File 2864-beam_ssa_dead-Add-elimination-of-redundant-tests.patch of Package erlang
From 837c56735c0dc9e5f4022c26c00596ca930d4855 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Fri, 20 Aug 2021 07:15:11 +0200
Subject: [PATCH 4/7] beam_ssa_dead: Add elimination of redundant tests
As a preparation for removing the beam_peep pass, reimplement
elimination of redundant tests in beam_ssa_dead. The optimization in
beam_ssa_dead is more general and will find many more redundant tests
than beam_peep ever did. For example, in Elixir's `Enum` module, it
will eliminate many unreachable inlined instances of the
`reduce/3` function.
---
lib/compiler/src/beam_ssa_dead.erl | 326 ++++++++++++++++++++++++++++-
lib/compiler/test/guard_SUITE.erl | 143 ++++++++++++-
2 files changed, 462 insertions(+), 7 deletions(-)
diff --git a/lib/compiler/src/beam_ssa_dead.erl b/lib/compiler/src/beam_ssa_dead.erl
index 7f00abd709..73626dd08a 100644
--- a/lib/compiler/src/beam_ssa_dead.erl
+++ b/lib/compiler/src/beam_ssa_dead.erl
@@ -27,8 +27,8 @@
-export([opt/1]).
-include("beam_ssa.hrl").
--import(lists, [append/1,keymember/3,last/1,member/2,
- reverse/1,takewhile/2]).
+-import(lists, [append/1,foldl/3,keymember/3,last/1,member/2,
+ reverse/1,reverse/2,takewhile/2]).
-type used_vars() :: #{beam_ssa:label():=sets:set(beam_ssa:var_name())}.
@@ -54,13 +54,13 @@
Label :: beam_ssa:label(),
Block :: beam_ssa:b_blk().
-opt(Linear) ->
- {Used,Skippable} = used_vars(Linear),
- Blocks0 = maps:from_list(Linear),
+opt(Linear0) ->
+ {Used,Skippable} = used_vars(Linear0),
+ Blocks0 = maps:from_list(Linear0),
St0 = #st{bs=Blocks0,us=Used,skippable=Skippable},
St = shortcut_opt(St0),
#st{bs=Blocks} = combine_eqs(St#st{us=#{}}),
- beam_ssa:linearize(Blocks).
+ opt_redundant_tests(Blocks).
%%%
%%% Shortcut br/switch targets.
@@ -1050,6 +1050,320 @@ lit_type(Val) ->
true -> none
end.
+
+%%%
+%%% Remove redundant tests.
+%%%
+%%% Repeated tests can be introduced by inlining, macros, or
+%%% complex guards such as:
+%%%
+%%% is_head(M, S) when M =:= <<1>>, S =:= <<2>> ->
+%%% true;
+%%% is_head(M, S) when M =:= <<1>>, S =:= <<3>> ->
+%%% false.
+%%%
+%%% The repeated test is not removed by any of the other optimizing
+%%% passes:
+%%%
+%%% 0:
+%%% _2 = bif:'=:=' _0, `<<1>>`
+%%% br _2, ^19, ^3
+%%%
+%%% 19:
+%%% _3 = bif:'=:=' _1, `<<2>>`
+%%% br _3, ^7, ^4
+%%%
+%%% 7:
+%%% ret `true`
+%%%
+%%% 4:
+%%% _4 = bif:'=:=' _0, `<<1>>`
+%%% br _4, ^15, ^3
+%%%
+%%% 15:
+%%% _5 = bif:'=:=' _1, `<<3>>`
+%%% br _5, ^11, ^3
+%%%
+%%% 11:
+%%% ret `false`
+%%%
+%%% 3:
+%%% %% Generate function clause error.
+%%% . . .
