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Optimize for comprehensions into: MapSet #14796

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Merged
merged 3 commits into from
Sep 29, 2025
Merged

Optimize for comprehensions into: MapSet #14796

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9afb425
Optimize for comprehensions into: MapSet
sabiwara Sep 29, 2025
c713cde
Even faster: build the map directly
sabiwara Sep 29, 2025
aeea3b5
Revert "Even faster: build the map directly"
sabiwara Sep 29, 2025
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9 changes: 9 additions & 0 deletions lib/elixir/src/elixir_erl_for.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,11 @@
-export([translate/3]).
-include("elixir.hrl").

-define(empty_map_set_pattern, {map, _, [
{map_field_assoc, _, {atom, _, '__struct__'}, {atom, _, 'Elixir.MapSet'}},
{map_field_assoc, _, {atom, _, map}, {map, _, []}}
]}).

translate(Meta, Args, S) ->
{Cases, [{do, Expr} | Opts]} = elixir_utils:split_last(Args),

Expand Down Expand Up @@ -156,6 +161,10 @@ build_inline_each(Ann, Clauses, Expr, {bin, _, []}, Uniq, S) ->
build_into(Ann, Clauses, Expr, {map, _, []}, Uniq, S) ->
{ReduceExpr, SR} = build_inline_each(Ann, Clauses, Expr, {nil, Ann}, Uniq, S),
{?remote(Ann, maps, from_list, [ReduceExpr]), SR};
build_into(Ann, Clauses, Expr, ?empty_map_set_pattern = _Into, Uniq, S) ->
InnerFun = fun(InnerExpr, InnerAcc) -> {cons, Ann, InnerExpr, InnerAcc} end,
{ReduceExpr, SR} = build_reduce(Ann, Clauses, InnerFun, Expr, {nil, Ann}, Uniq, S),
{?remote(Ann, 'Elixir.MapSet', new, [ReduceExpr]), SR};
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I think we could even optimize this further by changing the comprehension to return {Item, true} and then building the structure directly.

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@sabiwara sabiwara Sep 29, 2025 •
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Oh, do you mean this:

%MapSet{map: :maps.from_list(Enum.reduce([1, 2, 3], [], fn x, var3 -> [{x, []} | var3] end))}

Makes sense!

({Item, []}, not {Item, true}, right?)

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@sabiwara sabiwara Sep 29, 2025 •
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OK, interesting.

Done in c713cde, but while it speeds things up, it also seems to increase memory significantly. Maybe not worth it? sabiwara/elixir_benches@14c0876

(maybe because you implemented :sets.from_list(version: 2) efficiently as a BIF, and it doesn't need all these tuples?)

Screenshot 2025-09-29 at 17 07 17

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Will revert to the previous version and merge, WDYT?

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Keep the new version but use from_keys without the tuple. Although I guess that’s the same as MapSet.new anyway? So yeah, please revert!

build_into(Ann, Clauses, Expr, Into, Uniq, S) ->
{Fun, SF} = build_var(Ann, S),
{Acc, SA} = build_var(Ann, SF),
Expand Down