This book will not cover the programming language Erlang, but since the goal of the ERTS is to run Erlang code you will need to know how to compile Erlang code. In this chapter we will cover the compiler options needed to generate readable beam code and how to add debug information to the generated beam file. At the end of the chapter there is also a section on the Elixir compiler.
For those readers interested in compiling their own favorite language to ERTS this chapter will also contain detailed information about the different intermediate formats in the compiler and how to plug your compiler into the beam compiler backend. I will also present parse transforms and give examples of how to use them to tweak the Erlang language.
Erlang is compiled from source code modules in .erl files to fat binary .beam files.
The compiler can be run from the OS shell with the erlc command:
> erlc foo.erlAlternatively the compiler can be invoked from the Erlang shell with the default shell command c or by calling compile:file/{1,2}
1> c(foo).or
1> compile:file(foo).The optional second argument to compile:file is a list of compiler options. A full list of the options can be found in the documentation of the compile module: see http://www.erlang.org/doc/man/compile.html.
Normally the compiler will compile Erlang source code from a .erl file and write the resulting binary beam code to a .beam file. You can also get the resulting binary back as an Erlang term by giving the option binary to the compiler. This option has then been overloaded to mean return any intermediate format as a term instead of writing to a file. If you for example want the compiler to return Core Erlang code you can give the options [core, binary].
The compiler is made up of a number of passes as illustrated in Compiler Passes.
[] = Compiler options, () = files, {} = erlang terms, boxes = passes </title>
(.erl)
|
v
+---------------+
| Scanner |
| (Part of epp) |
+---------------+
|
v
+---------------+
| Pre-processor |
| epp |
+---------------+
|
v
+---------------+ +---------------+
| Parse | -> | user defined |
| Transform | <- | transformation|
+---------------+ +---------------+
|
+---------> (.Pbeam) [makedep]
+---------> {dep} [makedep, binary]
|
+---------> (.pp) [dpp]
+---------> {AST} [dpp, binary]
|
v
+---------------+
| Linter |
| |
+---------------+
|
+---------> (.P) ['P']
+---------> {AST} ['P',binary]
|
v
+---------------+
| Save AST |
| |
+---------------+
|
v
+---------------+
| Expand |
| |
+---------------+
|
+---------> (.E) ['E']
+---------> {.E} ['E', binary]
|
v
+---------------+
| Core |
| Erlang |
+---------------+
|
+---------> (.core) [dcore|to_core0]
+---------> {core} [to_core0,binary]
|
v
+---------------+
| Core |+
| Passes ||+
+---------------+||
+---------------+|
+---------------+
|
+---------> (.core) [to_core]
+---------> {core} [to_core,binary]
|
v
+---------------+
| Kernel |
| Erlang |
+---------------+
|
v
+---------------+
| Kernel |+
| Passes ||+
+---------------+||
+---------------+|
+---------------+
|
v
+---------------+
| BEAM Code |
| |
+---------------+
|
v
+---------------+
| ASM |+
| Passes ||+
+---------------+||
+---------------+|
+---------------+
|
+---------> (.S) ['S']
+---------> {.S} ['S', binary]
|
v
+---------------+
| Native Code |
| |
+---------------+
|
v
(.beam)</pre>
If you want to see a complete and up to date list of compiler passes you can run the function compile:options/0 in an Erlang shell. The definitive source for information about the compiler is of course the source: compile.erl
Looking at the code produced by the compiler is a great help in trying to understand how the virtual machine works. Fortunately, the compiler can show os the intermediate code after each compiler pass and the final beam code.
Let us try out our newfound knowledge to look at the generated code.
1> compile:options().
dpp - Generate .pp file
'P' - Generate .P source listing file...
'E' - Generate .E source listing file
...
'S' - Generate .S file
Let us try with a small example program "world.erl":
link:code/compiler_chapter/src/world.erl[role=include]And the include file "world.hrl"
link:code/compiler_chapter/src/world.hrl[role=include]If you now compile this with the 'P' option to get the parsed file you get a file "world.P":
2> c(world, ['P']).
** Warning: No object file created - nothing loaded **
okIn the resulting .P file you can see the a pretty printet version of the code after the preprocessor (and parse transformation) has been applied:
link:code/compiler_chapter/src/world.P[role=include]To see how the code looks after all source code transformations are done, you can compile the code with the 'E'-flag.
3> c(world, ['E']).
** Warning: No object file created - nothing loaded **
okThis gives us an .E file, in this case all compiler directives have been removed and the build in functions module_info/{1,2} have been added to the source:
link:code/compiler_chapter/src/world.E[role=include]We will make use of the 'P' and 'E' options when we look at parse transforms in Compiler Pass: Parse Transformations, but first we will take a look at an "assembler" view of generated BEAM code. Bu giving the option 'S' to the compiler you get a .S file with Erlang terms for each BEAM instruction in the code.