+%%%
+%%% This sub pass will keep track of all tests that are known to have
+%%% been executed at each block in the SSA code. If a repeated or
+%%% inverted test is seen, it can be eliminated. For the example
+%%% above, this sub pass will rewrite block 4 like this:
+%%%
+%%% 4:
+%%% _4 = bif:'=:=' `true`, `true`
+%%% br ^15
+%%%
+%%% This sub pass also removes redundant inverted test such as the
+%%% last test in this code:
+%%%
+%%% if
+%%% A < B -> . . . ;
+%%% A >= B -> . . .
+%%% end
+%%%
+%%% and this code:
+%%%
+%%% if
+%%% A < B -> . . . ;
+%%% A > B -> . . . ;
+%%% A == B -> . . .
+%%% end
+%%%
+
+opt_redundant_tests(Blocks) ->
+ All = #{0 => #{}, ?EXCEPTION_BLOCK => #{}},
+ RPO = beam_ssa:rpo(Blocks),
+ Linear = opt_redundant_tests(RPO, Blocks, All),
+ beam_ssa:trim_unreachable(Linear).
+
+opt_redundant_tests([L|Ls], Blocks, All0) ->
+ case All0 of
+ #{L := Tests} ->
+ Blk0 = map_get(L, Blocks),
+ Tests = map_get(L, All0),
+ Blk1 = opt_switch(Blk0, Tests),
+ #b_blk{is=Is0} = Blk1,
+ case opt_redundant_tests_is(Is0, Tests, []) of
+ none ->
+ All = update_successors(Blk1, Tests, All0),
+ [{L,Blk1}|opt_redundant_tests(Ls, Blocks, All)];
+ {new_test,Bool,Test,MustInvert} ->
+ All = update_successors(Blk1, Bool, Test, MustInvert,
+ Tests, All0),
+ [{L,Blk1}|opt_redundant_tests(Ls, Blocks, All)];
+ {old_test,Is,BoolVar,BoolValue} ->
+ Blk = case Blk1 of
+ #b_blk{last=#b_br{bool=BoolVar}=Br0} ->
+ Br = beam_ssa:normalize(Br0#b_br{bool=BoolValue}),
+ Blk1#b_blk{is=Is,last=Br};
+ #b_blk{}=Blk2 ->
+ Blk2#b_blk{is=Is}
+ end,
+ All = update_successors(Blk, Tests, All0),
+ [{L,Blk}|opt_redundant_tests(Ls, Blocks, All)]
+ end;
+ #{} ->
+ opt_redundant_tests(Ls, Blocks, All0)
+ end;
+opt_redundant_tests([], _Blocks, _All) -> [].
+
+opt_switch(#b_blk{last=#b_switch{arg=Arg,list=List0}=Sw}=Blk, Tests)
+ when map_size(Tests) =/= 0 ->
+ List = opt_switch_1(List0, Arg, Tests),
+ Blk#b_blk{last=Sw#b_switch{list=List}};
+opt_switch(Blk, _Tests) -> Blk.
+
+opt_switch_1([{Lit,_}=H|T], Arg, Tests) ->
+ case Tests of
+ #{{'=:=',Arg,Lit} := false} ->
+ opt_switch_1(T, Arg, Tests);
+ #{} ->
+ [H|opt_switch_1(T, Arg, Tests)]
+ end;
+opt_switch_1([], _, _) -> [].
+
+opt_redundant_tests_is([#b_set{op=Op,args=Args,dst=Bool}=I0], Tests, Acc) ->
+ case canonical_test(Op, Args) of
+ none ->
+ none;
+ {Test,MustInvert} ->
+ case old_result(Test, Tests) of
+ Result0 when is_boolean(Result0) ->
+ Result = #b_literal{val=Result0 xor MustInvert},
+ I = I0#b_set{op={bif,'=:='},args=[Result,#b_literal{val=true}]},
+ {old_test,reverse(Acc, [I]),Bool,Result};
+ none ->
+ {new_test,Bool,Test,MustInvert}
+ end
+ end;
+opt_redundant_tests_is([I|Is], Tests, Acc) ->
+ opt_redundant_tests_is(Is, Tests, [I|Acc]);
+opt_redundant_tests_is([], _Tests, _Acc) -> none.
