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QBE：一个新的编译器后端
Show HN: QBE – a new compiler back end
2016 — 2024，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
QBE 是一个用纯 C 写成、可嵌入的编译器后端：它定义了一套中间语言，负责 SSA 构造、优化和寄存器分配等后端工作，让写新语言的人不必自己处理后端，也不必背上 LLVM 的体量。作者的目标是用大约 10% 的代码量拿到高级编译器约 70% 的性能。
为什么经久不衰：它证明了“小而可读的编译器后端”这条路可行，多年来一直是 hobby 语言项目和编译器教学里最常被翻出来的参照实现之一。
HN 评论精选
反驳原文对 LLVM 栈槽的批评：栈槽是为可能逃逸的变量准备的，不逃逸的会被 mem2reg 提升为 SSA。
“实现 SSA 构造很难，为了让用户不必自己实现，LLVM 提供了栈槽，于是一次 `v++` 会被拆成 load、add、store 三条指令”——这个说法是错的。LLVM 提供栈槽有正当理由：分配在栈上的变量可能逃逸，你总得有一块内存放它。如果 mem2reg 能证明该栈变量不逃逸，它就会被提升为 SSA 值。
munin查看英文原文
> Implementing SSA construction is hard. To save its users from having to implement it, LLVM provides stack slots. This means that one increment of a variable v will be composed of three LLVM instructions: one load, one add, and one store. This is just wrong, though! LLVM provides stack slots for a good reason: stack allocated variables can escape. You need a place in memory to stick them. If mem2reg can prove that the stack allocated variable doesn't escape, it gets promoted into an SSA value.
赞赏“极小代码量换大部分能力”的设计哲学，并类比 vis 编辑器。
很棒的想法！作为编译器从业者我很期待看到它的实际效果。QBE 的理念——“纯 C、可嵌入，用 10% 的代码提供高级编译器 70% 的性能”——和 vis 编辑器很像（用 1% 的代码实现 vim 80% 的功能）。希望这类项目越来越多；我对更简单的程序和生态很感兴趣，哪怕代价是功能更少。
gnuvince查看英文原文
This is a wonderful idea! As a compiler person, I'm excited to see how well this works and wish you all the success in the world. > QBE aims to be a pure C embeddable backend that provides 70% of the performance of advanced compilers in 10% of the code. This philosophy is very similar to that of vis [1], a vim-like editor that aims to have 80% of vim's functionality in 1% of the code. I hope that more such project see the light of the day; I'm very interested in simpler programs and simpler ecosystems, even if that comes at the cost of features. [1] https://github.com/martanne/vis
建议趁项目小而简单时引入契约式设计、静态分析等高保障手段来提升正确性。
有趣的项目。我特别喜欢文档里讲优化的部分，尤其是那些能拿下最初 70% 收益的优化；我一直建议做优化的人去综述各种方法，找出能带来最大收益的最小集合，这能帮新编译器和形式验证社区起步。回到正确性：项目追求小、简单、严谨，这正是引入高保障方法的好时机——建议试试契约式设计加接口检查、静态分析工具，甚至 Softbound+CETS 这类自动让 C 更安全的东西；我也常建议用 OCaml 写编译器以减少 bug。或者干脆用安全语言和可移植 C 各写一份实现并行测试。
nickpsecurity查看英文原文
Interesting project mpu. I like the stuff here: http://c9x.me/compile/doc/llvm.html Especially the optimizations that get you the first 70%. I've been asking optimization people as I see them to do a survey of various methods to find the smallest collection that provide the hugest benefit. This will help people writing new compilers and formal verification community get a head start. Now, back to correctness. Your project aims to be small, simple, and rigorous. That's perfect time to use methods for higher assurance software or design yours to work with them easily. So, I suggest trying Design-by-Contract w/ interface checks, some static analysis tools, or even something like Softbound+CETS that makes C safer automatically. I also usually recommend writing compilers in something like Ocaml as it prevents lots of bugs and easier to do. Also, if you do Ocaml or safe imperative, you can always do two implementations side-by-side in the safe language and in portable C. You run tests, analysis, etc available for each one. Problems in one might be problems in the other. Regardless, though, you get the benefits of the safer language by default with a C implementation if you absolutely need it. Many of us have used this with Sandia Labs even doing it for hardware with ML and hardware language of a Java processor that came out great. So, I challenge you to try to push the envelope by using some available techniques and tools to boost the assurance of your code. Preferably getting it off C, too, due to all the errors it (esp pointers) introduce into the compilers.
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