3> c(world, ['S']).
** Warning: No object file created - nothing loaded **
okThe file world.S should look like this:
link:code/compiler_chapter/src/world.S[role=include]Since this is a file with dot (".") separated Erlang terms, you can read the file back into the Erlang shell with:
{ok, BEAM_Code} = file:consult("world.S").
The assembler code mostly follows the layout of the original source code.
The first instruction defines the module name of the code. The version mentioned in the comment (%% version = 0) is the version of the beam opcode format (as given by beam_opcodes:format_number/0).
Then comes a list of exports and any compiler attributes (none in this example) much like in any Erlang source module.
The first real beam-like instruction is {labels, 7} which tells the VM the number of lables in the code to make it possible to allocate room for all lables in one pass over the code.
After that there is the actual code for each function. The first instruction gives us the function name, the arity and the entry point as a label number.
You can use the 'S' option with great effect to help you understand how the BEAM works, and we will use it like that in later chapters. It is also invaluable if you develop your own language that you compile to the BEAM through Core Erlang, to see the generated code.
In the following sections we will go through most of the compiler passes shown in Compiler Passes.. For a language designer targeting the BEAM this is interesting since it will show you what you can accomplish with the different approaches: macros, parse transforms, core erlang, and BEAM code, and how they depend on each other.
When tuning Erlang code, it is good to know what optimizations are applied when, and how you can look at generated code before and after optimizations.
The compilation starts with a combined tokenizer (or scanner) and preprocessor. That is, the preprosessor drives the tokenizer. This means that macros are expanded as tokens, so it is not a pure string replacement (as for example m4 or cpp). You can not use Erlang macros to define your own syntax, a macro will expand as a separate token from its surrounding characters. You can not concatenate a macro and a character to a token:
-define(plus,+).
t(A,B) -> A?plus+B.This will expand to
t(A,B) -> A + + B.
and not
t(A,B) -> A ++ B.
On the other hand since macro expansion is done on the token level, you do not need to have a valid Erlang term in the right hand side of the macro, as long as you use it in a way that gives you a valid term. E.g.:
-define(p,o, o]). t() -> [f,?p.
There are few real useages for this other than to win the obfuscated Erlang code contest. The main point to remember is that you can not really use the Erlang preprocessor to define a language with a syntax that differs from Erlang. Fortunately there are other ways to do this, as you shall see later.
The easiest way to tweak the Erlang language is through Parse Transformations (or parse transforms). Parse Transformations comes with all sorts of warnings, like this note in the OTP documentation:
|
Warning
|
Programmers are strongly advised not to engage in parse transformations and no support is offered for problems encountered. |
When you use a parse transform you are basically writing an extra pass in the compiler and that can if you are not careful lead to very unexpected results. But to use a parse transform you have to declare the usage in the module using it, and it will be local to that module, so as far as compiler tweaks goes this one is quite safe.
The biggest problem with parse transforms as I see it is that you are inventing your own syntax, and it will make it more difficult for anyone else reading your code. At least until your parse transform has become as popular and widely used as e.g. QLC.
OK, so you know you shouldn’t use it, but if you have to, here is what you need to know. A parse transforms is a function that works on the abstract syntax tree (AST) (see http://www.erlang.org/doc/apps/erts/absform.html ). The compiler does preprocessing, tokenization and parsing and then it will call the parse transform function with the AST and expects to get back a new AST.
This means that you can’t change the Erlang syntax fundamentally, but you can change the semantics. Lets say for example that you for some reason would like to write json code directly in your Erlang code, then you are in luck since the tokens of json and of Erlang are basically the same. Also, since the Erlang compiler does most of its sanity checks in the linter pass which follows the parse transform pass, you can allow an AST which does not represent valid Erlang.
To write a parse transform you need to write an Erlang module (lets call it p) which exports the function parse_transform/2. This function is called by the compiler during the parse transform pass if the module being compiled (lets call it m) contains the compiler option {parse_transform, p}. The arguments to the funciton is the AST of the module m and the compiler options given to the call to the compiler.
|
Note
|
Note that you will not get any compiler options given in the file, this is a bit of a nuisance since you can’t give options to the parse transform from the code. The compiler does not expand compiler options until the expand pass which occures after the parse transform pass. |
The documenation of the abstract format is somewhat dense and it is quite hard to get a grip on the abstract format by reading the documentation. I encourage you to use the syntax_tools and especially erl_syntax_lib for any serious work on the AST.
Here we will develop a a simple parse transform just to get an understanding of the AST. Therefore we will work directly on the AST and use the old reliable io:format approach instead of syntax_tools.
First we create an example of what we would like to be able to compile json_test.erl:
-module(json_test).
