[llvm-dev] RFC: Representing unions in TBAA
Xin Tong via llvm-dev
llvm-dev at lists.llvm.org
Sun Apr 9 18:40:45 PDT 2017
I would like to see it disabled as well until fixed, we have problem
compiling HHVM with clang due to this union+TBAA problem.
-Xin
--
Software Engineer
Employee of Facebook Inc.
On Fri, Apr 7, 2017 at 1:29 PM, Daniel Berlin via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Ah.
> IMHO, yes we should disable it until it's correct.
>
>
> On Fri, Apr 7, 2017 at 1:28 PM, Krzysztof Parzyszek
> <kparzysz at codeaurora.org> wrote:
>>
>> I'm asking if people are ok with it. :)
>> We've had customer reports that can be tracked down to this issue, so this
>> is something we'd really like to working (at least in terms of correctness).
>>
>> I can come up with a patch.
>>
>> -Krzysztof
>>
>>
>> On 4/7/2017 3:25 PM, Daniel Berlin wrote:
>>>
>>> Not familiar with clang enough to know.
>>> Staring at it, it looks a bit annoying to do.
>>> You'd basically just want to stop decorating loads/stores with tbaa info
>>> if it's a union.
>>>
>>>
>>> On Fri, Apr 7, 2017 at 12:09 PM, Krzysztof Parzyszek via llvm-dev
>>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>
>>> Can we turn off TBAA info for union member accesses in clang before
>>> this gets fixed?
>>>
>>> -Krzysztof
>>>
>>>
>>> On 3/1/2017 5:30 PM, Daniel Berlin via llvm-dev wrote:
>>>
>>> So, https://bugs.llvm.org/show_bug.cgi?id=32056
>>> <https://bugs.llvm.org/show_bug.cgi?id=32056> is an example
>>> showing
>>> our current TBAA tree for union generation is definitely
>>> irretrievably
>>> broken.
>>> I'll be honest here. I'm pretty sure your proposal doesn't go
>>> far enough.
>>> But truthfully, I would rather see us come closer to a
>>> representation
>>> we know works, which is GCC's.
>>> Let me try to simplify what you are suggesting, and what we have.
>>> Our current representation is basically inverted from GCC, but
>>> we don't
>>> allow things that would enable it to work.
>>>
>>> Given
>>> union {int a, short b};
>>>
>>> GCC's will be:
>>>
>>> union
>>> / \
>>> short int
>>>
>>>
>>> Everything is implicitly a subset of alias set 0 (which for C/C++
>>> is
>>> char) just to avoid representing it.
>>>
>>> Our metadata has no child links, and only a single parent link.
>>>
>>> You can't represent the above form because you can't make a
>>> single short
>>> node a child of every union/struct it needs to be (lack of
>>> multiple
>>> parents means you can't just frob them all to offset zero).
>>>
>>> Your suggestion is to invert this as a struct
>>>
>>> short int
>>> | /
>>> union
>>>
>>> We don't allow multiple parents, however.
>>> Because of that, you suggest we follow all nodes for unions,
>>> special
>>> casing union-type nodes somehow
>>>
>>> Let me suggest something different:
>>>
>>> We know the current structure fails us in a number of ways.
>>> Instead of trying to shoehorn this into our current structure, I
>>> suggest: we stop messing around and just have a GCC style tree,
>>> and
>>> represent the children instead of the parents.
>>> We make contained types descendants instead of ancestors.
>>> We allow multiple children at each offset and for scalar type
>>> nodes.x`
>>>
>>> We could do that without changing to children, but honestly, i
>>> strongly
>>> believe nobody who ever looks at this really understands it
>>> right now.
>>> (If someone really does like the ancestor form, i'd love to
>>> understand
>>> why :P)
>>>
>>> So if we are going to change it, we should change it to
>>> something that
>>> is easier to understand.
>>>
>>> something like:
>>>
>>> scalar type node -> {name, children nodes}
>>> struct node -> {name, array of {offset, child node} }
>>>
>>> Paths are separate from the tbaa tree itself, and are just:
>>> path node -> {struct node, offset} or whatever.
>>>
>>> unions are just scalar type nodes with multiple children, not
>>> struct
>>> nodes with special-cased offset zero.
>>>
>>> This also has a well-defined upgrade path.
>>>
>>> For "old-style" DAGs, like exist now, we know we have to regen
>>> the
>>> metadata, and that it is wrong (So we could, for example, simply
>>> disable
>>> it for correctness reasons)
>>> .
>>> Anything with a "new-style" DAG, we know is usable.
>>>
>>> In this representation, the most generic tbaa node is just the
>>> one that
>>> contains the other.
>>> If neither contains the other, you can just make one that has
>>> both as
>>> children and use that.
>>> (instead of now, where you'd have to have multiple parents
>>> again).
