[llvm-dev] Propagation of debug information for variable into basic blocks.

[Daniel Berlin via llvm-dev]

>
>
>
>   // For all predecessors of this MBB, find the set of VarLocs that
>   // can be joined.
>   for (auto p : MBB.predecessors()) {
>     auto OL = OutLocs.find(p);
>     // Join is null in case of empty OutLocs from any of the pred.
>     if (OL == OutLocs.end())
>       return false;
>
>
> This is wrong  if the block is unvisited (as you say)
>

This is actually non-trivial to fix, IMHO.

For example, the following looks kinda right:

for (auto p : MBB.predecessors()) {
    if (p is not visited)
     continue;
    auto OL = OutLocs.find(p);
    // Join is null in case of empty OutLocs from any of the pred.
    if (OL == OutLocs.end())
      return false;

(and will work in a lot of cases)

This can be wrong, however, if you have more than a single backedge in the
program.
(it may be possible to have a block that has multiple uninited predecessors
that will require multiple iterations to resolve)

The problem here is that to get the maximal answer in an intersection
problem, you *must* initialize the sets to contain every value.  You can
look at dataflow algorithm papers for formal proofs if you want.

So you *actually* have to treat the default state of every unvisited block
as the equivalent of "containing every value" to get the maximal answer.

How to achieve this for real is ... not obvious at a glance.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160921/4c17d31c/attachment-0001.html>