[LLVMdev] Moving towards a singular pointer type

[David Blaikie]

On Wed, Feb 18, 2015 at 9:03 AM, Colin LeMahieu <colinl at codeaurora.org>
wrote:

> It’s a good point it wouldn’t help with the textual test churn.  It seemed
> like a good way to isolate issues related to depending on the
> pointed-to-type from issues related to converting to the opaque pointer
> class which might be in a different class inheritance tree and cause
> runtime issues due to dyn_cast failures etc.
>

Fair point - I think most of those "this is a completely different type"
will probably flush out as outright failures (either asserts, crashes, or
test failures) so that's mostly on me to fix those all up before committing
(or, where possible, I'll put the right changes in ahead of time - since by
the time I reach that point the code /shouldn't/ be caring about those
properties, so the changes will be incremental and easy to review).


>  I don’t have a full picture of what things will break
>

Neither do I - so I'm happy to have any/all ideas to keep in my toolkit
depending on how it all starts to shake out.

Thanks!

- David


> so I won’t double-down on the suggestion ;)
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Wednesday, February 18, 2015 10:53 AM
> *To:* Colin LeMahieu
> *Cc:* Chris Lattner; LLVM Developers Mailing List
>
> *Subject:* Re: [LLVMdev] Moving towards a singular pointer type
>
>
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>
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> On Wed, Feb 18, 2015 at 6:58 AM, Colin LeMahieu <colinl at codeaurora.org>
> wrote:
>
> It might help after adding explicit types where needed and before adding
> the opaque pointer type, to change all pointers to a single type in the
> existing type system, i8* could work or maybe a weird type that would shake
> out any issues like i99*.
>
>
> I'm not sure this would help things very much - it'd just add two major
> churns rather than one, no? (one to transform all the tests from T* to i42*
> then another to change from i42* to ptr)
>
> If we included back-compat textual IR support then it would be different
> (but we could include that while transitioning to ptr as easily as
> transitioning to i42*, I think).
>
>
>
>
> If all pointers were changed to this weird type and all GEP, load, store,
> etc were told to use their explicit type instead you might be able to find
> places where we’re still relying on pointer types before adding and
> converting to the opaque type.
>
>
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] *On
> Behalf Of *David Blaikie
> *Sent:* Wednesday, February 18, 2015 2:01 AM
> *To:* Chris Lattner
> *Cc:* LLVM Developers Mailing List
> *Subject:* Re: [LLVMdev] Moving towards a singular pointer type
>
>
>
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> On Tue, Feb 17, 2015 at 10:47 PM, David Blaikie <dblaikie at gmail.com>
> wrote:
>
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> On Tue, Feb 17, 2015 at 10:27 PM, Chris Lattner <clattner at apple.com>
> wrote:
>
> On Feb 17, 2015, at 1:58 PM, Chandler Carruth <chandlerc at gmail.com> wrote:
> > I'm not actually worried about this change though Chris, at least w.r.t.
> optimizer changes being necessary. There are a few reasons:
> >
> > 1) The old ScalarRepl pass cared a *lot* about pointer type, but the new
> SROA doesn't care at all, so the biggest offender is essentially handled.
>
> Why do you think that SRoA is the biggest “offender”?  This will pretty
> fundamentally changing the shape of the IR (in a good way) by presumably
> eliminating a ton of bitcasts etc.  This has the potential to provoke
> instcombine regressions, tickle things like globalopt and load/store
> elimination, etc.  I don’t think that any of these will be particularly
> difficult to fix, but I imagine that there will be a long tail of minor
> things.
>
> > 2) We've recently changed our pointer canonicalization rules several
> times and in different ways. Each of those changes helped shake out bugs
> where the optimizer was relying on the pointer type for something. The
> number of things found has dropped dramatically with each change, so I
> don't think there is a huge pile of problems left hiding somewhere.
>
> This is more reassuring for me.
>
> > 3) Almost all of the problems we found with the changes to
> canonicalization were actually cases where *casts* impeded optimizations,
> not the different pointer type. This change will be a strict reduction in
> the need for casts, and thus I expect it to actually be safer than the
> other changes. All evidence is that most of the remaining reliance on these
> kinds of things are actually relying on an absence of casts. With this
> change, the casts will all go away.
>
> Yes, I like this change for a number of reasons: reduction of casts,
> simplified type resolution stuff in libIR, etc.
>
> > So, I'm not as worried about having a very drawn out period of fixing
> the optimizer. I think we'll probably uncover a few minor things that we
> have to fix immediately, and then when we make the change some small number
> of benchmarks will regress (likely on some small number of platforms).
> We'll have to track those down, no doubt, but I'm not worried about it
> preventing progress for a long time.
>
> So long as the regressions are tracked down and fixed before the
> mega-patch is landed, I’m ok with making this change.  I just think that
> finding any ways to make it more incremental and stage it will be very well
> rewarded.  It will be impossible to review the resultant patch otherwise.
>
>
> It should come out somewhat incremental, I think. Here's how it's shaping
> up/I see it going:
>
> 1) add explicit types where required in IR instructions - gep, load,
> byval, anything else I can find...
>   a) Provide the mechanism to specify it (in textual IR, bitcode, and LLVM
> IR APIs)
>   b) Update callers to Clang, LLVM, and Polly, to pass that information
> (initially just asserting that it's the same information as was provided by
> the typed-pointer operands)
>   c) /rely/ on that information in LLVM - stop using the pointee types
>
> After that it might be monolithic - though I'll be trying to do it
> incrementally for my own sanity.
>
> <I was thinking I might remove ptr-to-ptr bitcasts here, before
> introducing the ptr type - since at this point they'll be pointless (har
> har) already, since the pointer-using instructions will no longer need the
> type from them anyway>
>
> 2) introduce opaque pointer type (initially unused)
> 3) Repeat:
>   a) choose an instruction or other source of pointer type
>   b) update type to opaque pointer
>   c) find & fix frontend bugs where it was relying on pointer type in the
> IR
>   d) commit LLVM change
>
> It could be monolithic there (do all the ptrs in one go), though I don't
> think it needs to be.
>
>
> On further reflection this step will probably be monolithic - it'd be
> really hard to script updating only some pointer types when the type is
> written at the use, not the def, so I'd lose the context from "this
> instruction now produces ptr". (why do we put the type on the use instead
> of the def? *shrug*)
>
> Ah well.
>
>
>
> 4) Remove non-opaque pointer types... - maybe. Might need to leave them in
> to make the back-compat bitcode reading easy, but I'm not sure.
>
>
>
> -Chris
>
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