Go 开发进程说明

浏览数：73 / 时间：2015年06月10日

Go in Go

随着 Go 1.5 版本的开发，现在整个系统都是使用 Go 编写的（有一小部分汇编）

C 已经成为过去时了。

注： gccgo 仍然很强大
这篇文章主要探讨原始编译器 gcc

为什么之前用 C 编写？

启动

（而且 Go 的主要目的不是作为一个编译器实现语言）

为什么编译器使用 Go 重写？

不单单是验证，我们还有更多实际的原因：

Go 比 C 容易编写（实际上）
Go 比 C 容易调试（即使没有调试器的情况下）
Go 将成为你唯一需要会的语言，鼓励贡献
Go 有更好的模块化，工具链，测试工具，配置工具等等
Go 很简单就能进行并行操作

虽然看起来很多优势，但是还是夸的太早了：）

设计文档： golang.org/s/go13compiler

为什么运行时也是用 Go 重写？

我们有自己的 C 编译器来编译运行时
我们需要一个带有跟 Go ABI 一样编译器，比如 segmented stacks。

使用 Go 编写可以摆脱 C 编译器的劣势。
这是比使用 Go 重写编译器还重要。

(所有使用 Go 重写编译器的理由都可以作为使用 Go 重写运行时的理由)

限制运行时只用一种语言编写，更容易进行集成，管理 stack 等等操作。

跟往常一样，简化是首要的考虑因素。

历史

为什么我们要有完全属于自己的工具链？
我们的 ABI？
我们自身的文件格式？

History, familiarity, and ease of moving forward. And speed.

Go 的大部分重大改进要比 GCC 或者 LLVM 要更困难。

news.ycombinator.com/item?id=8817990

重大改进

由于使用自身工具简化的功能和使用 Go 重写后的一些改进：

linker 重构
新垃圾收集器
堆叠图
连续栈
写屏障

最后三个都不可能用 C 实现：

C 是非类型安全的
因为最佳化而造成堆栈槽混淆

（Gccgo 很快会有 segmented stacks 和 imprecise (stack) collection ）

Goroutine 栈

Until 1.2: Stacks were segmented.
1.3: Stacks were contiguous unless executing C code (runtime).
1.4: Stacks made contiguous by restricting C to system stack.
1.5: Stacks made contiguous by eliminating C.

These were each huge steps, made quickly (led by khr@).

Converting the runtime

Mostly done by hand with machine assistance.

Challenge to implement the runtime in a safe language.
Some use of unsafe to deal with pointers as raw bits in the GC, for instance.
But less than you might think.

The translator (next sections) helped for some of the translation.

Converting the compiler

Why translate it, not write it from scratch? Correctness, testing.

Steps:

Write a custom translator from C to Go.
Run the translator, iterate until success.
Measure success by bit-identical output.
Clean up the code by hand and by machine.
Turn it from C-in-Go to idiomatic Go (still happening).

Translator

First output was C line-by-line translated to (bad!) Go.
Tool to do this written by rsc@ (talked about at GopherCon 2014).
Custom written for this job, not a general C-to-Go translator.

Steps:

Parse C code using new simple C parser (yacc)
Remove or rewrite C-isms such as *p++ as an expression
Walk the C parse tree, print the C code in Go syntax
Compile the output
Run, compare generated code
Repeat

The Yacc grammar was translated by sam-powered hands.

Translator configuration

Aided by hand-written rewrite rules, such as:

this field is a bool
this function returns a bool

Also diff-like rewrites for things such as using the standard library:

diff {
-    g.Rpo = obj.Calloc(g.Num*sizeof(g.Rpo[0]), 1).([]*Flow)
-    idom = obj.Calloc(g.Num*sizeof(idom[0]), 1).([]int32)
-    if g.Rpo == nil || idom == nil {
-        Fatal("out of memory")
-    }
+    g.Rpo = make([]*Flow, g.Num)
+    idom = make([]int32, g.Num)
}

Another example

This one due to semantic difference between the languages.

diff {
-    if nreg == 64 {
-        mask = ^0 // can't rely on C to shift by 64
-    } else {
-        mask = (1 << uint(nreg)) - 1
-    }
+    mask = (1 << uint(nreg)) - 1
}

Grind

Once in Go, new tool grind deployed (by rsc@):

parses Go, type checks
records a list of edits to perform: "insert this text at this position"
at end, applies edits to source (hard to edit AST).

Changes guided by profiling and other analysis:

removes dead code
removes gotos
removes unused labels, needless indirections, etc.
moves var declarations nearer to first use

rsc.io/grind

Performance problems

Output from translator was poor Go, and ran about 10X slower.
Most of that slowdown has been recovered.

Problems with C to Go:

C patterns can be poor Go; e.g.: complex for loops
C stack variables never escape; Go compiler isn't as sure
interfaces such as fmt.Stringer vs. C's varargs
no unions in Go, so use structs instead: bloat
variable declarations in wrong place

C compiler didn't free much memory, but Go has a GC.
Adds CPU and memory overhead.

Performance fixes

Profile! (Never done before!)

move vars closer to first use
split vars into multiple
replace code in the compiler with code in the library: e.g. math/big
use interface or other tricks to combine struct fields
better escape analysis (drchase@).
hand tuning code and data layout

Use tools like grind, gofmt -r and eg for much of this.

Removing interface argument from a debugging print library got 15% overall!

More remains to be done.

Technical benefits

Other benefits of the conversion:

Garbage collection means no more worry about introducing a dangling pointer.

Chance to clean up the back ends.

Unified 386 and amd64 architectures throughout the tool chain.

New architectures are easier to add.

Unified the tools: now one compiler, one assembler, one linker.

Compiler

GOOS=YYY GOARCH=XXX go tool compile

One compiler; no more 6g, 8g etc.

About 50K lines of portable code.
Even the registerizer is portable now; architectures well characterized.
Non-portable: Peepholing, details like registers bound to instructions.
Typically around 10% of the portable LOC.