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

[Friedman, Eli via llvm-dev]

You're probably barking up the wrong tree; the problem has nothing to do 
with code generation. Consider the following testcase:

template<typename T> struct Z {
   T t;
   Z(const Z&) { throw 1; }
   ~Z() { *t = 10; }
};
struct X {
    Z<int> x;
    Z<int> y;
};
void foo(Z<int>& a) {
    new X{a, a};
}

This should emit an error because "t" can't be dereferenced.  gcc 
correctly the error; clang doesn't.

Not exactly sure why the destructor doesn't get marked odr-used, but I'd 
start with Sema::BuildCXXNew.

-Eli

On 5/1/2018 11:40 PM, Keith Wyss via llvm-dev 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 
> <mailto: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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