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Cache mode printing to avoid repeated Format.asprintf
OxCaml mode annotations (locality, uniqueness, linearity, etc.) were
printed via Format.asprintf per argument type, allocating a fresh
Format buffer each time. With 150K items × multiple args, this was
24% of allocation in statmemprof.
Cache the result per mode constant value (small finite set — ~16
entries). Reduces minor heap allocation by 20%.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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src/loader/cmi.cppo.ml
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src/loader/cmi.cppo.ml
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#if defined OXCAML
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(** Extract non-default mode strings from an OxCaml argument mode.
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Replicates the logic from [Printtyp.tree_of_modes]. *)
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(* Cache for mode printing — mode constants are a small finite set so the
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cache stays tiny. Avoids allocating a fresh Format buffer per call. *)
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let mode_print_cache : (Obj.t, string) Hashtbl.t = Hashtbl.create 16
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let print_mode_cached (print : Format.formatter -> 'a -> unit) (v : 'a) : string =
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let key = Obj.repr v in
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match Hashtbl.find_opt mode_print_cache key with
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| Some s -> s
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| None ->
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let s = Format.asprintf "%a" print v in
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Hashtbl.replace mode_print_cache key s;
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s
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let extract_arg_modes marg =
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let modes = Mode.Alloc.zap_to_legacy marg in
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let diff = Mode.Alloc.Const.diff modes Mode.Alloc.Const.legacy in
···
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| _, _ -> diff.portability
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in
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let print_opt print a =
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Option.map (fun v -> Format.asprintf "%a" print v) a
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Option.map (fun v -> print_mode_cached print v) a
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in
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List.filter_map Fun.id
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[ print_opt Mode.Locality.Const.print diff.areality