[llvm-dev] [ThinLTO] Suggestions on how to traverse SCCs in the Module Summary Index bottom up

[Davide Italiano via llvm-dev]

On Wed, Aug 9, 2017 at 3:04 PM, Charles Saternos via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I just realized that I've been misunderstanding this (I think). If I'm
> understanding it now, the entry node for the graph should actually be the
> graph's root (not a bottom node), and that the scc_iterator works its way
> down. So for the ModuleSummary, I should find the main function and return
> that (or whatever function is the entry point for the entire program).
>

You can look at how the traits are used. Several passes in tree use
them, so you can get an idea (look for CGSCC, for example, the
inliner). Also, SCCs are discovered in post-order. If you want a
different visitation order you should built that yourself on top of
the existing one (see for example the code in FunctionAttrs.cpp which
discovers the SCC in RPO).

That said, I'm not sure why you need to find the entry point of the program.
SCCs (collapsed) create a DAG so you can first propagate the attribute
within the SCC (until convergence) and then propagate across SCCs (see
the strong component section of
http://www.imm.dtu.dk/~hrni/PPA/slides6.pdf ). Unless I misunderstood
your problem (which I might have :), you don't need to start from the
entry point, as long as you have a topological order.

--
Davide