From e7b49a3e657e1bd7bbf92050e6360b1e0f142a1e Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= Date: Mon, 22 Jan 2018 08:04:44 +0100 Subject: Apply common subexpression elimination in blocks Eliminate repeated evaluation of guard BIFs and building of cons cells in blocks. This optimization is applicable in more places than might be expected, because code generation for binaries and record can generate common sub expressions not visible in the original source code. For example, consider this function: make_binary(Term) -> Bin = term_to_binary(Term), Size = byte_size(Bin), <>. The compiler inserts a call to byte_size/2 to calculate the size of the binary being built: {function, make_binary, 1, 2}. {label,1}. {line,...}. {func_info,{atom,t},{atom,make_binary},1}. {label,2}. {allocate,0,1}. {line,...}. {call_ext,1,{extfunc,erlang,term_to_binary,1}}. {line,...}. {gc_bif,byte_size,{f,0},1,[{x,0}],{x,1}}. %Present in original code. {line,...}. {gc_bif,byte_size,{f,0},2,[{x,0}],{x,2}}. %Inserted by compiler. {bs_add,{f,0},[{x,2},{integer,4},1],{x,2}}. {bs_init2,{f,0},{x,2},0,2,{field_flags,[]},{x,2}}. {bs_put_integer,{f,0},{integer,32},1,{field_flags,[unsigned,big]},{x,1}}. {bs_put_binary,{f,0},{atom,all},8,{field_flags,[unsigned,big]},{x,0}}. {move,{x,2},{x,0}}. {deallocate,0}. return. Common sub expression elimination (CSE) eliminates the second call to byte_size/2: {function, make_binary, 1, 2}. {label,1}. {line,...}. {func_info,{atom,t},{atom,make_binary},1}. {label,2}. {allocate,0,1}. {line,...}. {call_ext,1,{extfunc,erlang,term_to_binary,1}}. {line,...}. {gc_bif,byte_size,{f,0},1,[{x,0}],{x,1}}. {move,{x,1},{x,2}}. {bs_add,{f,0},[{x,2},{integer,4},1],{x,2}}. {bs_init2,{f,0},{x,2},0,2,{field_flags,[]},{x,2}}. {bs_put_integer,{f,0},{integer,32},1,{field_flags,[unsigned,big]},{x,1}}. {bs_put_binary,{f,0},{atom,all},8,{field_flags,[unsigned,big]},{x,0}}. {move,{x,2},{x,0}}. {deallocate,0}. return. Note: A possible future optimization would be to include binary construction instructions in blocks. If that is done, the {move,{x,1},{x,2}} instruction could also be eliminated. --- lib/compiler/test/beam_block_SUITE.erl | 62 ++++++++++++++++++++++++++++++++-- 1 file changed, 60 insertions(+), 2 deletions(-) (limited to 'lib/compiler/test') diff --git a/lib/compiler/test/beam_block_SUITE.erl b/lib/compiler/test/beam_block_SUITE.erl index 55d5f2dbe8..fac18789e0 100644 --- a/lib/compiler/test/beam_block_SUITE.erl +++ b/lib/compiler/test/beam_block_SUITE.erl @@ -22,7 +22,7 @@ -export([all/0,suite/0,groups/0,init_per_suite/1,end_per_suite/1, init_per_group/2,end_per_group/2, get_map_elements/1,otp_7345/1,move_opt_across_gc_bif/1, - erl_202/1,repro/1]). + erl_202/1,repro/1,local_cse/1]). %% The only test for the following functions is that %% the code compiles and is accepted by beam_validator. @@ -40,7 +40,8 @@ groups() -> otp_7345, move_opt_across_gc_bif, erl_202, - repro + repro, + local_cse ]}]. init_per_suite(Config) -> @@ -237,6 +238,63 @@ find_operands(Cfg,XsiGraph,ActiveList,Count) -> [Count+1, length(NewActiveList), length(digraph:vertices(XsiGraph))], find_operands(NewCfg,XsiGraph,NewActiveList,Count+1). +%% Some tests of local common subexpression elimination (CSE). + +local_cse(_Config) -> + {Self,{ok,Self}} = local_cse_1(), + + local_cse_2([]), + local_cse_2(lists:seq(1, 512)), + local_cse_2(?MODULE:module_info()), + + {[b],[a,b]} = local_cse_3(a, b), + + {2000,Self,{Self,write_cache}} = local_cse_4(), + + ok. + +local_cse_1() -> + %% Cover handling of unsafe tuple construction in + %% eliminate_use_of_from_reg/4. It became necessary to handle + %% unsafe tuples when local CSE was introduced. + + {self(),{ok,self()}}. + +local_cse_2(Term) -> + case cse_make_binary(Term) of + <> -> + Term = binary_to_term(BinTerm); + <> -> + {'$size',TermSize} = binary_to_term(SizeTerm), + TermSize = byte_size(BinTerm), + Term = binary_to_term(BinTerm) + end. + +%% Copy of observer_backend:ttb_make_binary/1. During development of +%% the local CSE optimization this function was incorrectly optimized. + +cse_make_binary(Term) -> + B = term_to_binary(Term), + SizeB = byte_size(B), + if SizeB > 255 -> + SB = term_to_binary({'$size',SizeB}), + <<(byte_size(SB)):8, SB/binary, B/binary>>; + true -> + <> + end. + +local_cse_3(X, Y) -> + %% The following expression was incorrectly transformed to {[X,Y],[X,Y]} + %% during development of the local CSE optimization. + + {[Y],[X,Y]}. + +local_cse_4() -> + do_local_cse_4(2000, self(), {self(), write_cache}). + +do_local_cse_4(X, Y, Z) -> + {X,Y,Z}. + %%% %%% Common functions. %%% -- cgit v1.2.3