[llvm-dev] Opaque Pointers Help Wanted

[via llvm-dev]

| My expectation is that after the opaque pointer migration is complete, the time the non-opaque pointer mode remains available will be measured in days, not years.

That can be true for textual IR, but we need to retain backward compatibility (auto-upgrade) for bitcode for much longer.  I don’t have a clear idea of what’s involved (haven’t really been tracking this project) but many users/scenarios require bitcode compatibility for long periods.  I don’t think we’ve had a true format break since about 4.0?
--paulr

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Nikita Popov via llvm-dev
Sent: Wednesday, June 30, 2021 2:02 PM
To: David Blaikie <dblaikie at gmail.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Opaque Pointers Help Wanted

On Tue, Jun 22, 2021 at 7:12 PM David Blaikie via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:


On Tue, Jun 22, 2021 at 9:39 AM Kaylor, Andrew via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
> I don't think metadata can reference an LLVM type. My previous hacky suggestion was to add a new overloaded parameter with the type you want and pass undef/poison to it. In any case, we'll have to find a way to fix these sorts of intrinsics, we shouldn't block the opaque pointers project on some intrinsics.

It looks like I just missed your response before getting my response to David in flight. Yes, I see the metadata problem. I don’t like the undef/poison approach, but I agree that it would technically work.

I completely agree that we should solve this problem rather than block the opaque pointer project because of it. Nevertheless, we’ll need to solve the problem.


> LLVM already (mostly) treats memory as untyped, what is your intrinsic attempting to do?

It’s kind of like a GEP but more specialized. It’s doing a pointer calculation based on multidimensional composite types with a shape that is known at runtime but unknown at compile time, and we have a front end that’s able to supply enough info to make this useful.


> As for the timeline, we'll have to support mixed opaque pointers and legacy pointers for a while, we don't want out of tree users to immediately break as soon as the opaque pointers work is finished up in-tree.

Is there any consensus on the scope of “for a while”? Like, how many major releases after the opaque pointer work is complete? My manager would very much like to know the answer to this question. :-) I’ve been trying to prepare for the buffer being as little as one major release after the in-tree work is done but hoping for more. I expect there are others with a similar interest.

By default I'd vote for one major release - but likely the ongoing burden of carrying it for a bit longer if some significant contributors would benefit from extra time it's probably going to be pretty harmless to keep it around a bit longer, I think?

Opaque pointers are not like the pass manager switch, where we can retain support for the legacy pass manager at close to zero cost. Nearly all tests work the same on both pass managers, so there is little ongoing maintenance cost.

This is not going to be the case with opaque pointers. Doing the switch will require changes to nearly the whole test suite. This means that either typed pointers will end up being entirely untested (or very weakly tested), or we need to duplicate a very large fraction of our tests.

My expectation is that after the opaque pointer migration is complete, the time the non-opaque pointer mode remains available will be measured in days, not years.

Regards,
Nikita





From: Arthur Eubanks <aeubanks at google.com<mailto:aeubanks at google.com>>
Sent: Tuesday, June 22, 2021 12:15 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Opaque Pointers Help Wanted



On Mon, Jun 21, 2021 at 3:27 PM Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>> wrote:
This has probably been discussed somewhere, but I missed it. Can you elaborate a bit on this?


  *   Allow bitcode auto-upgrade of legacy pointer type to the new opaque pointer type (not to be turned on until ready)

     *   To support legacy bitcode, such as legacy stores/loads, we need to track pointee types for all values since legacy instructions may infer the types from a pointer operand's pointee type
I‘m specifically trying to understand what will happen when typed pointer support is removed. How will IR with typed pointers be auto-upgraded to pure opaque pointer IR? Will the bitcode reader keep some level of typed pointer support indefinitely?
Yes, the plan is something along the lines of associating each Value with a possible pointee type inside the bitcode reader.


Also, do you have a plan for replacing intrinsics that currently rely on pointee types? For example, the load instruction was updated to take an explicit type operand but I don’t think we can do the same thing for an intrinsic like llvm.masked.load since there is Value for Type. This is an easy problem to work around for something like masked.load, but more complicated if anyone has a downstream GEP-like intrinsic that needs more than the size of an element (spoiler alert: I do have such an intrinsic). Would you use a metadata argument?
I don't think metadata can reference an LLVM type. My previous hacky suggestion was to add a new overloaded parameter with the type you want and pass undef/poison to it. In any case, we'll have to find a way to fix these sorts of intrinsics, we shouldn't block the opaque pointers project on some intrinsics.

LLVM already (mostly) treats memory as untyped, what is your intrinsic attempting to do?
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