> G;
> };
>
> void foo(const std::function<int()>& a) {
>    new X{a, a};
> }
>
> int main() {
>   return 0;
> }
>
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