-compile({parse_transform, json_parser}).
-export([test/1]).
test(V) ->
<<{{
"name" : "Jack (\"Bee\") Nimble",
"format": {
"type" : "rect",
"widths" : [1920,1600],
"height" : (-1080),
"interlace" : false,
"frame rate": V
}
}}>>.Then we create a minimal parse transform module json_parser.erl:
-module(json_parser).
-export([parse_transform/2]).
parse_transform(AST, _Options) ->
io:format("~p~n", [AST]),
AST.This identity parse transform returns an unchanged AST but it also prints it out so that you can see what an AST looks like.
> c(json_parser).
{ok,json_parser}
2> c(json_test).
[{attribute,1,file,{"./json_test.erl",1}},
{attribute,1,module,json_test},
{attribute,3,export,[{test,1}]},
{function,5,test,1,
[{clause,5,
[{var,5,'V'}],
[],
[{bin,6,
[{bin_element,6,
{tuple,6,
[{tuple,6,
[{remote,7,{string,7,"name"},{string,7,"Jack (\"Bee\") Nimble"}},
{remote,8,
{string,8,"format"},
{tuple,8,
[{remote,9,{string,9,"type"},{string,9,"rect"}},
{remote,10,
{string,10,"widths"},
{cons,10,
{integer,10,1920},
{cons,10,{integer,10,1600},{nil,10}}}},
{remote,11,{string,11,"height"},{op,11,'-',{integer,11,1080}}},
{remote,12,{string,12,"interlace"},{atom,12,false}},
{remote,13,{string,13,"frame rate"},{var,13,'V'}}]}}]}]},
default,default}]}]}]},
{eof,16}]
./json_test.erl:7: illegal expression
./json_test.erl:8: illegal expression
./json_test.erl:5: Warning: variable 'V' is unused
error
The compilation of json_test fails since the module contains invalid Erlang syntax, but you get to see what the AST looks like. Now we can just write some functions to traverse the AST and rewrite the json code into Erlang code.[1]
-module(json_parser).
-export([parse_transform/2]).
parse_transform(AST, _Options) ->
json(AST, []).
-define(FUNCTION(Clauses), {function, Label, Name, Arity, Clauses}).
%% We are only interested in code inside functions.
json([?FUNCTION(Clauses) | Elements], Res) ->
json(Elements, [?FUNCTION(json_clauses(Clauses)) | Res]);
json([Other|Elements], Res) -> json(Elements, [Other | Res]);
json([], Res) -> lists:reverse(Res).
%% We are interested in the code in the body of a function.
json_clauses([{clause, CLine, A1, A2, Code} | Clauses]) ->
[{clause, CLine, A1, A2, json_code(Code)} | json_clauses(Clauses)];
json_clauses([]) -> [].
-define(JSON(Json), {bin, _, [{bin_element
, _
, {tuple, _, [Json]}
, _
, _}]}).
%% We look for: <<"json">> = Json-Term
json_code([]) -> [];
json_code([?JSON(Json)|MoreCode]) -> [parse_json(Json) | json_code(MoreCode)];
json_code(Code) -> Code.
%% Json Object -> [{}] | [{Lable, Term}]
parse_json({tuple,Line,[]}) -> {cons, Line, {tuple, Line, []}};
parse_json({tuple,Line,Fields}) -> parse_json_fields(Fields,Line);
%% Json Array -> List
parse_json({cons, Line, Head, Tail}) -> {cons, Line, parse_json(Head),
parse_json(Tail)};
parse_json({nil, Line}) -> {nil, Line};
%% Json String -> <<String>>
parse_json({string, Line, String}) -> str_to_bin(String, Line);
%% Json Integer -> Intger
parse_json({integer, Line, Integer}) -> {integer, Line, Integer};
%% Json Float -> Float
parse_json({float, Line, Float}) -> {float, Line, Float};
%% Json Constant -> true | false | null
parse_json({atom, Line, true}) -> {atom, Line, true};
parse_json({atom, Line, false}) -> {atom, Line, false};
parse_json({atom, Line, null}) -> {atom, Line, null};
%% Variables, should contain Erlang encoded Json
parse_json({var, Line, Var}) -> {var, Line, Var};
%% Json Negative Integer or Float
parse_json({op, Line, '-', {Type, _, N}}) when Type =:= integer
; Type =:= float ->
{Type, Line, -N}.
%% parse_json(Code) -> io:format("Code: ~p~n",[Code]), Code.
