Fix a performance issue when answering "is this tuple Copyable"? #65673
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xedin
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If we don't split up the tuple into individual constraints, we end up spending more time querying whether a tuple is Copyable in `lookupConformance`, because it will naively check the types of all elements of the tuple recursively with `lookupConformance`. This is inefficient because if we know some of the elements of the tuple are fixed types, we don't need to keep checking those again. For example, if we have `($T, Int, $U)`, and then try a binding for `$T`, we might ask again if the whole tuple conforms. Leading to `lookupConformance` to check whether `Int` (and all other elements of the tuple) conforms to Copyable, when we either already know that, or can't answer it yet because it's still a type variable. By splitting up a Copyable constraint on a tuple into invidivual constraints on each of its type elements, we can avoid this redundant work by `lookupConformance`. While today we could short-cut this even further to say that _all_ tuples are Copyable, since we emit an error if a noncopyable type appears in one, that won't be true in the near future. This is the nicer solution we'll want to keep around long-term. After discussing this with Pavel, we don't think there's a good way to add a regression test for this, because the performance issue primarily comes up in specific example programs that aren't amenable to scale tests. resolves rdar://107536402
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full test only failed the unrelated test |
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@swift-ci please smoke test macOS |
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If we don't split up the tuple into individual constraints, we end up spending more time querying whether a tuple is Copyable in
lookupConformance, because it will naively check the types of all elements of the tuple recursively withlookupConformance.This is inefficient because if we know some of the elements of the tuple are fixed types, we don't need to keep checking those again. For example, if we have
($T, Int, $U), and then try a binding for$T, we might ask again if the whole tuple conforms. Leading tolookupConformanceto check whetherInt(and all other elements of the tuple) conforms to Copyable, when we either already know that, or can't answer it yet because it's still a type variable.By splitting up a Copyable constraint on a tuple into invidivual constraints on each of its type elements, we can avoid this redundant work by
lookupConformance.While today we could short-cut this even further to say that all tuples are Copyable, since we emit an error if a noncopyable type appears in one, that won't be true in the near future. This is the nicer solution we'll want to keep around long-term.
After discussing this with Pavel, we don't think there's a good way to add a regression test for this, because the performance issue primarily comes up in specific example programs that aren't amenable to scale tests.
resolves rdar://107536402