[cfe-dev] Setting a FunctionDecl as 'uncallable':

Thu Jul 6 13:04:23 PDT 2017

Thanks for the link!  I definitely see that this is very similar then!  I DID try the first example on your link, and it fails because of ‘redefinition of foo’.

It actually seems that this cpu_dispatch/cpu_specific is perhaps even a sub-set of GCC’s target (since it only handles the ‘arch=’ tests).  The only real difference after that will be the linked-names, but perhaps that is something acceptable to us as well (or that could be kept internal).

Perhaps my best goal here would be instead to help you (or, have you guide me) to implement ‘target’ the rest of the way(IFunc, multiple defs, changing the name-mangling), then create cpu_dispatch/cpu_specific as an alias for ‘target’.

Thanks,
Erich

From: Eric Christopher [mailto:echristo at gmail.com]
Sent: Thursday, July 6, 2017 12:56 PM
To: Keane, Erich <erich.keane at intel.com>; Richard Smith <richard at metafoo.co.uk>; Eric Christopher <echristo at google.com>
Cc: Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] Setting a FunctionDecl as 'uncallable':

On Thu, Jul 6, 2017 at 12:53 PM Keane, Erich <erich.keane at intel.com<mailto:erich.keane at intel.com>> wrote:

From: Eric Christopher [mailto:echristo at gmail.com<mailto:echristo at gmail.com>]
Sent: Thursday, July 6, 2017 12:48 PM

To: Keane, Erich <erich.keane at intel.com<mailto:erich.keane at intel.com>>; Richard Smith <richard at metafoo.co.uk<mailto:richard at metafoo.co.uk>>; Eric Christopher <echristo at google.com<mailto:echristo at google.com>>
Cc: Clang Dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
Subject: Re: [cfe-dev] Setting a FunctionDecl as 'uncallable':

On Thu, Jul 6, 2017 at 12:11 PM Keane, Erich <erich.keane at intel.com<mailto:erich.keane at intel.com>> wrote:
The difference as far as I can tell is that the dispatch is done at Load/Runtime.  It checks on “CPUID” to determine which function to call.  As far as I can tell, target simply sends optimization hints to the backend, right?

You can set up automagic dispatch using ifunc or the rest of that (they do in gcc), just a lot of that functionality isn't wired up.
[Keane, Erich] I’m currently implementing in terms of ifunc, though I’m not sure what you mean here?  Are we missing some functionality of ‘target’ that would make it a lot more similar?

I think this is intended to be a superset of the ‘target’ functionality.

At a function definition level it's exactly the same :)
[Keane, Erich] The difference is that this allows multiple definitions of the same function, which does not seem to be the case in target.  Am I missing something else here?

At any rate, please do keep me on reviews for this. I don't believe the mechanics in general should be any different.
[Keane, Erich] I definitely will!  I was hoping to do ‘in progress’ reviews once I’m confident with the direction.

FWIW here's what our support is based upon:

https://gcc.gnu.org/wiki/FunctionMultiVersioning

I just stopped at "create your own dispatch function".

-eric

-eric

From: Eric Christopher [mailto:echristo at gmail.com<mailto:echristo at gmail.com>]
Sent: Thursday, July 6, 2017 12:02 PM
To: Keane, Erich <erich.keane at intel.com<mailto:erich.keane at intel.com>>; Richard Smith <richard at metafoo.co.uk<mailto:richard at metafoo.co.uk>>; Eric Christopher <echristo at google.com<mailto:echristo at google.com>>
Cc: Clang Dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>

Subject: Re: [cfe-dev] Setting a FunctionDecl as 'uncallable':

It's pretty much exactly attribute target :)

I'll definitely want to know if there are any differences in use between them.

-eric

On Thu, Jul 6, 2017 at 11:43 AM Keane, Erich via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
I’m using the ‘emit’ functionality from attribute-target to get the target-cpu emitted properly, however I hadn’t realized it allowed multiple function definitions.  I’ll look into that one as well.

Thanks Richard!
-Erich

From: metafoo at gmail.com<mailto:metafoo at gmail.com> [mailto:metafoo at gmail.com<mailto:metafoo at gmail.com>] On Behalf Of Richard Smith
Sent: Thursday, July 6, 2017 11:41 AM
To: Keane, Erich <erich.keane at intel.com<mailto:erich.keane at intel.com>>; Eric Christopher <echristo at google.com<mailto:echristo at google.com>>
Cc: Clang Dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>; Reid Kleckner <rnk at google.com<mailto:rnk at google.com>>

Subject: Re: [cfe-dev] Setting a FunctionDecl as 'uncallable':

+echristo This sounds similar to attribute target. Perhaps some of the work there could be reused?

On 6 Jul 2017 11:32 am, "Keane, Erich via cfe-dev" <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Thanks!  I’ll look into enable_if and others.

To spoil my RFC:
__attribute__((cpu_specific(atom)))
void foo(){} // atom specific impl
__attribute__((cpu_specific(p3)))
void foo(){} // p3 specific impl
__attribute__((cpu_dispatch(atom, p3)))
void foo(){}; // dispatch function.

Each ‘foo’ implementation is emitted with different name mangling.  They are generally ALL considered the same function, Function Pointers, references, etc will all be to the ‘dispatch’ version of the function.

The dispatch function will be implemented in terms of an iFunc.

-Erich

From: Reid Kleckner [mailto:rnk at google.com<mailto:rnk at google.com>]
Sent: Thursday, July 6, 2017 11:25 AM
To: Keane, Erich <erich.keane at intel.com<mailto:erich.keane at intel.com>>
Cc: cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] Setting a FunctionDecl as 'uncallable':

My first guess would be that you want to modify overload resolution. I think the CUDA host/device attributes are involved there. You can also look at the implementation of __attribute__((enable_if)) as well.

However, do we really need a FunctionDecl for every subtarget specialization? How does a user trigger specialization? Can they do something like take a specialization and instantiate a class template with it?

On Thu, Jul 6, 2017 at 9:31 AM, Keane, Erich via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Hi all-
I'm working on an attribute implementation (RFC coming once I get a better idea about the implementation details, cpu_dispatch/cpu_specific from ICC) that permits multiple definitions of a function.  All definitions of the function ARE emitted, however with different name-mangling.  1 version of the function keeps the normal name-mangling/linking, so all call-expressions should call THAT one.

My question is: Is there a way to mark a FunctionDecl as 'don't find me in lookup' such that it never ends up being a "Callee"?  Or should I be figuring out how to change the lookup-results here?

Thanks,
Erich
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