+
+old_result(Test, Tests) ->
+ case Tests of
+ #{Test := Val} -> Val;
+ #{} -> old_result_1(Test, Tests)
+ end.
+
+%%
+%% Remove the last test in a sequence of tests (in any order):
+%%
+%% if
+%% Val1 < Val2 -> . . .
+%% Val1 > Val2 -> . . .
+%% Val1 == Val2 -> . . .
+%% end
+%%
+%% NOTE: The same optimization is not possible to do with `=:=`, unless
+%% we have type information so that we know that `==` and `=:=` produces
+%% the same result.
+%%
+
+old_result_1({'==',A,B}, Tests) ->
+ case Tests of
+ #{{'<',A,B} := false, {'=<',A,B} := true} ->
+ %% not A < B, not A > B ==> A == B
+ true;
+ #{} ->
+ none
+ end;
+old_result_1({'=<',A,B}, Tests) ->
+ case Tests of
+ #{{'<',A,B} := false, {'==',A,B} := false} ->
+ %% not A < B, not A == B ==> A > B
+ false;
+ #{} ->
+ none
+ end;
+old_result_1({'<',A,B}, Tests) ->
+ case Tests of
+ #{{'=<',A,B} := true, {'==',A,B} := false} ->
+ %% not A < B, not A == B ==> A < B
+ true;
+ #{} ->
+ none
+ end;
+old_result_1({is_nonempty_list,A}, Tests) ->
+ case Tests of
+ #{{is_list,A} := false} -> false;
+ #{} -> none
+ end;
+old_result_1(_, _) -> none.
+
+%% canonical_test(Op0, Args0) -> {CanonicalTest, MustInvert}
+%% CanonicalTest = {Operator,Variable,Variable|Literal} |
+%% {TypeTest,Variable}
+%% Operation = '<' | '=<' | '=:=' | '=='
+%% TypeTest = is_atom | is_integer ...
+%% Variable = #b_var{}
+%% Literal = #b_literal{}
+%% MustInvert = true | false
+%%
+%% Canonicalize a test. Always make the register
+%% operand the first operand. If there are two registers,
+%% order the registers in lexical order. Invert four of
+%% the relation operators and indicate with MustInvert
+%% whether the operator was inverted.
+%%
+%% For example, this instruction:
+%%
+%% #b_set{op={bif,'=:='},args=[#b_literal{}, #b_var{}}
+%%
+%% will be canonicalized to:
+%%
+%% {{'=:=',#b_var{},#b_literal{}}, false}
+%%
+%% while:
+%%
+%% #b_set{op={bif,'>'},args=[#b_var{}, #b_literal{}}}
+%%
+%% will be canonicalized to:
+%%
+%% {{'=<',#b_var{},#b_literal{}}, true}
+%%
+canonical_test(Op, Args) ->
+ case normalize_test(Op, Args) of
+ none ->
+ none;
+ Test ->
+ Inv = case Test of
+ {'=/=',_,_} -> true;
+ {'/=',_,_} -> true;
+ {'>',_,_} -> true;
+ {'>=',_,_} -> true;
+ _ -> false
+ end,
+ case Inv of
+ true -> {invert_test(Test),true};
+ false -> {Test,false}
+ end
+ end.
+
+update_successors(#b_blk{last=#b_br{bool=Bool,succ=Succ,fail=Fail}},
+ Bool, Test, MustInvert, Tests, All0) ->
+ All1 = update_successor(Succ, Tests#{Test => not MustInvert}, All0),
+ update_successor(Fail, Tests#{Test => MustInvert}, All1);
+update_successors(Blk, _, _, _, TestsA, All) ->
+ update_successors(Blk, TestsA, All).
