-O2优化的printf（QUOT;％S \\ n＆QUOT ;, STR）到看跌期权（STR） [英] -O2 optimizes printf("%s\n", str) to puts(str)

问题描述

与玩弄周围铛，我编译包含此行C程序：

 的printf（％S \\ N的argv [0]）;
 

在没有优化的编译，汇编输出名为的printf 设置寄存器后：

  MOVQ（RCX％），RSI％
MOVQ％RAX，％RDI
MOVB $ 0％人
callq _printf
 

我试着用编译铛-O2 。在的printf 调用替换为一个看跌电话：

  MOVQ（％RSI），％RDI
callq _puts
 

虽然这非常有意义，在这种情况下，提出了两个问题：

1. 多久函数调用替换优化的编译发生？这是频繁或标准输入输出异常？


2. 我可以写我自己的库编译器优化？我会怎么做呢？

解决方案

  

  
1. 多久函数调用替换优化的编译发生？这是频繁或标准输入输出异常？

  

替换的printf 与看跌的优化LLVM是班上 LibCallSimplifier 。你可以看到在头文件<一个href=\"https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Transforms/Utils/SimplifyLibCalls.h\"相对=nofollow> LLVM /有/ LLVM /变换/ utils的/ SimplifyLibCalls.h 和的 LLVM / lib中/变换/ utils的/ SimplifyLibCalls.cpp 。纵观文件将给出一些完成其它库调用替换优化的例子（头文件是可能更容易下手）。当然，还有一些LLVM呢，你可以通过看的 LLVM通过。

  <醇开始=2>
  

我可以写我自己的库编译器优化？我会怎么做呢？

  

是的，你可以。 LLVM是非常模块化并在一系列传球执行对红外转换。所以，如果你想添加自定义自己传递了自己的图书馆，你可以这样做（尽管它仍然是了解如何在LLVM编译器流程的工作原理相当数量的工作）。一个很好的起点是文档：写一个LLVM通

Toying around with clang, I compiled a C program containing this line:

printf("%s\n", argv[0]);

When compiling without optimization, the assembly output called printf after setting up the registers:

movq    (%rcx), %rsi
movq    %rax, %rdi
movb    $0, %al
callq   _printf

I tried compiling with clang -O2. The printf call was replaced to a puts call:

movq    (%rsi), %rdi
callq   _puts

While this makes perfect sense in this case, it raises two questions:

1. How often does function call substitution happen in optimized compilation? Is this frequent or is stdio an exception?
2. Could I write compiler optimizations for my own libraries? How would I do that?

解决方案

1. How often does function call substitution happen in optimized compilation? Is this frequent or is stdio an exception?

The optimization that replaces printf with puts in LLVM is in the class LibCallSimplifier. You can see the header file in llvm/include/llvm/Transforms/Utils/SimplifyLibCalls.h and the implementation in llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp. Looking at the files will give an example of some of the other library call replacement optimizations that are done (the header file is probably easier to start with). And of course there are other many other optimizations that LLVM does, you can get an idea of some of them by looking at the list of LLVM passes.

2. Could I write compiler optimizations for my own libraries? How would I do that?

Yes you could. LLVM is very modular and performs transformation on the IR in a series of passes. So if you wanted to add a custom pass yourself for your own library you could do so (although it is still a fair amount of work to understand how the LLVM compiler flow works). A good starting point is the document: Writing an LLVM Pass.

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