Compiling principles written for front end

zxg_ God says light 2021-05-04 16:53:39
compiling principles written end


Hao Hao is a front-end engineer , Recently, it involves the field of engineering , Want to know something about compiling . It happens that I studied earlier than he did , So I introduced what I knew to him , So here's the conversation .

What is compilation ?

Haohao : Recently, I want to know something about compiling , light , What is compiler ?

I : Compiling is a kind of transformation technology , From one programming language to another , From high-level language to low-level language , Or from high-level language to high-level language , Such conversion technology .

Haohao : What is advanced language , What is low level language ?

I Low level language It's about machines , When it comes to registers 、cpu Instruction, etc , special “ low ”, Describe the execution process on the machine , Like machine language 、 assembly language 、 Bytecode, etc . High-level language There is no such thing as concrete implementation , It's mainly used to express logic , And it provides the conditions 、 loop 、 function 、 Object oriented and so on , And then through compiling, these described high-level language logic can be automatically converted into low-level language instructions , In this way, we can express logic conveniently , Without affecting the specific implementation . It's not right to say that it doesn't affect execution , Because if you write assembly directly , Write the most efficient code , But if it is a high-level language, it will be automatically converted to a low-level language by compiling , Then it is difficult to guarantee the execution efficiency of the generated code , Various compilation optimizations are required , This is the difficulty in compiling .

In fact, think of , We put ideas in our heads , Translate the plan into high-level language code , Is this process also a transformation , Can it be automated , This involves ai 了 . Now we have the research direction of intelligent technology for understanding requirements document and generating code .

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Haohao : How is it converted ?

I : To transform, we must first understand the two sides of the transformation , What is to be converted , To what . Such as high-level language to high-level language , It's a string to convert , Organized in a certain format , These formats are called morphology 、 grammar , The whole is called grammar , The target to be changed , If the target is also a high-level language, understand the format of the target language , If the target is a low-level language , For example, assembly , It's about understanding what each command does . Then we need to transform the semantic equivalence , Pay attention to this “ Semantic equivalence ”, Explain one language to another , You can't lose or add some semantics , It has to be consistent .

Know what both sides of the conversion are , You can do the conversion , First, let the computer understand what to transform , What do you mean “ Computer understanding “ Well ? It's just to put the words that we prescribe 、 The syntax format tells the computer , How to tell ? It's data structure , According to a certain data structure, the results of source string parsing should be organized , The computer can handle it . This process is called parse, First of all, participle , Reconstruct the idiom tree .

In fact, it's not just the compilation field that needs “ understand ”, There are many other fields that need to be “ understand ”:

Full text search engine also need to first search the string through the word segmentation machine segmentation , Then according to these words, use the same word segmentation machine to segment words and index them in the database , Score and sort the matching results of words , This is full text search .

The field of artificial intelligence deals with natural language , He also has to follow the morphology 、 grammar 、 And so on “ understand ”, After becoming a certain data structure , The computer can understand and process , Then there are various processing algorithms .

Word segmentation is based on the state machine ( Finite state machine DFA), What's this for , Why does participle need it , I know you must have questions . Because morphology describes the format of the smallest word , For example, an identifier cannot begin with a number , And then followed by alphanumeric underscores and so on , such , And keywords if、while、continue etc. , These can't be subdivided any more , It doesn't make sense to subdivide . Word segmentation is to change a string into the smallest unit of words that can no longer be split , Also called token, Because different words have different formats , You can't write if else Let's deal with different formats . Actually, it can be ,wenyan Namely if else, Make complaints about it . But when there is 100 The format of words in English should be handled , It's all written in if else, My day, , Can you still read the code . So treat the processing of each word as a state , When we deal with different word formats, we jump to different states , The way to jump is naturally based on the character currently being processed , Process a string from the start state to a different state to process , So that's state automata , Every token After identification, it can be thrown out , The final product is a token Array .

In fact, the state is more than one level , Think about it, like a html The beginning tag of the tag , It can be treated as a state , But in this state, we have to deal with properties 、 Start tagging, etc , This is the secondary state , Attributes can be subdivided into several States to handle , It's a three-level state , This is the idea of partition , Layer by layer .