+
+update_successors(#b_blk{last=#b_ret{}}, _Tests, All) ->
+ All;
+update_successors(#b_blk{last=#b_switch{arg=Arg,fail=Fail,list=List}},
+ Tests, All0) ->
+ All1 = update_successors_sw_fail(List, Arg, Fail, Tests, All0),
+ update_successors_sw(List, Arg, Tests, All1);
+update_successors(Blk, Tests, All) ->
+ foldl(fun(L, A) ->
+ update_successor(L, Tests, A)
+ end, All, beam_ssa:successors(Blk)).
+
+update_successors_sw_fail(List, Arg, Fail, Tests0, All) ->
+ Tests = foldl(fun({Lit,_}, A) ->
+ A#{{'=:=',Arg,Lit} => false}
+ end, Tests0, List),
+ update_successor(Fail, Tests, All).
+
+update_successors_sw([{Lit,L}|T], Arg, Tests, All0) ->
+ All = update_successor(L, Tests#{{'=:=',Arg,Lit} => true}, All0),
+ update_successors_sw(T, Arg, Tests, All);
+update_successors_sw([], _, _, All) -> All.
+
+update_successor(?EXCEPTION_BLOCK, _Tests, All) ->
+ All;
+update_successor(L, TestsA, All0) ->
+ case All0 of
+ #{L := TestsB} ->
+ All0#{L := maps_intersect_kv(TestsA, TestsB)};
+ #{} ->
+ All0#{L => TestsA}
+ end.
+
+maps_intersect_kv(Map, Map) ->
+ Map;
+maps_intersect_kv(Map1, Map2) ->
+ if
+ map_size(Map1) < map_size(Map2) ->
+ map_intersect_kv_1(Map1, Map2);
+ true ->
+ map_intersect_kv_1(Map2, Map1)
+ end.
+
+map_intersect_kv_1(SmallMap, BigMap) ->
+ Next = maps:next(maps:iterator(SmallMap)),
+ case maps_is_subset_kv(Next, BigMap) of
+ true -> SmallMap;
+ false -> map_intersect_kv_2(Next, BigMap, [])
+ end.
+
+map_intersect_kv_2({K, V, Iterator}, BigMap, Acc) ->
+ Next = maps:next(Iterator),
+ case BigMap of
+ #{K := V} ->
+ map_intersect_kv_2(Next, BigMap, [{K,V}|Acc]);
+ #{} ->
+ map_intersect_kv_2(Next, BigMap, Acc)
+ end;
+map_intersect_kv_2(none, _BigMap, Acc) ->
+ maps:from_list(Acc).
+
+maps_is_subset_kv({K, V, Iterator}, BigMap) ->
+ Next = maps:next(Iterator),
+ case BigMap of
+ #{K := V} ->
+ maps_is_subset_kv(Next, BigMap);
+ #{} ->
+ false
+ end;
+maps_is_subset_kv(none, _BigMap) -> true.
+
%%%
%%% Calculate used variables for each block.
%%%
diff --git a/lib/compiler/test/guard_SUITE.erl b/lib/compiler/test/guard_SUITE.erl
index af7d8299d5..3d7da5075f 100644
--- a/lib/compiler/test/guard_SUITE.erl
+++ b/lib/compiler/test/guard_SUITE.erl
@@ -1424,7 +1424,9 @@ rel_op_combinations(Config) when is_list(Config) ->
Red = gb_trees:from_orddict(Red0),
rel_op_combinations_3(100, Red),
- rel_op_combinations_4().
+ rel_op_combinations_4(),
+
+ rel_op_combinations_5().
rel_op_combinations_1(0, _) ->
ok;
@@ -1677,6 +1679,145 @@ rel_op_vars_1(X, N) when X =< N -> le.
rel_op_vars_2(X, N) when X =/= N -> ne;
rel_op_vars_2(X, N) when X >= N -> ge.
