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

[Dean Michael Berris via llvm-dev]

I could be wrong here, but AFAICT, the implementation is allowed to elide
initialisation/allocation if it can prove that the allocation is not
observable. In this case, I would look at the generated IR and whether the
call to ‘new’ turns into a ‘malloc’ that gets elided later. -O0 and/or
other optimisation levels might do different things too.

If you change this to return the pointer and print it, do you get the
destructor code-gen’ed?
On Wed, 2 May 2018 at 4:40 pm, Keith Wyss via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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-- 
-- Dean
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