File 2861-Eliminate-redundant-branches.patch of Package erlang
From 3e4b86a954a24aed31f1bc17d22a1f4e1e05e8aa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Mon, 28 Jun 2021 15:23:22 +0200
Subject: [PATCH 1/7] Eliminate redundant branches
`sys_core_fold` has a weak optimization that rewrites:
case BoolExpr of
true -> true
false -> false
end
to simply:
BoolExpr
That optimization is weak because it will only handle a boolean
expression in a Core Erlang `case` expression, not other boolean
expressions resulting from, for example, guards.
Implement the optimization in a more effective way in the
`beam_ssa_opt` pass.
---
lib/compiler/src/beam_ssa_opt.erl | 124 ++++++++++++++++++++++++++-
lib/compiler/src/sys_core_fold.erl | 47 +++-------
lib/compiler/test/beam_ssa_SUITE.erl | 16 +++-
3 files changed, 149 insertions(+), 38 deletions(-)
diff --git a/lib/compiler/src/beam_ssa_opt.erl b/lib/compiler/src/beam_ssa_opt.erl
index 3ffa8ecc0a..72e320749e 100644
--- a/lib/compiler/src/beam_ssa_opt.erl
+++ b/lib/compiler/src/beam_ssa_opt.erl
@@ -39,7 +39,7 @@
-include("beam_ssa_opt.hrl").
--import(lists, [all/2,append/1,duplicate/2,flatten/1,foldl/3,
+-import(lists, [all/2,append/1,droplast/1,duplicate/2,flatten/1,foldl/3,
keyfind/3,last/1,mapfoldl/3,member/2,
partition/2,reverse/1,reverse/2,
splitwith/2,sort/1,takewhile/2,unzip/1]).
@@ -301,6 +301,7 @@ epilogue_passes(Opts) ->
?PASS(ssa_opt_bsm_shortcut),
?PASS(ssa_opt_sink),
?PASS(ssa_opt_blockify),
+ ?PASS(ssa_opt_redundant_br),
?PASS(ssa_opt_merge_blocks),
?PASS(ssa_opt_get_tuple_element),
?PASS(ssa_opt_tail_calls),
@@ -3101,6 +3102,127 @@ is_tail_call_is([I|Is], Bool, Ret, Acc) ->
is_tail_call_is(Is, Bool, Ret, [I|Acc]);
is_tail_call_is([], _Bool, _Ret, _Acc) -> no.
+%%%
+%%% Eliminate redundant branches.
+%%%
+%%% Redundant `br` instructions following calls to guard BIFs such as:
+%%%
+%%% @bif_result = bif:Bif ...
+%%% br @bif_result, ^100, ^200
+%%%
+%%% 100:
+%%% ret `true`
+%%%
+%%% 200:
+%%% ret `false`
+%%%
+%%% can can be rewritten to:
+%%%
+%%% @bif_result = bif:Bif ...
+%%% ret @bif_result
+%%%
+%%% A similar rewriting is possible if the true and false branches end
+%%% up at a phi node.
+%%%
+%%% A code sequence such as:
+%%%
+%%% @ssa_bool = bif:'=:=' Var, Other
+%%% br @ssa_bool, ^100, ^200
+%%%
+%%% 100:
+%%% ret Other
+%%%
+%%% 200:
+%%% ret Var
+%%%
+%%% can be rewritten to:
+%%%
+%%% ret Var
+%%%
+
+ssa_opt_redundant_br({#opt_st{ssa=Blocks0}=St, FuncDb}) ->
+ Blocks = redundant_br(beam_ssa:rpo(Blocks0), Blocks0),
+ {St#opt_st{ssa=Blocks}, FuncDb}.
