[llvm-dev] [RFC] Polly Status and Integration

[Saito, Hideki via llvm-dev]

>I think we need to get that story right for both cases up front.

Renato, I kicked off this secondary discussion, borrowing the opportunity from Michael's RFC,
but to the point of reviewing https://reviews.llvm.org/D38676, I'd like the review to proceed
separately from this bigger (and most likely longer) discussion. We intentionally made the interfaces
similar such that whatever the outcome of this discussion would be, the changes we have to make later,
if any, is small and mechanical. We just need to agree that
	VPValue/VPUser/VPInstruction
is not a precedence, i.e., still subject to ongoing discussion and is expected to abide by the eventual
outcome of this discussion.

To the best of my understanding, if we do not want to modify the IR (i.e., CFG, Instructions, and Uses/Defs hooked up
in the Function) before we decide to let vectorizer transform (i.e., cost modeling phase of LV), we really don't have anything
that we can use today in the LLVM infrastructure. If the collective wisdom concludes investment into that, we are more than
happy to contribute our share of effort, but that longer term work (one year is probably too optimistic) should not block shorter
term development of vectorizer.

More inputs from other optimizers would greatly help build up the context of this discussion. Please speak up
if you felt similarly to us in the past. I'll be at the LLVM Conference if anyone is interested in in-person discussions.

Thanks,
Hideki

-----Original Message-----
From: Renato Golin [mailto:renato.golin at linaro.org] 
Sent: Monday, October 16, 2017 4:59 AM
To: Hal Finkel <hfinkel at anl.gov>
Cc: Daniel Berlin <dberlin at dberlin.org>; Michael Kruse <llvmdev at meinersbur.de>; llvm-dev at lists.llvm.org; Saito, Hideki <hideki.saito at intel.com>; Rapaport, Gil <gil.rapaport at intel.com>; Zaks, Ayal <ayal.zaks at intel.com>
Subject: Re: [llvm-dev] [RFC] Polly Status and Integration

On 15 October 2017 at 00:57, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> I think the way that we generally look at this is: Our IR data types 
> (Value, User, Instruction, Constant, and so on) are the most efficient 
> representation we have of the IR. There are a lot of capabilities that 
> come along with that hierarchy (use lists, value handles, metadata, 
> names). Value has a non-inline destructor to deal with all of these 
> capabilities, as does Instruction. Each individual instance of these 
> types allocate memory. The question is then: For any particular use 
> case, do you need those capabilities/properties? If not, is there a 
> more succinct representation that can be used efficiently?

There's also the case where we want *more* information than what the IR constructs can provide us (like temporary state for the vectoriser), in which case we can easily wrap the IR values in a small class with a pointer to the Instruction/Value and the extra bits.

However, the current proposal for the VPlan goes well beyond that:

https://reviews.llvm.org/D38676

and makes VPValue and VPInstruction full-fledged implementations that have some parts in common of what we already have (like Users).

I think we need to get that story right for both cases up front.

cheers,
--renato