>>>
>>> (The above also may be better for other languages)
>>>
>>> --Dan
>>>
>>>
>>>
>>>
>>> On Tue, Feb 28, 2017 at 4:44 PM, Steven Perron
>>> <perrons at ca.ibm.com <mailto:perrons at ca.ibm.com>
>>> <mailto:perrons at ca.ibm.com <mailto:perrons at ca.ibm.com>>> wrote:
>>>
>>> Seems like the comments have stopped. I'll try to get a
>>> patch
>>> together. Then we can continue the discussion from there.
>>>
>>> Hubert, as for the issue with the llvm optimizations losing
>>> the TBAA
>>> information, it should be the responsibility to make sure the
>>> aliasing is changed in the correct way. One function related
>>> to
>>> this has already been mentioned: getMostGenericTBAA.
>>>
>>> Later,
>>> Steven Perron
>>>
>>>
>>>
>>> From: Hubert Tong <hubert.reinterpretcast at gmail.com
>>> <mailto:hubert.reinterpretcast at gmail.com>
>>> <mailto:hubert.reinterpretcast at gmail.com
>>> <mailto:hubert.reinterpretcast at gmail.com>>>
>>> To: Steven Perron/Toronto/IBM at IBMCA
>>> Cc: Daniel Berlin <dberlin at dberlin.org
>>> <mailto:dberlin at dberlin.org>
>>> <mailto:dberlin at dberlin.org <mailto:dberlin at dberlin.org>>>,
>>> llvm-dev <llvm-dev at lists.llvm.org
>>> <mailto:llvm-dev at lists.llvm.org>
>>> <mailto:llvm-dev at lists.llvm.org
>>> <mailto:llvm-dev at lists.llvm.org>>>, Sanjoy Das
>>> <sanjoy at playingwithpointers.com
>>> <mailto:sanjoy at playingwithpointers.com>
>>> <mailto:sanjoy at playingwithpointers.com
>>>
>>> <mailto:sanjoy at playingwithpointers.com>>>
>>> Date: 2017/02/15 07:44 AM
>>> Subject: Re: [llvm-dev] RFC: Representing unions in
>>> TBAA
>>>
>>>
>>> ------------------------------------------------------------------------
>>>
>>>
>>>
>>> On Tue, Feb 14, 2017 at 11:22 PM, Steven Perron
>>> <_perrons at ca.ibm.com_ <mailto:perrons at ca.ibm.com
>>> <mailto:perrons at ca.ibm.com>>> wrote:
>>> 3) How should we handle a reference directly through a
>>> union, and a
>>> reference that is not through the union?
>>>
>>> My solution was to look for each member of the union
>>> overlaps the
>>> given offset, and see if any of those members aliased the
>>> other
>>> reference. If no member aliases the other reference, then
>>> the
>>> answer is no alias. Otherwise the answer is may alias. The
>>> run
>>> time for this would be proportional to "distance to the
>>> root" *
>>> "number of overlapping members". This could be slow if
>>> there are
>>> unions with many members or many unions of unions.
>>>
>>> Another option is to say that they do not alias. This would
>>> mean
>>> that all references to unions must be explicitly through the
>>> union.
>>> From what I gather from the thread so far, the access
>>> through the
>>> union may be lost because of LLVM transformations. I am not
>>> sure
>>> how, in the face of that, TBAA could indicate NoAlias safely
>>> (without the risk of functional-correctness issues in correct
>>> programs) between types which overlap within any union
>>> (within some
>>> portion of the program).
>>>
>>> As for the standards, it is definitely not true that all
>>> references
>>> to unions must be explicitly through the union. However, if
>>> you are
>>> trying to perform union-based type punning (under C11), then
>>> it
>>> appears that it is intended that the read must be through
>>> the union.
>>>
>>> This would be the least restrictive aliasing allowing the
>>> most
>>> optimization. The implementation would be simple. I believe
>>> we
>>> make the parent of the TBAA node for the union to be
>>> "omnipotent
>>> char". This might be similar to treating the union TBAA
>>> node more
>>> like a scalar node instead of a struct-path. Then the
>>> traversal of
>>> the TBAA nodes will be quick. I'll have to work this out a
>>> bit
>>> more, but, if this is good enough to meet the requirements
>>> of the
>>> standard, I can try to think this through a little more.
>>> I'll need
>>> Hubert and Daniel to comment on that since I am no expert on
>>> the C
>>> and C++ standards.
>>>
>>> The third option is to be pessimistic and say "may alias"
>>> all of the
>>> time (conservatively correct), and rely on other alias
>>> analysis to
>>> improve it. This will have good compile time, but could
>>> hinder
>>> optimization. Personally I do not like this option. Most
>>> of the
>>> time it will not have a negative effect, but there will be a
>>> reasonable number of programs where this will hurt
>>> optimization more
>>> that it needs to.
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>>
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>>>
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
>> by The Linux Foundation
>
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