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

[Reid Kleckner]

To implement http://llvm.org/PR16959<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. 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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