# Reverse Fusion of Node Pairs in Scheduler

Hi,

I’m interested in PyTorch’s operator fusion. I have been reading the Scheduler’s source code and noticed a strange case where it seems to consider the fusability of a pair of nodes `node1, node2` in the normal order: `self.can_fuse(node1, node2)` but also the reverse order: `self.can_fuse(node2, node1)` . Specifically, I’m referring to this code here:

``````def check_all_pairs(nodes):
for node1_index, node1 in enumerate(nodes):
for node2 in nodes[node1_index + 1 :]:
key = (node1, node2)
if key in seen:
continue

if self.can_fuse(node1, node2):
possible_fusions.append(key)
elif (node2.is_template() or node2.is_foreach()) and self.can_fuse(
node2, node1
):
# foreach fusions and epilogue fusions are order dependent
possible_fusions.append((node2, node1))
``````

I inspected this function on some trivial functions via `torch.compile()` and it seems that `nodes` is typically a set containing a graph node `n` and all the nodes it outputs to `m_1,...m_k` (it’s probably not that simple, so feel free to correct me there). So we have a set (`n`, `m_1`, … `m_k`). So in the `self.can_fuse(node1, node2)` case we have:

1. Check whether we can fuse `n` with some `m_i+1` (propbably vertical fusion)
2. Check whether we can fuse `m_i` with `m_i+1` (probably horizontal fusion)
for some `i` in `1...k`.

Now my question is, when is `self.can_fuse(node2, node1)` meaningful? Is this specifically for cases where the order of operations doesn’t affect the correctness? Although in that case `self.can_fuse(node1, node2)` should also be sufficient.

Would appreciate someone’s insight here. Many thanks!

FYI: this is a re-post of this question on the pytorch forum. I’m hoping that’s ok given it didn’t see any response there.