After word segmentation, we got the words one by one , And then we're going to assemble these words , Generate ast, Why do you have to ast Well ? I know you want to ask . In fact, high-level language code is nested , You see low-level languages like assembly , It's just instructions , Linear structure , But high level languages , Have function 、if、else、while And so on , Blocks can be nested . So it's natural to organize a tree like data structure for the computer to understand , Namely Abtract Syntaxt Tree, Grammar tree 、 And it's abstract , That is, some meaningless separators are ignored , such as html Of <、>、</ Equal character ,js Of { }() [] ; It's the details , Don't care , Comments are also ignored , Notes are just participles that will come out , But don't put it in ast Inside .

How to assemble it , Or nested assembly , Is that recursive assembly , Yes , You're right. You need recursion , It's not just here ast Assembly requires recursion , There is also a lot of recursion in the following processing , Except when it comes to linear code , It's like assembly , What are you doing , There is no nested structure to recurse .

We just analyzed the morphology , It's a string format , Grammar , It's the assembly format , It's the combination of words . That's why we're just going to participle , If you assemble it directly from a string ast, So we're dealing with the string level , And from token It starts at the word level , It's like building a castle out of building blocks , But building blocks also need to be made of mud , How do you make it , You can build the blocks first , And then assemble it into a castle , You can also build blocks and assemble them at the same time . But for cars, you can make building blocks at the same time , Edge assembly , Castle level building blocks are assembled at the same time. Can you figure out what building blocks to build , It's hard , So it still depends . Do it both ways parser Both have , Simple analysis can be side word method , Analyze the hot words and immediately assemble them into ast in , such as html、css such , But like js、c++ such , If you don't segment first , Just start with a string ast, I can only say it's too fierce .

Talking about building blocks and assembly for a long time , So how to assemble it , Processing from left to right token, Meet a token How do you know his grammar , It's like how to know which part a building block belongs to . There are also two ways of thinking , One is to make sure that the building block belongs to that part , And find the drawing of that part , Assemble according to the drawing , The other is that you assemble first , After assembly, let's see what this part is . There are two ways , First, determine which part is based on one or two building blocks , Then assemble the part according to the drawing , This is a ll The way , Assemble first , After assembly, let's see what the parts are , This is a lr The way .ll The way to determine what is assembled ast The nodes need to look down a few , According to the number to see what is assembled, they are LL(1),LL(2) And so on .ll That's recursion descent , This is the easiest way to assemble , Of course, some people think that lr It's also very simple .ll There's a problem that has to be solved lr solve , That is, recursion falls down, and recursion on the left side doesn't reach the end , To eliminate left recursion , That's when you can't assemble according to the drawings , Let's first assemble and then see what's assembled . It's actually very similar to life , One way is to look down two steps and decide what to do right now , Another way is to go first , We'll see where we go . Actually, I belong to the second kind , There's no planning .

After morphology 、 After the analysis of grammar, it came into being ast. Use a tree like data structure to describe the source code , From here on, computers can understand , It can be explained and executed later 、 The transformation can be compiled . Both interpretation and compilation need to be done first parse, That is, let the computer understand what it is first , And then decide what to do with it .

Put the tree in the back ast Convert to another ast, And then print it as a string of object code , This is the translator , hold ast Interpretation and execution or special linear intermediate code and then interpretation and execution , This is the interpreter , hold ast Turn to linear intermediate code , And then generate assembly code , Then do the assembly and link , Generate machine code , This is the compiler .

How does the compiler handle AST Of ?

Haohao : light , How does the compiler handle ast ah ?

I : With ast after , Computers can understand high-level language code , But compilers have to produce low-level languages , For example, assembly code , Directly from ast It's a long way to start . Because one is nested 、 Tree shaped , One is linear 、 Sequential , So! , It needs to be converted to a linear code first , And then generate low-level code . I think ast It's also a tree shape IR,IR yes immediate representation The meaning of the middle . First of all AST It's linear IR, And then generate the assembly 、 Bytecode, etc .

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How to translate , How can the tree structure become linear ? Obviously recursion , Recursion according to grammatical structure ast, Translate each node , This is called grammar guided translation , With linearity IR To translate AST Properties of a node . How to translate each node ,if How to translate 、while How to translate can go to see the relevant information , Search for intermediate code generation .

however ast If you can't come up, just turn to the intermediate code .

Haohao : Why? ,ast Can't represent the source code information , Why can't it be translated directly into linear ir?

I : Because we haven't done semantic checking yet , Structure doesn't mean right , It's like “ Haohao is a pig ”, This is grammatically correct , But the semantics is obviously wrong , It's a curse , So we need to do semantic checking first . There is also the need to derive some information to , In order to do the follow-up translation .

