[Mlir-commits] [mlir] [mlir][vector] Make gather/scatter index dimensions separately (PR #194395)

[Andrzej Warzyński]

banach-space wrote:

> Perhaps we should start an RFC and schedule an ODM meeting for high-bandwidth discussion?

You are of course free and welcome to send an RFC and/or organise an ODM discussion. My main concern is that, based on the visible engagement so far, I am not sure this is primarily a "discussion bandwidth" problem. It looks more like limited evidence of broad stakeholder interest in changing the semantics/API of these ops at this time.

Before starting a new thread, I think it would be useful to first establish that there are several stakeholders who want to engage with the design space and who have concrete use cases that require this semantic/API change rather than just fixing/documenting the existing op.

> This topic keeps coming up every few months

I may have missed earlier discussions, but I don't think the record shows this exact topic recurring every few months. What has come up recently is that the semantics of `vector.gather` / `vector.scatter` in the presence of strided MemRefs are underspecified. That was raised here:

* https://discourse.llvm.org/t/rfc-semantics-of-vector-gather-indices-with-strided-memrefs

With underspecified semantics, it is unsurprising that we ended up with inconsistent lowerings, as documented here:

* https://github.com/llvm/llvm-project/issues/187215

But AFAICT this specific discussion only really started while discussing the indexed-access interface work in:

* https://github.com/llvm/llvm-project/pull/177014

and then crystallised in the March issue above. That earlier PR also struggled with review, which suggests to me that this area is complex and currently has limited reviewer bandwidth/engagement.

So my preference would still be: first fix the existing inconsistency and document the current semantics clearly; then revisit a semantic/API change once there are concrete upstream users and broader stakeholder engagement.

https://github.com/llvm/llvm-project/pull/194395