[llvm] [CostModel][AArch64] Make extractelement, with fmul user, free whenev… (PR #111479)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Hi - I think what this patch is trying to do makes a lot of sense. I guess there might be a reluctance due to the way it has to do it. There is quite a lot of new code. In the past we have tried to have the cost model functions able to test generic (non-existent) instructions. Sometimes they need to know more info (especially about uses and operands), so operands get added and sometimes a CxtI instruction to get the uses from. Am I right that that approach of passing a CxtI instruction doesn't work very well in this case because there is no instruction yet? The extract is new and the mul the new user of it. It would be nice if the interface was fairly generic if we an make it, not specific to the SLP vectorizer.

https://github.com/llvm/llvm-project/pull/111479