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

[Chris Lattner via llvm-dev]

Disclaimer: I haven’t been following this discussion closely, nor do I know what is going on in the modern AA/MemorySSA/GVN infra, but:

I’d prefer to avoid abusing the Value* class hierarchy if reasonable.  Adding new subclasses that only occur in special cases makes it more difficult to reason about what can occur and when.  A historical example that always grated against my sensibilities was CodeGen/PseudoSourceValue, which originally inherited from Value in order to make it “fit better” into the AA infra.  Thankfully Nick Lewycky broke that tie back in a patch on Tue Apr 15 07:22:52 2014, and the codebase has been better off for that change.

All this is to say that I’d prefer to avoid tying it into Value just because that is an expedient way to get something done.  If tying into Value is the “right” thing to do for some reason then that is a completely different story.

-Chris





> On Oct 16, 2017, at 5:33 PM, Daniel Berlin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> (I have no beef with using Value, FWIW)
> I believe one of the objections is, for example, lib/IR should not have to depend on, say, lib/Analysis, which it does if you extend value ;)
> I'm more surfacing the objections i've heard from others, than ones i have myself.
> I'll forward this thread to them and let them speak for themselves if they want
>  
> 
> On Sat, Oct 14, 2017 at 4:57 PM, Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
> 
> On 10/14/2017 05:28 PM, Daniel Berlin via llvm-dev wrote:
>> 
>> 
>> On Sat, Oct 14, 2017 at 2:54 PM, Michael Kruse <llvmdev at meinersbur.de <mailto:llvmdev at meinersbur.de>> wrote:
>> 2017-10-14 5:03 GMT+02:00 Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org>>:
>> > FWIW: We hit a subset of this issue with MemorySSA (which subclasses value
>> > for the MemoryAccess's, etc), and it was discussed as well during
>> > PredicateInfo.
>> >
>> > NewGVN has a variant the same issue as well, where it actually creates
>> > unattached (IE not in a basic block) new instructions just so it can analyze
>> > them.
>> >
>> > IMHO, some nice way to make virtual forms over the instructions that didn't
>> > require reimplementing the tons of existing functionality that works with
>> > Value would be much appreciated.
>> >
>> > But maybe the right answer at some point is to just sit down and build out
>> > such an infrastructure. It certainly sounds like there are enough clients.
>> 
>> What would be different in such an infrastructure? IMHO the
>> llvm::Value hierarchy is already relatively thin,
>> 
>> I think most people would disagree with "Relatively thin".
>> Given there have also been multiple threads in the past year with most people not wanting non-actual-IR to derive from Value, i think you may be in the minority here.
> 
> 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?
> 
>  -Hal
> 
>> 
>> 
>> 
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> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
> 
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