Avoid O(N^2) behavior in widenreturn #44437
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In functions with lots of slots that construct PartialStructs with lots of fields, `widenreturn` would attempt to repeatedly find an interesting slot for Interconditional to use. This is unnecessary, because the slot selected does not depend on the value of the PartialStruct field. Memoize this computation and only do it once per return statement.
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Is there a simple example where this is a performance win? |
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I've been working through all the pathologies of this testcase: https://gist.github.com/Keno/5587fbfe89bff96c863c8eeb1477defa. This was one of them. |
vtjnash
reviewed
Mar 4, 2022
| @@ -2075,7 +2086,7 @@ function widenreturn(@nospecialize(rt), @nospecialize(bestguess), nslots::Int, s | |||
| local anyrefine = false | |||
| for i in 1:length(fields) | |||
| a = fields[i] | |||
| a = isvarargtype(a) ? a : widenreturn(a, bestguess, nslots, slottypes, changes) | |||
| a = isvarargtype(a) ? a : widenreturn(a, bestguess, nslots, slottypes, changes, interconditional_slot) | |||
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Fields are prohibited from containing Conditional/InterConditional values
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Then the code that is here now is wrong, because it likes to add them.
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Replaced by #44438 |
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In functions with lots of slots that construct PartialStructs with
lots of fields,
widenreturnwould attempt to repeatedly find aninteresting slot for Interconditional to use. This is unnecessary,
because the slot selected does not depend on the value of the
PartialStruct field. Memoize this computation and only do it once
per return statement.