+
+redundant_br([L|Ls], Blocks0) ->
+ Blk0 = map_get(L, Blocks0),
+ case Blk0 of
+ #b_blk{is=Is,
+ last=#b_br{bool=#b_var{}=Bool,
+ succ=Succ,
+ fail=Fail}} ->
+ case Blocks0 of
+ #{Succ := #b_blk{is=[],last=#b_ret{arg=#b_literal{val=true}}},
+ Fail := #b_blk{is=[],last=#b_ret{arg=#b_literal{val=false}}}} ->
+ case redundant_br_safe_bool(Is, Bool) of
+ true ->
+ Blk = Blk0#b_blk{last=#b_ret{arg=Bool}},
+ Blocks = Blocks0#{L => Blk},
+ redundant_br(Ls, Blocks);
+ false ->
+ redundant_br(Ls, Blocks0)
+ end;
+ #{Succ := #b_blk{is=[],last=#b_br{succ=PhiL,fail=PhiL}},
+ Fail := #b_blk{is=[],last=#b_br{succ=PhiL,fail=PhiL}}} ->
+ case redundant_br_safe_bool(Is, Bool) of
+ true ->
+ Blocks = redundant_br_phi(L, Blk0, PhiL, Blocks0),
+ redundant_br(Ls, Blocks);
+ false ->
+ redundant_br(Ls, Blocks0)
+ end;
+ #{Succ := #b_blk{is=[],last=#b_ret{arg=Other}},
+ Fail := #b_blk{is=[],last=#b_ret{arg=Var}}} when Is =/= [] ->
+ case last(Is) of
+ #b_set{op={bif,'=:='},args=[Var,Other]} ->
+ Blk = Blk0#b_blk{is=droplast(Is),
+ last=#b_ret{arg=Var}},
+ Blocks = Blocks0#{L => Blk},
+ redundant_br(Ls, Blocks);
+ #b_set{} ->
+ redundant_br(Ls, Blocks0)
+ end;
+ #{} ->
+ redundant_br(Ls, Blocks0)
+ end;
+ _ ->
+ redundant_br(Ls, Blocks0)
+ end;
+redundant_br([], Blocks) -> Blocks.
+
+redundant_br_phi(L, Blk0, PhiL, Blocks) ->
+ #b_blk{is=Is0} = PhiBlk0 = map_get(PhiL, Blocks),
+ case Is0 of
+ [#b_set{op=phi},#b_set{op=phi}|_] ->
+ Blocks;
+ [#b_set{op=phi,args=PhiArgs0}=I0|Is] ->
+ #b_blk{last=#b_br{succ=Succ,fail=Fail}} = Blk0,
+ BoolPhiArgs = [{#b_literal{val=false},Fail},
+ {#b_literal{val=true},Succ}],
+ PhiArgs1 = ordsets:from_list(PhiArgs0),
+ case ordsets:is_subset(BoolPhiArgs, PhiArgs1) of
+ true ->
+ #b_blk{last=#b_br{bool=Bool}} = Blk0,
+ PhiArgs = ordsets:add_element({Bool,L}, PhiArgs1),
+ I = I0#b_set{args=PhiArgs},
+ PhiBlk = PhiBlk0#b_blk{is=[I|Is]},
+ Br = #b_br{bool=#b_literal{val=true},succ=PhiL,fail=PhiL},
+ Blk = Blk0#b_blk{last=Br},
+ Blocks#{L := Blk, PhiL := PhiBlk};
+ false ->
+ Blocks
+ end
+ end.
+
+redundant_br_safe_bool([], _Bool) ->
+ true;
+redundant_br_safe_bool(Is, Bool) ->
+ case last(Is) of
+ #b_set{op={bif,_}} -> true;
+ #b_set{op=has_map_field} -> true;
+ #b_set{dst=Dst} -> Dst =/= Bool
+ end.
+
%%%
%%% Common utilities.
%%%
diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 01e9987196..3653d5e647 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -324,19 +324,16 @@ expr(#c_case{}=Case0, Ctxt, Sub) ->
%% (in addition to any warnings that may have been emitted
%% according to the rules above).
