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Diffstat (limited to 'lib/compiler/src/beam_ssa_share.erl')
| -rw-r--r-- | lib/compiler/src/beam_ssa_share.erl | 374 |
1 files changed, 374 insertions, 0 deletions
diff --git a/lib/compiler/src/beam_ssa_share.erl b/lib/compiler/src/beam_ssa_share.erl new file mode 100644 index 0000000000..73983bd34a --- /dev/null +++ b/lib/compiler/src/beam_ssa_share.erl @@ -0,0 +1,374 @@ +%% +%% %CopyrightBegin% +%% +%% Copyright Ericsson AB 2018. All Rights Reserved. +%% +%% Licensed under the Apache License, Version 2.0 (the "License"); +%% you may not use this file except in compliance with the License. +%% You may obtain a copy of the License at +%% +%% http://www.apache.org/licenses/LICENSE-2.0 +%% +%% Unless required by applicable law or agreed to in writing, software +%% distributed under the License is distributed on an "AS IS" BASIS, +%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +%% See the License for the specific language governing permissions and +%% limitations under the License. +%% +%% %CopyrightEnd% +%% + +%% +%% Share code for semantically equivalent blocks referred to +%% to by `br` and `switch` instructions. +%% +%% A similar optimization is done in beam_jump, but doing it here as +%% well is beneficial as it may enable other optimizations. If there +%% are many semantically equivalent clauses, this optimization can +%% substanstially decrease compilation times. +%% +%% block/2 is called from the liveness optimization pass in +%% beam_ssa_opt, as code sharing helps the liveness pass and vice +%% versa. +%% + +-module(beam_ssa_share). +-export([module/2,block/2]). + +-include("beam_ssa.hrl"). + +-import(lists, [keyfind/3,reverse/1,sort/1]). + +-spec module(beam_ssa:b_module(), [compile:option()]) -> + {'ok',beam_ssa:b_module()}. + +module(#b_module{body=Fs0}=Module, _Opts) -> + Fs = [function(F) || F <- Fs0], + {ok,Module#b_module{body=Fs}}. + +-spec block(Blk0, Blocks0) -> Blk when + Blk0 :: beam_ssa:b_blk(), + Blocks0 :: beam_ssa:block_map(), + Blk :: beam_ssa:b_blk(). + +block(#b_blk{last=Last0}=Blk, Blocks) -> + case share_terminator(Last0, Blocks) of + none -> Blk; + Last -> Blk#b_blk{last=beam_ssa:normalize(Last)} + end. + +%%% +%%% Local functions. +%%% + +function(#b_function{anno=Anno,bs=Blocks0}=F) -> + try + PO = reverse(beam_ssa:rpo(Blocks0)), + {Blocks1,Changed} = blocks(PO, Blocks0, false), + Blocks = case Changed of + true -> + beam_ssa:trim_unreachable(Blocks1); + false -> + Blocks0 + end, + F#b_function{bs=Blocks} + catch + Class:Error:Stack -> + #{func_info:={_,Name,Arity}} = Anno, + io:fwrite("Function: ~w/~w\n", [Name,Arity]), + erlang:raise(Class, Error, Stack) + end. + +blocks([L|Ls], Blocks, Changed) -> + #b_blk{last=Last0} = Blk0 = map_get(L, Blocks), + case block(Blk0, Blocks) of + #b_blk{last=Last0} -> + blocks(Ls, Blocks, Changed); + #b_blk{}=Blk -> + blocks(Ls, Blocks#{L:=Blk}, true) + end; +blocks([], Blocks, Changed) -> + {Blocks,Changed}. + +share_terminator(#b_br{bool=#b_var{},succ=Succ0,fail=Fail0}=Br, Blocks) -> + {Succ,SuccBlk} = shortcut_nonempty_block(Succ0, Blocks), + {Fail,FailBlk} = shortcut_nonempty_block(Fail0, Blocks), + case are_equivalent(Succ, SuccBlk, Fail, FailBlk, Blocks) of + true -> + %% The