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cmd/compile: use hashing for switch statements #34381

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mdempsky opened this issue Sep 18, 2019 · 2 comments

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@mdempsky
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commented Sep 18, 2019

Currently we implement switch statements with binary search. This works well for values that fit into a register, but for strings it might be better to compute a hash function.

Some thoughts:

  1. Even in the simple case of using a full hash function, exactly two passes over the switched value is probably better than O(log N) possible passes over it.

  2. Since we have the expected keys statically, we can use a simple, cheap universal hash (e.g., FNV? djb2?) and keep trying different seeds until we get one without any collisions.

  3. Instead of hashing all of the input bytes, we can hash just enough of them to avoid collisions.

  4. We can try using a minimal (or near minimal) perfect hash to emit a jump table instead of binary search. (This might even be useful for sparse integer values too.)

If someone can help figure out code that can take a []string and figures out an appropriate and cheap hash function to use, I can help wire it into swt.go.

@mdempsky mdempsky added this to the Unplanned milestone Sep 18, 2019
@josharian

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commented Sep 19, 2019

Drive-by comment from phone: It wouldn’t surprise me if “use the first word of the string data” turns out to in practice to be a decent first-cut hash, and it is certainly cheap (can be generated inline, should be just a few instructions).

@pierrec

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commented Sep 19, 2019

After reading minimal perfect hash function which does have an implementation go-mph, maybe that can just be used as is?

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