%%
- case opt_bool_case(Case0, Sub) of
- #c_case{anno=Anno,arg=Arg0,clauses=Cs0}=Case1 ->
- Arg1 = body(Arg0, value, Sub),
- LitExpr = cerl:is_literal(Arg1),
- {Arg2,Cs1} = case_opt(Arg1, Cs0, Sub),
- Cs2 = clauses(Arg2, Cs1, Ctxt, Sub, LitExpr, Anno),
- Case = Case1#c_case{arg=Arg2,clauses=Cs2},
- warn_no_clause_match(Case1, Case),
- Expr = eval_case(Case, Sub),
- move_case_into_arg(Expr, Sub);
- Other ->
- expr(Other, Ctxt, Sub)
- end;
+ Case1 = opt_bool_case(Case0, Sub),
+ #c_case{anno=Anno,arg=Arg0,clauses=Cs0} = Case1,
+ Arg1 = body(Arg0, value, Sub),
+ LitExpr = cerl:is_literal(Arg1),
+ {Arg2,Cs1} = case_opt(Arg1, Cs0, Sub),
+ Cs2 = clauses(Arg2, Cs1, Ctxt, Sub, LitExpr, Anno),
+ Case = Case1#c_case{arg=Arg2,clauses=Cs2},
+ warn_no_clause_match(Case1, Case),
+ Expr = eval_case(Case, Sub),
+ move_case_into_arg(Expr, Sub);
expr(#c_apply{anno=Anno,op=Op0,args=As0}=Apply0, _, Sub) ->
Op1 = expr(Op0, value, Sub),
As1 = expr_list(As0, value, Sub),
@@ -1593,34 +1590,12 @@ opt_bool_not(#c_case{arg=Arg,clauses=Cs0}=Case0) ->
Case = Case0#c_case{arg=Expr,clauses=Cs},
opt_bool_not(Case);
_ ->
- opt_bool_case_redundant(Case0)
+ Case0
end.
opt_bool_not_invert(#c_clause{pats=[#c_literal{val=Bool}]}=C) ->
C#c_clause{pats=[#c_literal{val=not Bool}]}.
-%% opt_bool_case_redundant(Core) -> Core'.
-%% If the sole purpose of the case is to verify that the case
-%% expression is indeed boolean, we do not need the case
-%% (since we have already verified that the case expression is
-%% boolean).
-%%
-%% case BoolExpr of
-%% true -> true ==> BoolExpr
-%% false -> false
-%% end.
-%%
-opt_bool_case_redundant(#c_case{arg=Arg,clauses=Cs}=Case) ->
- case all(fun opt_bool_case_redundant_1/1, Cs) of
- true -> Arg;
- false -> Case
- end.
-
-opt_bool_case_redundant_1(#c_clause{pats=[#c_literal{val=B}],
- body=#c_literal{val=B}}) ->
- true;
-opt_bool_case_redundant_1(_) -> false.
-
%% eval_case(Case) -> #c_case{} | #c_let{}.
%% If possible, evaluate a case at compile time. We know that the
%% last clause is guaranteed to match so if there is only one clause
diff --git a/lib/compiler/test/beam_ssa_SUITE.erl b/lib/compiler/test/beam_ssa_SUITE.erl
index 590ee231ff..6bada57a2a 100644
--- a/lib/compiler/test/beam_ssa_SUITE.erl
+++ b/lib/compiler/test/beam_ssa_SUITE.erl
@@ -25,7 +25,8 @@
cover_ssa_dead/1,combine_sw/1,share_opt/1,
beam_ssa_dead_crash/1,stack_init/1,
mapfoldl/0,mapfoldl/1,
- grab_bag/1,coverage/1]).
+ grab_bag/1,redundant_br/1,
+ coverage/1]).
suite() -> [{ct_hooks,[ts_install_cth]}].
@@ -45,6 +46,7 @@ groups() ->
beam_ssa_dead_crash,
stack_init,
grab_bag,
+ redundant_br,
coverage
]}].
@@ -1084,6 +1086,19 @@ grab_bag_23(#{page_title := unset} = State1) ->
end},
State2}.
+redundant_br(_Config) ->
+ {false,{x,y,z}} = redundant_br_1(id({x,y,z})),
+ {true,[[a,b,c]]} = redundant_br_1(id([[[a,b,c]]])),
+ ok.
+
+redundant_br_1(Specs0) ->
+ {Join,Specs} =
+ if
+ is_list(hd(hd(Specs0))) -> {true,hd(Specs0)};
+ true -> {false,Specs0}
+ end,
+ id({Join,Specs}).
+
coverage(_Config) ->
%% Cover beam_ssa_codegen:force_reg/2
--
2.31.1