blocks are semantically equivalent. + Br#b_br{succ=Succ,fail=Succ}; + false -> + if + Succ =:= Succ0, Fail =:= Fail0 -> + %% None of blocks were cut short. + none; + true -> + %% One or both labels were cut short + %% to avoid jumping to an empty block. + Br#b_br{succ=Succ,fail=Fail} + end + end; +share_terminator(#b_switch{}=Sw, Blocks) -> + share_switch(Sw, Blocks); +share_terminator(_Last, _Blocks) -> none. + +%% Test whether the two blocks are semantically equivalent. This +%% function is specially optimized to return `false` as fast as +%% possible if the blocks are not equivalent, as that is the common +%% case. + +are_equivalent(_Succ, _, ?BADARG_BLOCK, _, _Blocks) -> + %% ?BADARG_BLOCK is special. Sharing could be incorrect. + false; +are_equivalent(_Succ, #b_blk{is=Is1,last=#b_ret{arg=RetVal1}=Ret1}, + _Fail, #b_blk{is=Is2,last=#b_ret{arg=RetVal2}=Ret2}, _Blocks) -> + case {RetVal1,RetVal2} of + {#b_literal{},#b_literal{}} -> + case RetVal1 =:= RetVal2 of + true -> + %% The return values are identical literals. We + %% only need to compare the canonicalized bodies. + Can1 = canonical_is(Is1), + Can2 = canonical_is(Is2), + Can1 =:= Can2; + false -> + %% Non-equal literals. + false + end; + {#b_var{},#b_var{}} -> + %% The return values are varibles. We must canonicalize + %% the blocks (including returns) and compare them. + Can1 = canonical_is(Is1 ++ [Ret1]), + Can2 = canonical_is(Is2 ++ [Ret2]), + Can1 =:= Can2; + {_,_} -> + %% One literal and one variable. + false + end; +are_equivalent(Succ, + #b_blk{is=Is1, + last=#b_br{bool=#b_literal{val=true}, + succ=Target}}, + Fail, + #b_blk{is=Is2, + last=#b_br{bool=#b_literal{val=true}, + succ=Target}}, + Blocks) -> + %% Both blocks end with an unconditional branch to the + %% same target block. If the target block has phi nodes, + %% we must pick up the values from the phi nodes and + %% compare them. + #b_blk{is=Is} = map_get(Target, Blocks), + Phis1 = canonical_terminator_phis(Is, Succ), + Phis2 = canonical_terminator_phis(Is, Fail), + case {Phis1,Phis2} of + {[#b_set{args=[#b_literal{}]}|_],_} when Phis1 =/= Phis2 -> + %% Different values are used in the phi nodes. + false; + {_,[#b_set{args=[#b_literal{}]}|_]} when Phis1 =/= Phis2 -> + %% Different values are used in the phi nodes. + false; + {_,_} -> + %% The values in the phi nodes are variables or identical + %% literals. We must canonicalize the blocks and compare + %% them. + Can1 = canonical_is(Is1 ++ Phis1), + Can2 = canonical_is(Is2 ++ Phis2), + Can1 =:= Can2 + end; +are_equivalent(Succ0, #b_blk{is=Is1,last=#b_br{bool=#b_var{},fail=Same}}, + Fail0, #b_blk{is=Is2,last=#b_br{bool=#b_var{},fail=Same}}, + Blocks) -> + %% Two-way branches with identical failure labels. First compare the + %% canonicalized bodies of the blocks. + case canonical_is(Is1) =:= canonical_is(Is2) of + false -> + %% Different bodies. + false; + true -> + %% Bodies were equal. That is fairly uncommon, so to keep + %% the code simple we will rewrite the `br` to a `switch` + %% and let share_switch/2 do the work of following the + %% branches. + Sw = #b_switch{arg=#b_var{name=not_used},fail=Fail0, + list=[{#b_literal{},Succ0}]}, + #b_switch{fail=Fail,list=[{_,Succ}]} = share_switch(Sw, Blocks), + Fail =:= Succ + end; +are_equivalent(_, _, _, _, _) -> false. + +share_switch(#b_switch{fail=Fail0,list=List0}=Sw, Blocks) -> + Prep = share_prepare_sw([{value,Fail0}|List0], Blocks, 0, []), + Res = do_share_switch(Prep, Blocks, []), + [{_,Fail}|List] = [VL || {_,VL} <- sort(Res)], + Sw#b_switch{fail=Fail,list=List}. + +share_prepare_sw([{V,L0}|T], Blocks, N, Acc) -> + {L,_Blk} = shortcut_nonempty_block(L0, Blocks), + share_prepare_sw(T, Blocks, N+1, [{{L,#{}},{N,{V,L}}}|Acc]); +share_prepare_sw([], _, _, Acc) -> Acc. + +do_share_switch(Prep, Blocks, Acc) -> + Map = share_switch_1(Prep, Blocks, #{}), + share_switch_2(maps:values(Map), Blocks, Acc). + +share_switch_1([{Next0,Res}|T], Blocks, Map) -> + {Can,Next} = canonical_block(Next0, Blocks), + case Map of + #{Can:=Ls} -> + share_switch_1(T, Blocks, Map#{Can:=[{Next,Res}|Ls]}); + #{} -> + share_switch_1(T, Blocks, Map#{Can=>[{Next,Res}]}) + end; +share_switch_1([], _Blocks, Map) -> Map. + +share_switch_2([[{_,{N,Res}}]|T], Blocks, Acc) -> + %% This block is not equivalent to any other block. + share_switch_2(T, Blocks, [{N,Res}|Acc]); +share_switch_2([[{done,{_,{_,Common}}}|_]=Eqs|T], Blocks, Acc0) -> + %% Two or more blocks are semantically equivalent, and all blocks + %% are either terminated with a `ret` or a `br` to the same target + %% block. Replace the labels in the `switch` for all of those + %% blocks with the label for the first of the blocks. + Acc = [{N,{V,Common}} || {done,{N,{V,_}}} <- Eqs] ++ Acc0, + share_switch_2(T, Blocks, Acc); +share_switch_2([[{_,_}|_]=Prep|T], Blocks, Acc0) -> + %% Two or more blocks are semantically equivalent, but they have + %% different successful successor blocks. Now we must check + %% recursively whether the successor blocks are equivalent too. + Acc = do_share_switch(Prep, Blocks, Acc0), + share_switch_2(T, Blocks, Acc); +share_switch_2([], _, Acc) -> Acc. + +canonical_block({L,VarMap0}, Blocks) -> + #b_blk{is=Is,last=Last0} = map_get(L, Blocks), + case canonical_terminator(L, Last0, Blocks) of + none -> + %% The block has a terminator that we don't handle. + {{none,L},done}; + {Last,done} -> + %% The block ends with a `ret` or an unconditional `br` to + %% another block. + {Can,_VarMap} = canonical_is(Is ++ Last, VarMap0, []), + {Can,done}; + {Last,Next} -> + %% The block ends with a conditional branch. + {Can,VarMap} = canonical_is(Is ++ Last, VarMap0, []), + {Can,{Next,VarMap}} + end. + +%% Translate a sequence of instructions to a canonical representation. If the +%% canonical representation of two blocks compare equal, the blocks are +%% semantically equivalent. The following translations are done: +%% +%% * Variables defined in the instruction sequence are replaced with +%% {var,0}, {var,1}, and so on. Free variables are not changed. +%% +%% * `location` annotations that would produce a `line` instruction are +%% kept. All other annotations are cleared. +%% +%% * Instructions are repackaged into tuples instead of into the +%% usual records. The main reason is to avoid violating the types for +%% the SSA records. We can simplify things a little by linking the +%% instructions directly instead of putting them into a list. + +canonical_is(Is) -> + {Can,_} = canonical_is(Is, #{}, []), + Can. + +canonical_is([#b_set{op=Op,dst=Dst,args=Args0}=I|Is], VarMap0, Acc) -> + Args = [canonical_arg(Arg, VarMap0) || Arg <-Args0], + Var = {var,map_size(VarMap0)}, + VarMap = VarMap0#{Dst=>Var}, + LineAnno = case Op of + bs_match -> + %% The location annotation for a bs_match instruction + %% is only used in warnings, never to emit a `line` + %% instruction. Therefore, it should not be included. + []; + _ -> + %% The location annotation will be used in a `line` + %% instruction. It must be included. + beam_ssa:get_anno(location, I, none) + end, + canonical_is(Is, VarMap, {Op,LineAnno,Var,Args,Acc}); +canonical_is([#b_ret{arg=Arg}], VarMap, Acc0) -> + Acc1 = case Acc0 of + {call,_Anno,Var,[#b_local{}|_]=Args,PrevAcc} -> + %% This is a tail-recursive call to a local function. + %% There will be no line instruction generated; + %% thus, the annotation is not significant. + {call,[],Var,Args,PrevAcc}; + _ -> + Acc0 + end, + {{ret,canonical_arg(Arg, VarMap),Acc1},VarMap}; +canonical_is([#b_br{bool=#b_var{}=Arg,fail=Fail}], VarMap, Acc) -> + %% A previous buggy version of this code omitted the canonicalized + %% argument in the return value. Unfortunately, that worked most + %% of the time, except when `br` terminator referenced a variable + %% defined in a previous block instead of in the same block. + {{br,canonical_arg(Arg, VarMap),succ,Fail,Acc},VarMap}; +canonical_is([#b_br{succ=Succ}], VarMap, Acc) -> + {{br,Succ,Acc},VarMap}; +canonical_is([], VarMap, Acc) -> + {Acc,VarMap}. + +canonical_terminator(_L, #b_ret{}=Ret, _Blocks) -> + {[Ret],done}; +canonical_terminator(L, #b_br{bool=#b_literal{val=true},succ=Succ}=Br, Blocks) -> + #b_blk{is=Is} = map_get(Succ, Blocks), + case canonical_terminator_phis(Is, L) of + [] -> + {[],Succ}; + [_|_]=Phis -> + {Phis ++ [Br],done} + end; +canonical_terminator(_L, #b_br{bool=#b_var{},succ=Succ}=Br, _Blocks) -> + {[Br],Succ}; +canonical_terminator(_, _, _) -> none. + +canonical_terminator_phis([#b_set{op=phi,args=PhiArgs}=Phi|Is], L) -> + {Value,L} = keyfind(L, 2, PhiArgs), + [Phi#b_set{op=copy,args=[Value]}|canonical_terminator_phis(Is, L)]; +canonical_terminator_phis([#b_set{op=peek_message}=I|_], L) -> + %% We could get stuck into an infinite loop if we allowed the + %% comparisons to continue into this block. Force an unequal + %% compare with all other predecessors of this block. + [I#b_set{op=copy,args=[#b_literal{val=L}]}]; +canonical_terminator_phis(_, _) -> []. + +canonical_arg(#b_var{}=Var, VarMap) -> + case VarMap of + #{Var:=CanonicalVar} -> + CanonicalVar; + #{} -> + Var + end; +canonical_arg(#b_remote{mod=Mod,name=Name}, VarMap) -> + {remote,canonical_arg(Mod, VarMap), + canonical_arg(Name, VarMap)}; +canonical_arg(Other, _VarMap) -> Other. + +%% Shortcut branches to empty blocks if safe. + +shortcut_nonempty_block(L, Blocks) -> + case map_get(L, Blocks) of + #b_blk{is=[],last=#b_br{bool=#b_literal{val=true},succ=Succ}}=Blk -> + %% This block is empty. + case is_forbidden(Succ, Blocks) of + false -> + shortcut_nonempty_block(Succ, Blocks); + true -> + {L,Blk} + end; + #b_blk{}=Blk -> + {L,Blk} + end. + +is_forbidden(L, Blocks) -> + case map_get(L, Blocks) of + #b_blk{is=[#b_set{op=phi}|_]} -> true; + #b_blk{is=[#b_set{op=peek_message}|_]} -> true; + #b_blk{} -> false + end. |