[LLVMdev] how to detect if block N is reachable from block M ?

[Kenneth Uildriks]

On Wed, Mar 30, 2011 at 11:35 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Wed, Mar 30, 2011 at 10:14 AM, Jimborean Alexandra
> <xinfinity_a at yahoo.com> wrote:
>> Hi,
>>
>> Is there any method to check if there is a path in the CFG from block M to
>> block N, but M does not necessarily dominate block N?
>> In other words, if N is reachable from M.
>
> I don't think there's any method built in to LLVM.  It's pretty easy
> to write, though.
>
> -Eli
A simple depth-first or breadth-first search is good unless you end up
querying all pairs - each query is O(blocks + edges), and the number
of pairs is O(blocks ^ 2).  If you need to answer queries such as "all
blocks reachable from X", especially using several X's in the course
of an analysis, you'll want more sophisticated algorithms.  I found a
quick comparison of several algorithms at
http://www.seas.upenn.edu/~mengmeng/presentations/reachability.pdf