martong added a comment.
Hi Aleksei,
Unfortunately, it seems that it is very to hard synthesize a lit or unit test
for this fix.
I have found this flaw during the CTU analysis of Bitcoin, where the AST was
immense.
Because of the lack of tests, now I am trying to persuade you based on some
theory.
Let's consider this simple BFS algorithm from the `s` source:
void bfs(Graph G, int s)
{
Queue<Integer> queue = new Queue<Integer>();
marked[s] = true; // Mark the source
queue.enqueue(s); // and put it on the queue.
while (!q.isEmpty()) {
int v = queue.dequeue(); // Remove next vertex from the queue.
for (int w : G.adj(v))
if (!marked[w]) // For every unmarked adjacent vertex,
{
marked[w] = true;
queue.enqueue(w);
}
}
}
I believe , the structural equivalence check could be implemented as a parallel
BFS on a pair of graphs.
And, I think that must have been the original approach at the beginning.
Indeed, it has it's queue, which holds pairs of nodes, one from each graph,
this is the `DeclsToCheck` and it's pair is in `TentativeEquivalences`.
`TentativeEquivalences` also plays the role of the marking (`marked`)
functionality above, we use it to check whether we've already seen a pair of
nodes.
We put in the elements into the queue only in the toplevel decl check function:
static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
Decl *D1, Decl *D2);
The `while` loop where we iterate over the children is implemented in
`Finish()`.
And `Finish` is called only from the two **member** functions which check the
equivalency of two Decls or two Types. ASTImporter (and other clients) call
only these functions.
The `static` implementation functions are called from `Finish`, these push the
children nodes to the queue.
So far so good, this is almost like the BFS.
However, if we let a static implementation function to call `Finish` via
another **member** function that means we end up with two nested while loops
each of them working on the same queue. This is wrong and nobody can reason
about it's doing.
So, now `TentativeEquivalences` plays two roles. It is used to store the second
half of the decls which we want to compare, plus it plays a role in closing the
recursion. On a long term, I think, (after this change) we could refactor
structural equivalency to be more alike to the traditional BFS.
Repository:
rC Clang
https://reviews.llvm.org/D49300
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