[LLVMdev] clarification on the semantics of AliasAnalysis queries

[Christian Convey]

Hello,

I'm trying to improve my understanding of the meaning of
AliasAnalysis::alias queries and their results.  There was a conversation
on this topic about a year ago:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-August/076068.html

Gerolf Hoflehner wrote:

> I think I see the fundamental issue you are looking at. It is mentioned
> implicitly in the discussions, but not called out. In your CFG example
> there is no backedge (head always dominates tail), only retreat edges. So
> your graph is irreducible. For such graphs “simultaneous live” means that
>  there be can be two dynamic execution paths where the variables (memory
> locations, objects etc)  are simultaneously live, but they may not be live
> at the same time on all execution paths.  Since LLVM uses SSA,  all
> variables (memory locations, objects, … ) are strictly defined, so there
> cannot be irreducible dependence graphs. I believe this assumption is built
> into the alias queries. So to catch cases like in your scenario I think you
> need to base your queries on classical dataflow analysis.


I was wondering if anyone knows....

(1) Was it intentional that AliasAnalysis allows queries in which neither
address dominates the other?  Since AA queries don't sound well-defined for
such cases, I'm surprised they're allowed to go unreported.

(2) When such AA queries are made, do we have an ideal answer is to the
query?  Empirically it seems to be "may-alias".

Thanks,
Christian
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150607/3ac8a294/attachment.html>