Semantic analysis should check out semantic errors , Such as whether the types match 、 Whether the referenced variable exists 、break Whether in while secondary , The main thing to do Scope analysis Reference resolution Type derivation and checking Correctness check etc. .

Scope analysis is analyzing functions 、 Block, etc , What are the variables in these scopes , What is the connection between scopes , In fact, the scope is a tree , From the top scope to the sub scope, a tree data structure can be generated . I remember one who did scope Analytical webpack plug-in unit , He linked the modules as well , Formed a big scope graph, And then do the analysis .

There are various declarations in the scope , We need to sort them out 、 Initial value 、 Access modifiers and other information are recorded , The structure that holds this information is called the symbol table , This is equivalent to a cache , Later, when processing this symbol, just look up the symbol table , Don't start from ast Come looking for .

Reference resolution is to check whether definitions can be found for each symbol , If you can't find it, report an error . You are familiar with the type ,js It's impossible to write all types in the source code of , Many places can be deduced directly , according to ast You can get a declaration of the type , Record it in the symbol table , And then go through ast, Check the type of each node when it takes out the declaration , If you don't agree, you report an error . There are other trivial checks , such as continue、break Can only appear in while Medium and so on .

Haohao : Semantic analysis, I get it , Is to check for errors and record some of the analyzed information to the symbol table , After semantic analysis ?

After semantic analysis, it means that the program has no syntax and semantic errors , It's safe to do all kinds of subsequent conversions , There will be no more developer mistakes . Then translate it into linear IR, And then for linearity IR To optimize , Need to optimize is because automatically generated code inevitably has a lot of redundancy , We need to get rid of all kinds of unnecessary treatment . But keep the semantics unchanged . Like dead code deletion 、 Common subexpression delete 、 Constant propagation and so on .

linear IR The analysis of the flow chart to establish , It's the control flow diagram , control flow It is based on if、while、 Program jump caused by function call, etc , Connecting the code executed in sequence with the code that jumps to is a graph , Sequential execution of code as a whole , It's called basic fast . Then do data flow analysis according to the flow chart , That is to analyze the code that a variable flows through , And then do all kinds of optimizations based on these .

This part is called program analysis , perhaps Static analysis , It's a special direction , Can be used for static checking of code vulnerabilities , Can be used for compilation optimization , This is more difficult . There are fewer doctors studying this . In China, only Peking University and Nanjing University offer program analysis courses .

The optimized linearity IR You can generate assembly code , And then through the assembler into machine code , Link to some standard libraries , such as v8 You can see builtins Catalog , Here are all kinds of compiled machine code files , Can be statically linked into an executable file .

Haohao : Oh , I don't think the front end can touch the assembly and link .

I : Right , because js It's interpreted language , Explain and execute directly from the source code , Don't say js 了 ,java And the bytecode doesn't need static links . image c、c++ Only those who generate executable files need to make code into machine code through assembler, and then link it into a file . And if the target platform has these libraries , So there's no need to link statically together , You can dynamically link . You may have heard of .dll and .so These are respectively windows and linux For runtime dynamic loading to save the machine code file .

You're right , The front-end domain basically doesn't need assembly and linking , Even if it is wasm, Also generate wasm Bytecode , Then explain and execute . The front end is mainly translator .

How does the translator handle AST Of ?

Haohao : The translator is ast And what did you do after that ?

I : The target code of the translator is also a high-level language , It's also a nested structure , So from high-level language to high-level language is from tree structure to tree structure , It's not like translating into a language organized in a low-level way , It has to be translated into linear first IR, High level to high level language conversion , It only needs ast, Yes ast After all kinds of transformations , You can do code generation .

Haohao : I said , I didn't hear about it babel There's a linearity in it IR The concept of .

I : Right , Whether it's cross language conversion , such as ts turn rust, It's the same language conversion js turn js You don't need a linear structure , What kind of linear intermediate code does the conversion of two trees need . So most translators are parsetransformgenerate this 3 Stages .

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parse In a broad sense, it includes morphology 、 The analysis of grammar and semantics , The narrow sense parse Grammatical analysis . This doesn't have to be tangled up .

transform That's right ast Addition, deletion and modification of , after generator And then ast Print it as a string , We analyze ast When the time is right []{} () The bisector has been removed ,generate Add the details back when you need to .

