c-lang无限尾递归优化

#include <iostream> 

int foo(int i){ 
     return foo(i + 1);
} 

int main(int argc,char * argv[]){ 
     if(argc != 2){ 
         return 1; 
     } 
     std::cout << foo(std::atoi(argv[1])) << std::endl; 
} 

％clang -O2 test.cc

％时间./a.out 42

1490723512

./a.out 42 0.00s用户0.00s系统69％cpu 0.004总计

％时间./a.out 42

1564058296

./a.out 42 0.00s用户0.00s系统56％cpu 0.006总计

％g -O2 test.cc

％./a.out 42#无限递归

^ C

% clang++ --version 
clang version 3.3 (tags/RELEASE_33/final) 
Target: x86_64-apple-darwin12.4.0 
Thread model: posix 
% g++ --version 
i686-apple-darwin11-llvm-g++-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00) 
Copyright (C) 2007 Free Software Foundation, Inc. 
This is free software; see the source for copying conditions.  There is NO 
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 

那么它是bug还是clang的功能？

解决方法:

尽管g和clang都能够编译C 98和C 11代码,但是clang从一开始就被设计为C 11编译器,并且在其DNA中嵌入了一些C 11行为(可以这么说).

有了C 11,C标准就可以识别线程了,这意味着现在有了一些特定的线程行为.特别是1.10 / 24指出：

The implementation may assume that any thread will eventually do one of the following:

— terminate,

— make a call to a library I/O function,

— access or modify a volatile object, or

— perform a synchronization operation or an atomic operation.

[Note: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. — end note ]

而这正是clang在优化时正在做的事情.可以看到该函数没有副作用,即使没有终止也将其删除.

标签：c,clang-2,tail-recursion
来源： https://codeday.me/bug/20191013/1906477.html