+rel_op_combinations_5() ->
+ lt = lt_gt_eq(a, b),
+ lt = lt_gt_eq(1.0, 42),
+ lt = lt_gt_eq(1, 42.0),
+
+ eq = lt_gt_eq(a, a),
+ eq = lt_gt_eq(42, 42),
+ eq = lt_gt_eq(42.0, 42),
+ eq = lt_gt_eq(42, 42.0),
+ eq = lt_gt_eq(42.0, 42.0),
+
+ gt = lt_gt_eq(b, a),
+ gt = lt_gt_eq(42.0, 1),
+ gt = lt_gt_eq(42, 1.0),
+
+ lt = eq_exact_lt_gt(a, b),
+ lt = eq_exact_lt_gt(1.0, 42),
+ lt = eq_exact_lt_gt(1, 42.0),
+
+ eq = eq_exact_lt_gt(a, a),
+ eq = eq_exact_lt_gt(42, 42),
+ none = eq_exact_lt_gt(42, 42.0),
+
+ gt = eq_exact_lt_gt(b, a),
+ gt = eq_exact_lt_gt(42.0, 1),
+ gt = eq_exact_lt_gt(42, 1.0),
+
+ ok.
+
+lt_gt_eq(A, B) ->
+ Res = lt_gt_eq_1(A, B),
+ Res = lt_gt_eq_2(A, B),
+ Res = lt_gt_eq_3(A, B),
+ Res = lt_gt_eq_4(A, B),
+ Res = lt_gt_eq_5(A, B),
+ lt_gt_eq_6(A, B).
+
+%% The last test in each 'if' is unnecessary.
+lt_gt_eq_1(A, B) ->
+ if
+ A < B -> lt;
+ A == B -> eq;
+ A > B -> gt
+ end.
+
+lt_gt_eq_2(A, B) ->
+ if
+ A > B -> gt;
+ A == B -> eq;
+ A < B -> lt
+ end.
+
+lt_gt_eq_3(A, B) ->
+ if
+ A == B -> eq;
+ A < B -> lt;
+ A > B -> gt
+ end.
+
+lt_gt_eq_4(A, B) ->
+ if
+ A == B -> eq;
+ A > B -> gt;
+ A < B -> lt
+ end.
+
+lt_gt_eq_5(A, B) ->
+ if
+ A < B -> lt;
+ A > B -> gt;
+ A == B -> eq
+ end.
+
+lt_gt_eq_6(A, B) ->
+ if
+ A > B -> gt;
+ A < B -> lt;
+ A == B -> eq
+ end.
+
+eq_exact_lt_gt(A, B) ->
+ Res = eq_exact_lt_gt_1(A, B),
+ Res = eq_exact_lt_gt_2(A, B),
+ Res = eq_exact_lt_gt_3(A, B),
+ Res = eq_exact_lt_gt_4(A, B),
+ Res = eq_exact_lt_gt_5(A, B),
+ Res = eq_exact_lt_gt_6(A, B).
+
+%% Not possible to optimize (unless we have type information so we
+%% know that A == B and A =:= B produces the same result).
+
+eq_exact_lt_gt_1(A, B) ->
+ if
+ A < B -> lt;
+ A =:= B -> eq;
+ A > B -> gt;
+ true -> none
+ end.
+
+eq_exact_lt_gt_2(A, B) ->
+ if
+ A > B -> gt;
+ A =:= B -> eq;
+ A < B -> lt;
+ true -> none
+ end.
+
+eq_exact_lt_gt_3(A, B) ->
+ if
+ A =:= B -> eq;
+ A < B -> lt;
+ A > B -> gt;
+ true -> none
+ end.
+
+eq_exact_lt_gt_4(A, B) ->
+ if
+ A =:= B -> eq;
+ A > B -> gt;
+ A < B -> lt;
+ true -> none
+ end.
+
+eq_exact_lt_gt_5(A, B) ->
+ if
+ A < B -> lt;
+ A > B -> gt;
+ A =:= B -> eq;
+ true -> none
+ end.
+
+eq_exact_lt_gt_6(A, B) ->
+ if
+ A > B -> gt;
+ A < B -> lt;
+ A =:= B -> eq;
+ true -> none
+ end.
+
%% Exhaustively test all combinations of relational operators
%% to ensure the correctness of the optimizations in beam_ssa_dead.
--
2.31.1
