File 2853-Fix-exponential-time-algorithm.patch of Package erlang
From 7d79897d1f1424aaf97802869ae5298a1aa1703d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Wed, 26 Apr 2023 15:45:09 +0200
Subject: [PATCH] Fix exponential time algorithm
The function `beam_ssa_bool:covered/1` ran in exponential time.
The new implementation does not always produce the same list of
covered vertices, but the same code is generated for every module
compiled by `scripts/diffable`.
Closes #7338
---
lib/compiler/src/beam_ssa_bool.erl | 41 ++++++++++++++--------------
lib/compiler/test/andor_SUITE.erl | 43 ++++++++++++++++++++++++++++--
2 files changed, 61 insertions(+), 23 deletions(-)
diff --git a/lib/compiler/src/beam_ssa_bool.erl b/lib/compiler/src/beam_ssa_bool.erl
index 7ad38dc3ca..4c25769478 100644
--- a/lib/compiler/src/beam_ssa_bool.erl
+++ b/lib/compiler/src/beam_ssa_bool.erl
@@ -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).
diff --git a/lib/compiler/test/andor_SUITE.erl b/lib/compiler/test/andor_SUITE.erl
index 250f2c5803..22cf4a2cb8 100644
--- a/lib/compiler/test/andor_SUITE.erl
+++ b/lib/compiler/test/andor_SUITE.erl
@@ -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;
@@ -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) ->
--
2.35.3
