[PATCH] D99675: RFC [llvm][clang] Create new intrinsic llvm.arith.fence to control FP optimization at expression level

[Pengfei Wang via Phabricator via cfe-commits]

pengfei added a comment.

In D99675#2695424 <https://reviews.llvm.org/D99675#2695424>, @kpn wrote:

> What changes are needed for a backend, and what happens if they aren't done?

As far as I understand it, backend does optimizations based on patterns of the known nodes and MIs. Inserting a new node/MI will block any optimizations across the fence. So it respects the semantics of the intrinsic without target special chenges.
I'm not sure if there's room for optimization cross the `arithmetic.fence`. If there is and no changes for it, backend may have some performance loss under these circumstances.

> Having something needed for correctness silently not work seems ... sub-optimal.

I think backend is conservative for optimizations when use the intrinsic. It won't have correctness issue silently, but performance loss might.


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