[llvm-dev] Opaque Pointers Help Wanted

Nikita Popov via llvm-dev
Thu, 01 Jul 2021 15:00:14 -0000

On Thu, Jul 1, 2021 at 4:39 PM <paul.robinson at sony.com> wrote:

> | 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
>

Sure, we definitely need to retain bitcode compatibility (i.e. auto-upgrade
to opaque pointers).

My comment was targeted at maintainers of out-of-free frontends and
targets, who shouldn't assume there will be a long-term period of dual
compatibility, like there was/is with the new pass manager. If you don't
want to be stuck on an old toolchain, you need to be ready by the time the
switch happens.

Regards,
Nikita


>  *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> wrote:
>
>
>
>
>
> On Tue, Jun 22, 2021 at 9:39 AM Kaylor, Andrew via llvm-dev <
> 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>
> *Sent:* Tuesday, June 22, 2021 12:15 PM
> *To:* Kaylor, Andrew <andrew.kaylor at intel.com>
> *Cc:* 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>
> 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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