On Mon, 28 Oct 2024 03:55:57 GMT, Jasmine Karthikeyan <jkarthike...@openjdk.org> wrote:
>> I mean we might want to run another kind of `Ideal` that will replace the >> normal `Ideal` on a node after its lowering. For example, consider this: >> >> vector<int,8> v; >> u = v.withLane(0, a).withLane(1, b); >> >> This will be parsed into: >> >> vector<int,8> v; >> v0 = InsertI(v, 4, a); >> u = InsertI(v0, 5, b); >> >> And can be lowered to: >> >> vector<int,8> v; >> vector<int,4> v1 = VectorExtract(v, 1); >> v2 = InsertI(v1, 0, a); >> v0 = VectorInsert(v, 1, v2); >> vector<int,4> v3 = VectorExtract(v0, 1); >> v4 = InsertI(v3, 1, b); >> u = VectorInsert(v0, 1, v4); >> >> Which represents this sequence: >> >> ymm0; >> vextracti128 xmm1, ymm0, 1; >> vpinsrd xmm1, xmm1, a, 0; >> vinserti128 ymm0, ymm0, xmm1, 1; >> vextracti128 xmm1, ymm0, 1; >> vpinsrd xmm1, xmm1, b, 1; >> vinserti128 ymm0, ymm0, xmm1, 1; >> >> As you can imagine this sequence is pretty inefficient, what we really want >> is: >> >> ymm0; >> vextracti128 xmm1, ymm0, 1; >> vpinsrd xmm1, xmm1, a, 0; >> vpinsrd xmm1, xmm1, b, 1; >> vinserti128 ymm0, ymm0, xmm1, 1; >> >> Looking back at the graph, we can `Identity` `v3` into `v2` since it is >> pretty obvious that we just do an insert and extract from the same place. >> However, to transform `u = VectorInsert(v0, 1, v4)` into `u = >> VectorInsert(v, 1, v4)`, we would need an `Ideal`-like transformation to >> see that we just insert into a location twice and remove the intermediate >> `VectorInsert`. >> >> As a result, in addition to ease of implementation, I think you may extend >> `PhaseIterGVN` and override its `PhaseGVN::apply_ideal` to return `nullptr` >> for now, and take advantages of `PhaseIterGVN::optimize` to do the iterative >> transformation for you. > > Ah, I see what you mean now. I think this makes extending IGVN more appealing > because we could continue to do Ideal on lowered nodes, as you mentioned. We > could override `PhaseGVN::apply_ideal` to return `nullptr` when processing > regular nodes, but run the other `Ideal` type when encountering lowered > nodes. Do you think it would be better to add another method to `Node` or > should we re-use the existing Ideal call, but lowering specific nodes are > guarded with a new node flag? I think having a new method in `Node` would be more manageable, I can imagine it allows us to reuse pre-lowered nodes for lowering. The example I gave above we reuse `ExtractI` since the semantics is still the same, the only difference is that from here `ExtractI` can only appear with the index parameter being smaller than 4. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1818374892
