github.com/hikaru7719/go@v0.0.0-20181025140707-c8b2ac68906a/src/cmd/compile/README.md (about) 1 <!--- 2 // Copyright 2018 The Go Authors. All rights reserved. 3 // Use of this source code is governed by a BSD-style 4 // license that can be found in the LICENSE file. 5 --> 6 7 ## Introduction to the Go compiler 8 9 `cmd/compile` contains the main packages that form the Go compiler. The compiler 10 may be logically split in four phases, which we will briefly describe alongside 11 the list of packages that contain their code. 12 13 You may sometimes hear the terms "front-end" and "back-end" when referring to 14 the compiler. Roughly speaking, these translate to the first two and last two 15 phases we are going to list here. A third term, "middle-end", often refers to 16 much of the work that happens in the second phase. 17 18 Note that the `go/*` family of packages, such as `go/parser` and `go/types`, 19 have no relation to the compiler. Since the compiler was initially written in C, 20 the `go/*` packages were developed to enable writing tools working with Go code, 21 such as `gofmt` and `vet`. 22 23 It should be clarified that the name "gc" stands for "Go compiler", and has 24 little to do with uppercase "GC", which stands for garbage collection. 25 26 ### 1. Parsing 27 28 * `cmd/compile/internal/syntax` (lexer, parser, syntax tree) 29 30 In the first phase of compilation, source code is tokenized (lexical analysis), 31 parsed (syntax analysis), and a syntax tree is constructed for each source 32 file. 33 34 Each syntax tree is an exact representation of the respective source file, with 35 nodes corresponding to the various elements of the source such as expressions, 36 declarations, and statements. The syntax tree also includes position information 37 which is used for error reporting and the creation of debugging information. 38 39 ### 2. Type-checking and AST transformations 40 41 * `cmd/compile/internal/gc` (create compiler AST, type checking, AST transformations) 42 43 The gc package includes an AST definition carried over from when it was written 44 in C. All of its code is written in terms of it, so the first thing that the gc 45 package must do is convert the syntax package's syntax tree to the compiler's 46 AST representation. This extra step may be refactored away in the future. 47 48 The AST is then type-checked. The first steps are name resolution and type 49 inference, which determine which object belongs to which identifier, and what 50 type each expression has. Type-checking includes certain extra checks, such as 51 "declared and not used" as well as determining whether or not a function 52 terminates. 53 54 Certain transformations are also done on the AST. Some nodes are refined based 55 on type information, such as string additions being split from the arithmetic 56 addition node type. Some other examples are dead code elimination, function call 57 inlining, and escape analysis. 58 59 ### 3. Generic SSA 60 61 * `cmd/compile/internal/gc` (converting to SSA) 62 * `cmd/compile/internal/ssa` (SSA passes and rules) 63 64 65 In this phase, the AST is converted into Static Single Assignment (SSA) form, a 66 lower-level intermediate representation with specific properties that make it 67 easier to implement optimizations and to eventually generate machine code from 68 it. 69 70 During this conversion, function intrinsics are applied. These are special 71 functions that the compiler has been taught to replace with heavily optimized 72 code on a case-by-case basis. 73 74 Certain nodes are also lowered into simpler components during the AST to SSA 75 conversion, so that the rest of the compiler can work with them. For instance, 76 the copy builtin is replaced by memory moves, and range loops are rewritten into 77 for loops. Some of these currently happen before the conversion to SSA due to 78 historical reasons, but the long-term plan is to move all of them here. 79 80 Then, a series of machine-independent passes and rules are applied. These do not 81 concern any single computer architecture, and thus run on all `GOARCH` variants. 82 83 Some examples of these generic passes include dead code elimination, removal of 84 unneeded nil checks, and removal of unused branches. The generic rewrite rules 85 mainly concern expressions, such as replacing some expressions with constant 86 values, and optimizing multiplications and float operations. 87 88 ### 4. Generating machine code 89 90 * `cmd/compile/internal/ssa` (SSA lowering and arch-specific passes) 91 * `cmd/internal/obj` (machine code generation) 92 93 The machine-dependent phase of the compiler begins with the "lower" pass, which 94 rewrites generic values into their machine-specific variants. For example, on 95 amd64 memory operands are possible, so many load-store operations may be combined. 96 97 Note that the lower pass runs all machine-specific rewrite rules, and thus it 98 currently applies lots of optimizations too. 99 100 Once the SSA has been "lowered" and is more specific to the target architecture, 101 the final code optimization passes are run. This includes yet another dead code 102 elimination pass, moving values closer to their uses, the removal of local 103 variables that are never read from, and register allocation. 104 105 Other important pieces of work done as part of this step include stack frame 106 layout, which assigns stack offsets to local variables, and pointer liveness 107 analysis, which computes which on-stack pointers are live at each GC safe point. 108 109 At the end of the SSA generation phase, Go functions have been transformed into 110 a series of obj.Prog instructions. These are passed to the assembler 111 (`cmd/internal/obj`), which turns them into machine code and writes out the 112 final object file. The object file will also contain reflect data, export data, 113 and debugging information. 114 115 ### Further reading 116 117 To dig deeper into how the SSA package works, including its passes and rules, 118 head to [cmd/compile/internal/ssa/README.md](internal/ssa/README.md).