[PATCH] D150670: [WebAssembly] Disable generation of fshl/fshr for rotates

[Nikita Popov via Phabricator via llvm-commits]

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This doesn't looks like a wasm specific problem. You get essentially the same issue on any target that has a rotate instruction but no funnel shift instruction. Here are just a couple examples: https://godbolt.org/z/8v6nfaax9

I believe this needs to be either solved by preventing demanded bits simplifications that break a rotate pattern (though I'm not sure if that would break any other optimizations we care about) or by adding a special case for this in the backend when lowering FSH to ROT.

Lowering to a rotate intrinsic only "solves" this for wasm and will at the same time make these rotates completely opaque to optimization -- heck, it looks like we don't even support constant folding for these intrinsics (https://llvm.godbolt.org/z/hMWG16b9W).


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