llvm dev - Sep 2013 - [LLVMdev] Proposal: Adding an optional association field to llvm.global_ctors

On Mon, Sep 2, 2013 at 5:18 AM, Duncan Sands <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?

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

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.
>>
>>
>> ______________________________**_________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>
http://lists.cs.uiuc.edu/**mailman/listinfo/llvmdev<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>>
>>
> ______________________________**_________________
> LLVM Developers mailing list
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>
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>
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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.
>
>
>         _________________________________________________
>         LLVM Developers mailing list
>         LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
http://llvm.cs.uiuc.edu
>         http://lists.cs.uiuc.edu/__mailman/listinfo/llvmdev
>         <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>
>
>     _________________________________________________
>     LLVM Developers mailing list
>     LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
http://llvm.cs.uiuc.edu
>     http://lists.cs.uiuc.edu/__mailman/listinfo/llvmdev
>     <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>
>

On Wed, Sep 4, 2013 at 4:11 AM, Duncan Sands <baldrick at free.fr> wrote:
>  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?
>
For the Microsoft C++ ABI, yes, they use the same mangling (?__E*) and are
linkonce_odr.

For the Itanium C++ ABI, no, they are all internal and hence different.  It
uses a guard variable to avoid double initialization.

> 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?
>
Richard and I considered this, but cl.exe associates the .CRT$XCU function
pointer with the global, and not the initializer function.  I could try to
recover the name of the global from the name of the initializer, but this
seems fairly terrible, and possibly invalid in the face of GlobalOpt.

Consider a constructor which does nothing but tail call to 'void foo()'.
 It should be valid to make the .CRT$XCU function pointer point directly to
foo and delete the initializer.  We could outlaw this optimization when the
unique bit is set, but then I still have to recover the global's symbol to
get the MS C++ ABI right.

That said, if I only needed one bit, I could steal it from the i32 priority
field which only uses 16 bits.  =D
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