[LLVMdev] Proposal: Adding an optional association field to llvm.global_ctors

Duncan Sands baldrick at free.fr
Wed Sep 4 01:11:02 PDT 2013

Hi Reid,

On 03/09/13 20:08, Reid Kleckner wrote:
> On Mon, Sep 2, 2013 at 5:18 AM, Duncan Sands <baldrick at free.fr
> <mailto:baldrick at free.fr>> wrote:
>     Hi Reid,
>     On 26/08/13 23:43, Reid Kleckner wrote:
>         To implement http://llvm.org/PR16959
>         <http://llvm.org/bugs/show___bug.cgi?id=16959
>         <http://llvm.org/bugs/show_bug.cgi?id=16959>>, I need to add a new field to
>         global_ctors.
>         Static data members of class template instantiations can have
>         initializers that
>         must only run once on program startup.  Itanium solves this with guard
>         variables
>         and the emission of multiple initializers, only one of which actually
>         initializes the data at runtime.
>         Microsoft solves this by using a COFF comdat feature called
>         IMAGE_COMDAT_SELECT___ASSOCIATIVE. The semantics are that the section
>         with this
>         attribute is only linked if the associated section is chosen for the
>         final link.
>         Otherwise it is discarded.
>         In this way, only one initializer is linked and only one entry is
>         produced in
>         the initializer array (.CRT$XCU). Everyone I've spoken with so far
>         thinks this
>         is basically awesome and is much cleaner than using guard variables. :)
>         I propose changing LangRef to make llvm.global_ctors and
>         llvm.global_dtors be
>         arrays of { i32, void()*, i8* }, where the last field is an optional
>         pointer to
>         a GlobalValue. Old modules containing { i32, void()*} elements will also be
>         accepted, and the missing third field will be assumed to be null.
>         When performing LTO, if there are multiple entries with the same associated
>         global, only one of them will be kept in the linked module.
>     will two different globals have different initialization functions too, or will
>     the same function be reused for many different globals?
> I'm not sure I follow.  Do you mean, are initialization functions reused for two
> globals in the same TU?

yes, or even in different TU (eg by using linkonce linkage).

> Even today, every global with a dynamic initializer gets its own void()
> initialization function.  We don't use a single function to initialize everything.

OK, thanks for explaining.  In that case, will multiple instances of the same
global (the ones you want to eliminate) have the same initialization function?
If so, then I guess right now, without your changes, llvm.global_ctors will
have multiple copies of the same function.  Then instead of introducing a third
"global variable" i8* field in llvm.global_ctors, there could instead by a
third boolean field meaning: collapse multiple instances of this initialization
function.  In fact, maybe it is always OK to remove duplicate initialization
functions in llvm.global_ctors?

Ciao, Duncan.

> We need to enforce an ordering on initializers in a single TU, so we have a
> function (usually _GLOBAL_I_a) which calls each stub in turn.  After inlining,
> most stubs are probably eliminated.
>     Thanks, Duncan.
>       Similarly, on
>         targets supporting this feature (COFF for the moment, but maybe ELF one
>         day as
>         an optimization), LLVM will emit the appropriate comdat bits so that the
>         system
>         linker behaves similarly. On platforms lacking this support, the old
>         behavior
>         will be used, in which case guard variables are still necessary.
>         This seems fairly uncontroversial, but let me know if anyone objects.
>         I'll try
>         to send patches later this week.
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