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Fix exponential time algorithm #7340

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Merged
bjorng merged 1 commit into from
Jun 2, 2023
Merged

Fix exponential time algorithm #7340

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41 changes: 20 additions & 21 deletions lib/compiler/src/beam_ssa_bool.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -1631,35 +1631,34 @@ del_out_edges(V, G) ->
beam_digraph:del_edges(G, beam_digraph:out_edges(G, V)).

covered(From, To, G) ->
Seen0 = sets:new([{version, 2}]),
Seen0 = #{},
{yes,Seen} = covered_1(From, To, G, Seen0),
sets:to_list(Seen).
[V || {V,reached} <- maps:to_list(Seen)].

covered_1(To, To, _G, Seen) ->
{yes,Seen};
covered_1(From, To, G, Seen0) ->
Vs0 = beam_digraph:out_neighbours(G, From),
Vs = [V || V <- Vs0, not sets:is_element(V, Seen0)],
Seen = sets:union(sets:from_list(Vs, [{version, 2}]), Seen0),
case Vs of
[] ->
no;
[_|_] ->
covered_list(Vs, To, G, Seen, false)
end.
covered_1(From, To, G, Seen) ->
Vs = beam_digraph:out_neighbours(G, From),
covered_list(Vs, To, G, Seen, no).

covered_list([V|Vs], To, G, Seen0, AnyFound) ->
case covered_1(V, To, G, Seen0) of
{yes,Seen} ->
covered_list(Vs, To, G, Seen, true);
no ->
covered_list(Vs, To, G, Seen0, AnyFound)
case Seen0 of
#{V := reached} ->
covered_list(Vs, To, G, Seen0, yes);
#{V := not_reached} ->
covered_list(Vs, To, G, Seen0, AnyFound);
#{} ->
case covered_1(V, To, G, Seen0) of
{yes,Seen1} ->
Seen = Seen1#{V => reached},
covered_list(Vs, To, G, Seen, yes);
{no,Seen1} ->
Seen = Seen1#{V => not_reached},
covered_list(Vs, To, G, Seen, AnyFound)
end
end;
covered_list([], _, _, Seen, AnyFound) ->
case AnyFound of
true -> {yes,Seen};
false -> no
end.
{AnyFound,Seen}.

digraph_roots(G) ->
digraph_roots_1(beam_digraph:vertices(G), G).
Expand Down
43 changes: 41 additions & 2 deletions lib/compiler/test/andor_SUITE.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -486,11 +486,25 @@ in_case_1_guard(LenUp, LenDw, LenN, Rotation, Count) ->
end.

-record(state, {stack = []}).
-record(conf, {e1=[], e2=[], e3=[], e4=[], e5=[], e6=[]}).

slow_compilation(_) ->
%% The function slow_compilation_1 used to compile very slowly.
ok = slow_compilation_1({a}, #state{}).
ok = slow_compilation_1({a}, #state{}),

{'EXIT', {function_clause,_}} = catch slow_compilation_2(#{}),
{'EXIT', {function_clause,_}} = catch slow_compilation_2(true),

true = #conf{} =:= slow_compilation_3(#conf{}, #conf{}),
#conf{e1=a, e2=[], e3=[], e4=[], e5=[], e6=[]} =
slow_compilation_3(#conf{e1=a}, #conf{}),
#conf{e1=[], e2=[], e3=c, e4=[], e5=[], e6=[]} =
slow_compilation_3(#conf{e3=c}, #conf{}),
#conf{e1=[], e2=[], e3=[], e4=[], e5=[], e6=f} =
slow_compilation_3(#conf{e6=f}, #conf{}),

ok.

%% This function used to compile very slowly.
slow_compilation_1(T1, #state{stack = [T2|_]})
when element(1, T2) == a, element(1, T1) == b, element(1, T1) == c ->
ok;
Expand All @@ -514,6 +528,31 @@ slow_compilation_1(_, T) when element(1, T) == b ->
slow_compilation_1(T, _) when element(1, T) == a ->
ok.

%% The following function used to compile really slowly (about one and
%% a half minutes on my computer). The culprit was
%% beam_ssa_bool:covered/1. (Thanks to Robin Morisset and erlfuzz.)
slow_compilation_2(X)
when X or is_function(ok, ok);
X#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}
#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}
#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}
#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}#{ok := ok}
#{ok := ok} ->
ok.

%% GH-7338. Very slow compilation time (the culprit was beam_ssa_bool:covered/1).
slow_compilation_3(Old, New) ->
if Old#conf.e1 =/= New#conf.e1;
Old#conf.e2 =/= New#conf.e2;
Old#conf.e3 =/= New#conf.e3;
Old#conf.e4 =/= New#conf.e4;
Old#conf.e5 =/= New#conf.e5;
Old#conf.e6 =/= New#conf.e6 ->
Old;
true ->
New
end.

%% Utilities.

echo(X) ->
Expand Down