-define(FIELD(Lable, Code), {remote, L, {string, _, Label}, Code}).
parse_json_fields([], L) -> {nil, L};
%% Label : Json-Term --> [{<<Label>>, Term} | Rest]
parse_json_fields([?FIELD(Lable, Code) | Rest], _) ->
cons(tuple(str_to_bin(Label, L), parse_json(Code), L)
, parse_json_fields(Rest, L)
, L).
tuple(E1, E2, Line) -> {tuple, Line, [E1, E2]}.
cons(Head, Tail, Line) -> {cons, Line, Head, Tail}.
str_to_bin(String, Line) ->
{bin
, Line
, [{bin_element
, Line
, {string, Line, String}
, default
, default
}
]
}.And now we can compile json_test without errors:
1> c(json_parser).
{ok,json_parser}
2> c(json_test).
{ok,json_test}
3> json_test:test(42).
[{<<"name">>,<<"Jack (\"Bee\") Nimble">>},
{<<"format">>,
[{<<"type">>,<<"rect">>},
{<<"widths">>,[1920,1600]},
{<<"height">>,-1080},
{<<"interlace">>,false},
{<<"frame rate">>,42}]}]The AST generated by parse_teansfom/2 must correspond to valid Erlang code. Unless you apply several parse transforms, which is possible. The validity of the code is checked by the following compiler pass.
The linter (erl_lint.erl) generaters warnings for syntactically correct but otherwise bad code, like "export_all flag enabled".
In order to enable debugging of a module, you can "debug compile" the module, that is pass the option debug_info to the compiler. The abstract syntax tree will then be saved by the "Save AST" until the end of the compilation, where it will be written to the .beam file.
It is important to note that the code is saved before any optimisations are appilied, so if there is a bug in an optimisation pass in the compiler and you run code in the debugger you will get a diffferent behavior. If you are implementing your own compiler optimisations this can trick you up badly.
In the expand phase source erlang constructs, such as records, are expanded to lower level erlang constructs. Compiler options, "-compile(...)", are also expanded to meta data.
Core Erlang is a strict functional language suitable for compiler optimizations. It makes code transformations easier by reducing the number of ways to express the same operation. One way it does this is by introducing let and letrec expressions to make scoping more explicit.
Core Erlang is the best target for a language you want to run in ERTS. It changes very seldom and it contains all aspects of Erlang in a clean way. If you target the beam instruction set directly you will have to deal with much more detail, and that instruction set usually changes slightly between each major release of ERTS. If you on the other hand target Erlang directly you will be more restricted in what you can describe, and you will also have to deal with more details, since Core Erlang is a cleaner language.
To compile an Erlang file to core you can give the option "to_core", note though that this writes the Erlang core program to a file with the ".core" extension. To compile an Erlang core program from a ".core" file you can give the option "from_core" to the compiler.
1> c(world, to_core).
** Warning: No object file created - nothing loaded **
ok
2> c(world, from_core).
{ok,world}
Note that the .core files are text files written in the human readable core format. To get the core program as an Erlang term you can add the binary option to the compilation.
Kernel Erlang is a flat version of Core Erlang with a few differences. For example, each variable is unique and the scope is a whole function. Pattern matching is compiled to more primitive operations.
The last step of a normal compilation is the external beam code format. Some low level optimizations such as dead code elimination and peep hole optimisations are done on this level.
The BEAM code is described in detail in [CH-Instructions] and [AP-Instructions]
There are a number of tools available to help you work with code generation and code manipulation. These tools are written in Erlang and not really part of the runtime system but they are very nice to know about if you are implementing another language on top of the BEAM.
In this section we will cover three of the most useful code tools: the lexer — Leex, the parser generator — Yecc, and a general set of functions to manipulate abstract forms — Syntax Tools.
Leex is the Erlang lexer generator. The lexer generator takes a description of a DFA from a definitions file (<fileextension>xrl</fileextension>) and produces an Erlang program that matches tokens described by the DFA.
The details of how to write a DFA definition for a tokenizer is beyond the scope of this book. For a thorough explanation I recommend the "Dragon book" (Compiler … by Aho, Sethi and Ullman). Other good resources are the man and info entry for "flex" the lexer program t inspired leex, and the leex documentation itself. If you have info and flex installed you can read the full manual by typing:
> info flex
The online Erlang documentation also has the leex manual (see [yecc.html](http://erlang.org/doc/man/yecc.html)).
We can use the lexer generator to create an Erlang program which recognizes JSON tokens. By looking at the JSON definition http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf we can see that there are only a handful of tokens that we need to handle.
include::code/compiler_chapter/src/json_tokens.xrl
By using the Leex compiler we can compile this DFA to Erlang code, and by giving the option dfa_graph we also produce a dot-file which can be viewed with e.g. Graphviz.
1> leex:file(json_tokens, [dfa_graph]).
{ok, "./json_tokens.erl"}
2>You can view the DFA graph using for example dotty.
> dotty json_tokens.dotimage::code/compiler_chapter/json_tokens.png
We can try our tokenizer on an example json file (test.json).
include::code/compiler_chapter/src/test.json