[llvm-dev] RFC: Representing unions in TBAA

[Hubert Tong via llvm-dev]

On Tue, Feb 14, 2017 at 11:22 PM, Steven Perron <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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