Re: C++ constructors and callgraph interaction ("__ct_comp " vs constructor function body)

[David Malcolm <dmalcolm at redhat dot com>]

On Wed, 2020-02-05 at 23:07 -0500, David Malcolm wrote:
> When debugging PR analyzer/93288, I'm compiling this test C++ file:
> 
> struct a {
>   a();
> };
> class foo {
>   a b;
> } c;
> 
> I see a cgraph_edge from __static_initialization_and_destruction_0 to
> a "__ct_comp ", which I see is a DECL_COMPLETE_CONSTRUCTOR_P.
> 
> The "__ct_comp " prints as a "a::a()/2", but its DECL_STRUCT_FUNCTION
> has a NULL CFG (which is what's causing the ICE in the analyzer).
> 
> The simple fix is to check for NULL, and skip such cases (but that
> would fail to analyze the calls).
> 
> However, there seems to be a separate a::a in the cgraph with a CFG
> containing the body of the ctor.
> 
> Looking at the __ct_comp:
> 
> (gdb) call edge->callee->debug() 
> _ZN3fooC1Ev/2 (foo::foo()) @0x7fffeaaaa2d0
>   Type: function definition analyzed alias cpp_implicit_alias
>   Visibility: externally_visible public weak comdat
> comdat_group:_ZN3fooC5Ev one_only artificial
>   Same comdat group as: _ZN3fooC2Ev/1
>   References: _ZN3fooC2Ev/1 (alias)
>   Referring: 
>   Availability: available
>   Function flags:
>   Called by: _Z41__static_initialization_and_destruction_0ii/4 (can
> throw external) 
>   Calls: 
> 
> and I note that we have _ZN3fooC1Ev/2 (the __ct_comp) and
> _ZN3fooC2Ev/1, which demangle to foo::foo()/2 and foo::foo()/1
> respectively.
> 
> I'm a bit hazy on the details here, sorry.
> 
> Presumably this "__ct_comp " fndecl's cgraph_node is thus linked to
> the
> "real" fndecl as a "cpp_implicit_alias"?
> 
> What's the best strategy for the analyzer to follow when it
> encounters
> a DECL_STRUCT_FUNCTION with a NULL CFG?
> 
> Should the analyzer:
> 
> (a) special-case this (e.g. assume that "cpp_implicit_alias" will
> have
> a single reference I can look up, or get it from the comdat groups,
> or
> somesuch), or
> 
> (b) are there some more general semantics I need to implement? (e.g.
> if
> it's weak, look in the comdat group for a non-weak alias, or
> somesuch?)

Answering my own question, calling ultimate_alias_target on the
__ct_comp cgraph_node seems to find the "right" cgraph_node.

Is that the right general-purpose answer here?

Thanks
Dave

> (c) not bother exploring those paths


> Thanks, and sorry if I'm missing something obvious here (I'm less
> familiar with this part of the compiler)
> 
> Dave
>