[llvm-dev] RFC: Representing unions in TBAA

Daniel Berlin via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 1 17:17:42 PST 2017

And, fwiw, the path walking issues are easy to fix.
What we did in GCC, and could be done here, is to generate the transitive
closure of the children (or parents) as we saw them.
The height of the TBAA tree is not that large.
For scalar type nodes, this answers queries in O(1) time.
Even in offset land, you can use this, because if none of your children
(parents) have that type, it can't alias, no matter what.
So it will cut off the query at the first point it is possible to discover
that your path does not contain the type.
The worst case is that the exact path does contain the type, in which case
you just made a few wasted queries.

On Wed, Mar 1, 2017 at 3:30 PM, Daniel Berlin <dberlin at dberlin.org> 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> 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>
>> To:        Steven Perron/Toronto/IBM at IBMCA
>> Cc:        Daniel Berlin <dberlin at dberlin.org>, llvm-dev <
>> llvm-dev at lists.llvm.org>, Sanjoy Das <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*
>> <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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