lang无限尾递归优化

问题描述：

#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

% clang++ -O2 test.cc

％time ./a.out 42

% time ./a.out 42

1490723512

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

./a.out 42 0.00s user 0.00s system 69% cpu 0.004 total

％time ./a.out 42

% time ./a.out 42

1564058296

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

./a.out 42 0.00s user 0.00s system 56% cpu 0.006 total

％g ++ -O2 test.cc

% g++ -O2 test.cc

％./a.out 42＃无限递归

% ./a.out 42 #infinte recursion

^ 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？

So is it a bug or a feature of clang++?

答

尽管g ++和clang ++都能够编译C ++ 98和C ++ 11代码，但是clang ++最初是作为C ++ 11编译器设计的，并且具有（可以这么说）一些C ++ 11行为嵌入其DNA中。

While both g++ and clang++ are able to compile C++98 and C++11 code, clang++ was designed from the start as a C++11 compiler and has some C++11 behaviors embedded in its DNA (so to speak).

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状态：

With C++11 the C++ standard became thread aware, and that means that now there are some specific thread behavior. In particular 1.10/24 states:

该实现可能假定任何线程最终都会执行以下操作之一：

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

-终止，

-调用库I / O函数，

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

-访问或修改易失性对象，或者

— access or modify a volatile object, or

-执行同步操作或原子操作。

— perform a synchronization operation or an atomic operation.

[注意：这旨在允许编译器进行转换，例如删除空循环，即使无法证明终止也是如此。 —尾注]

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

这正是clang ++在优化时所做的。它看到该函数没有副作用，并且即使没有终止也将其删除。

And that is precisely what clang++ is doing when optimizing. It sees that the function has no side effects and removes it even if it does not terminate.

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