Erlang match_spec引擎介紹和應用
阿新 • • 發佈：2018-12-31
match_spec是什麼呢？
A “match specification” (match_spec) is an Erlang term describing a small “program” that will try to match something (either the parameters to a function as used in the erlang:trace_pattern/2 BIF, or the objects in an ETS table.). The match_spec in many ways works like a small function in Erlang, but is interpreted/compiled by the Erlang runtime system to something much more efficient than calling an Erlang function. The match_spec is also very limited compared to the expressiveness of real Erlang functions.// erl_db_util.c:L5038
#ifdef DMC_DEBUG
/*
** Disassemble match program
*/
void db_match_dis(Binary *bp)
{
...
}
#endif /* DMC_DEBUG */
$ cd otp $ export ERL_TOP=`pwd` $ cd erts $ make debug FLAVOR=smp DMC_DEBUG=1 $ sudo make install有了支援反彙編的執行期，我們來試驗看下matchspec的opcode：
$ erl
Erlang R14B04 (erts-5.8.5) 1[/source] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.8.5 (abort with ^G)
1> Spec=ets:fun2ms(fun({A, tag}) when A>1 ->A end).
[{{'$1',tag},[{'>','$1',1}],['$1']}]
%%指示開啟反彙編標誌
2> erlang:match_spec_test({1234,tag}, Spec, dis).
true
3> erlang:match_spec_test({1234,tag}, Spec, table).
Tuple 2
Bind 1
Eq tag
PushC 1
PushV 1
Call2 '>'
True
Catch
PushVResult 1
Return
Halt
term_save: {}
num_bindings: 2
heap_size: 24
stack_offset: 12
text: 0x006e3ecc
stack_size: 12 (words)
{ok,1234,[],[]}
4>
-compile({parse_transform,ms_transform}).
%%ms_transform.erl
copy({call,Line,{remote,_Line2,{atom,_Line3,ets},{atom,_Line4,fun2ms}},
As0},Bound) ->
{transform_call(ets,Line,As0,Bound),Bound};
copy({call,Line,{remote,_Line2,{record_field,_Line3,
{atom,_Line4,''},{atom,_Line5,ets}},
{atom,_Line6,fun2ms}}, As0},Bound) ->
%% Packages...
{transform_call(ets,Line,As0,Bound),Bound};
copy({call,Line,{remote,_Line2,{atom,_Line3,dbg},{atom,_Line4,fun2ms}},
As0},Bound) ->
{transform_call(dbg,Line,As0,Bound),Bound};
從程式碼可以看出，ms_transform只對dbg:fun2ms或者ets:fun2ms做matchspec的編譯期計算,
我們來驗證下我們的理解：
$ cat veri.erl
-module(veri).
-export([start/0]).
-include_lib("stdlib/include/ms_transform.hrl").
start()->
ets:fun2ms(fun({M,N}) when N > 3 -> M end).
$ erlc +"'S'" veri.erl
$ cat veri.S
{module, veri}. %% version = 0
{exports, [{module_info,0},{module_info,1},{start,0}]}.
{attributes, []}.
{labels, 7}.
{function, start, 0, 2}.
{label,1}.
{func_info,{atom,veri},{atom,start},0}.
{label,2}.
{move,{literal,[{{'$1','$2'},[{'>','$2',3}],['$1']}]},{x,0}}.
return.
{function, module_info, 0, 4}.
{label,3}.
{func_info,{atom,veri},{atom,module_info},0}.
{label,4}.
{move,{atom,veri},{x,0}}.
{call_ext_only,1,{extfunc,erlang,get_module_info,1}}.
{function, module_info, 1, 6}.
{label,5}.
{func_info,{atom,veri},{atom,module_info},1}.
{label,6}.
{move,{x,0},{x,1}}.
{move,{atom,veri},{x,0}}.
{call_ext_only,2,{extfunc,erlang,get_module_info,2}}.
$ erl
Erlang R14B04 (erts-5.8.5) 1 [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.8.5 (abort with ^G)
1> veri:start().
[{{'$1','$2'},[{'>','$2',3}],['$1']}]
從彙編碼，我們可以看出 ets:fun2ms(fun({M,N}) when N > 3 -> M end). 的結果是編譯期間已經出來了，也就是說它在執行期不會佔用執行時間的，用的時候不用擔心效能問題的。
那麼如何使用呢？ETS給我們很好的例子：
match_spec_compile(MatchSpec) -> CompiledMatchSpec
Types:
MatchSpec = match_spec()
CompiledMatchSpec = comp_match_spec()
This function transforms a match_spec into an internal representation that can be used in subsequent calls to ets:match_spec_run/2. The internal representation is opaque and can not be converted to external term format and then back again without losing its properties (meaning it can not be sent to a process on another node and still remain a valid compiled match_spec, nor can it be stored on disk). The validity of a compiled match_spec can be checked using ets:is_compiled_ms/1.
If the term MatchSpec can not be compiled (does not represent a valid match_spec), a badarg fault is thrown.
Note
This function has limited use in normal code, it is used by Dets to perform the dets:select operations.
match_spec_run(List,CompiledMatchSpec) -> list()
Types:
List = [ tuple() ]
CompiledMatchSpec = comp_match_spec()
This function executes the matching specified in a compiled match_spec on a list of tuples. The CompiledMatchSpec term should be the result of a call to ets:match_spec_compile/1 and is hence the internal representation of the match_spec one wants to use.
The matching will be executed on each element in List and the function returns a list containing all results. If an element in List does not match, nothing is returned for that element. The length of the result list is therefore equal or less than the the length of the parameter List. The two calls in the following example will give the same result (but certainly not the same execution time…):
Table = ets:new…
MatchSpec = ….
% The following call…
ets:match_spec_run(ets:tab2list(Table),
ets:match_spec_compile(MatchSpec)),
% …will give the same result as the more common (and more efficient)
ets:select(Table,MatchSpec),
Note
This function has limited use in normal code, it is used by Dets to perform the dets:select operations and by Mnesia during transactions.
總結：match_spec可以用在我們自己實現的driver模組裡面用來做過濾和匹配。
祝玩得開心！