[llvm-dev] RFC: Representing unions in TBAA
Daniel Berlin via llvm-dev
llvm-dev at lists.llvm.org
Fri Apr 7 13:25:13 PDT 2017
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> 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 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>> 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>>
>> To: Steven Perron/Toronto/IBM at IBMCA
>> Cc: Daniel Berlin <dberlin at dberlin.org
>> <mailto:dberlin at dberlin.org>>, llvm-dev <llvm-dev at lists.llvm.org
>> <mailto:llvm-dev at lists.llvm.org>>, Sanjoy Das
>> <sanjoy at playingwithpointers.com <mailto:sanjoy at playingwithpoin
>> ters.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>> 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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