In fact, the front-end domain is mainly translator , Because the mainstream js The engine executes the source code , But the source code is not the same as the one we wrote , So many source to source translators at the front end do this kind of conversion , such as babel、typescript、terser、eslint、postcss、prettier etc. .

babel It's a higher version es The code is converted to a lower version , And injection polyfill.typescript It's type checking and turning into js Code .eslint It's checking according to the specifications , but --fix You can also generate the repaired code .prettier Also used to format code , Than eslint More to deal with , It's not limited to js.postcss Mainly dealing with css Of ,posthtml Used for processing html. I believe you have used it a lot .taro This kind of small program translator is based on babel Packaged .

How the interpreter handles AST Of ?

Haohao : Oh , light , I probably know that both compilers and translators are right ast What have you done to deal with , Both of them generate code , And the interpreter ?

I : Yes , First of all, translator is also a kind of compiler , It's just something special , be called transpiler, A general compiler is called compiler. The difference between an interpreter and a compiler is really whether to generate code , A machine code compiled in advance is called AOT compiler , What compiles into machine code at run time is called JIT compiler ,

The interpreter doesn't generate machine code , So how does it work ? I know you have questions .

In fact, the interpreter uses one high-level language to explain another high-level language , such as c++, Commonly used c++ To write an interpreter , Because you can do memory management . use c++ To write js Interpreter , image v8、spidermonkey Are all . We have ast And after semantic analysis, you can traverse ast, And then use c++ To execute different nodes , This is called. tree walker Interpreter , Direct interpretation execution ast,v8 The engine is 17 That's what I did years ago . But in 17 Bytecode was introduced two years later , Because bytecode can be cached , So next time you execute bytecode directly, you don't need to parse 了 . Bytecode is a linear structure , Also do ast To linear ir Transformation , After the vm Execute bytecode on .

General explanation of linear code, such as assembly code 、 Bytecode and other programs are called virtual machine , Because machine code is linear , Actually from ast It can be explained from the beginning , But not vm, I think that's why , The interpreter of linear code, which is similar to machine code, is called vm.

Whether it's explanation ast Good , It's better to translate it into bytecode and explain it again , It's not particularly efficient , Because it uses other high-level languages to execute the code of the current language , So to improve efficiency, we have to compile it into machine code , This kind of runtime compilation is JIT compiler , Compiling is time consuming , So it's not all code JIT, Do heat statistics , You don't do it until you reach the threshold JIT. Then cache the machine code , Of course, it could be cached assembly code , When it is used, it can be converted into machine code by assembler , Because machine code takes up a lot of space .

You can contrast v8 To understand the ,v8 Yes parser、ignation Interpreter 、turbofan compiler , also gc.

ignation The interpreter is to put parse Out of ast Into bytecode , Then explain the execution bytecode , When the heat reaches the threshold, it will be handed over to turbofan Compile into assembly code and generate machine code , To speed up .gc It's independent of memory management .

turbofan It's the turbocharger , The name can reflect JIT The meaning of . but JIT Improved execution speed , There are also shortcomings. , For example, it will make js The engine is bigger , More memory , So lightweight js The engine does not contain jit, That's the speed and the packet size 、 The trade-off between memory space . Architecture design often needs to be done. Both sides are OK , But to make a choice trade off, We call it scheme blending .

When it comes to trade-offs , I think of rn Of js engine hermes Instead, it supports direct bytecode execution , Put... During compilation js Code compiled into bytecode , And then execute the bytecode directly , This is in the cross domain js Engine trade off.

Where does the front-end domain use compilation knowledge ?

Haohao : Oh , light , I understand the interpreter 、 compiler 、 What do translators do , Where does the front-end domain use the knowledge of compiler principles ?

I : In fact, you must have a general understanding , But it's not clear , Let me make a list of what I know .

Various translators in the field of engineering : babel、typescript、eslint、terser、prettier、postcss、posthtml、taro、vue template compiler etc.

js engine : v8、javascriptcore、quickjs、hermes etc.

wasm: llvm Can generate wasm Bytecode , therefore c++、rust Etc. can be converted into llvm ir You can do it in any language wasm Development

ide Of lsp: Syntax of programming languages 、 Smart tips 、 Error checking, etc language service protocol Protocol to communicate , and lsp The server is mainly based on parser Analyze the text being edited

How do you realize a language ?

Haohao : I learned the principle of compilation, can I realize a language ?

I : In fact, programming language is mainly design , If it is realized, it should be realized first parser And semantic analysis , There are two roads in the back , One is interpreter coordination for interpretation and execution JIT The way of compiler , One is compiled into assembly code , Then generate the machine code and link it to the executable file .

parser Part of it is cumbersome , It can be used antlr such parser Generator to generate , Semantic analysis should be written by oneself , This is not too difficult , It's mainly about ast All kinds of treatment . And then if you want to build a compiler , It can be used llvm This kind of general optimizer and code generator ,clang、rust、swift All based on it , So it's very reliable , It can be used directly . If you're an interpreter, you can write tree walker Interpreter , Or to further generate linear bytecode , And then write one vm To explain bytecode .JIT Compilers can also use llvm To do it . To put ast Turn into llvm ir, It's also a tree structure to a linear structure , This is a very common operation in the field of compilation .

In fact, the compiler principle just tells you how to implement it , Language design doesn't care about implementation , A language can be either compiled or interpreted , It can also be made java That kind of compile before explain , You see hermes(react native Realized js engine ) No, it's just putting js Compile into bytecode and then explain the bytecode . Language does not translate , This concept should have ,c There are also interpreters ,js There are also compilers , The main way to explain is to translate or translate .

Programming languages can be divided into GPL and DSL Two kinds of .

GPL It's a universal programming language , It's Turing's complete , That is, it can describe any computable problem , image c++、java、python、go、rust These languages are Turing's complete , So whatever one language can do, another language can do , It's just that it's very difficult to implement . such as go Language built-in coroutine implementation , So it's easy to write high concurrency programs ,java There is no language level coroutine , Then it's up to the top . You may have heard that design patterns are a complement to language defects, which means , Different languages have different design ideas , The built-in things are also different , Sometimes it needs runtime to make up for .、

Programming languages have different design ideas , The big direction is the programming paradigm , Like imperative 、 declarative 、 Functional expression 、 Logic, etc , These big ideas lead to the grammar of language , The built-in implementations are different , The ability to express is also different . This basically sets the tone of the language , It's hard to make up for it later , It's like js It's functional , You can't restrict people from using imperative , It's hard to write pure functional code .

DSL Not Turing complete , But in exchange for a stronger expression ability in a certain field , such as html、css、 Regular expressions ,jq And so on , It's more like pseudo code , Strong expression in specific areas , But it's not Turing's complete and can't describe all computable problems .

Compiling principles are the steps to realize programming languages , There's a lot to learn from the higher level of language design , It's better to be familiar with the features of multiple programming languages .

How can I learn compiler principles ?

Haohao : light , How can I learn compiler principles ?

I : First of all, you need to understand what compiler learns , Look at the compiler above me 、 translation 、 The explanation of popular science may have an impression , Then check the relevant information . When you know what you can do, write first parser, Because whatever you do, you have to parse become ast Can be “ understand ” And subsequent processing , Learn the finite state machine to segment words and recursive descent construction ast. It's recommended to have a look at vue template compiler Of parser, such xml Of parser Relatively simple , Suitable for entry . At the language level parser There are many details , We still have to find one debug see . But I don't think it's necessary , Generally, I just write html parser, If the language is , It can be used antlr Generate . The translator must understand babel, This is a very good translator in the front-end field .

js The engine can try to use babel do parser, Do your own semantic analysis , Explain to perform ast try , After that, it further generates bytecode or other linear ir, And then write one vm To explain bytecode .

You can also learn wasm Related technology , That is related to other languages compiling to wasm Bytecode process .

I'm writing a 《babel Plug in clearance secrets 》 A small volume of , There will be js Interpreter 、 Translator 、type cheker、linter wait , A lot of knowledge about compiler principles , Maybe it can help you get started with compiling principles .

When you're done with compiling principles , You probably know how to implement a programming language , Later, if you want to go deep into language design, you can learn more languages of other programming paradigms , Learn about language features , How to design a strong expressive gpl perhaps dsl.

You can also learn more about the operating system and architecture , Because the compiled code still needs to run as a process on the operating system , So how to design the runtime depends on understanding the operating system . then cpu How instruction sets are implemented in circuits , If you want to go deeper, you can take a look at the computer architecture .

however , Front end engineers don't need to reach that depth , But there's no harm in having an open mind .

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