[英]clang infinite tail recursion optimization
#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 %时间./a.out 42
1490723512 1490723512
./a.out 42 0.00s user 0.00s system 69% cpu 0.004 total ./a.out 42 0.00s用户0.00s系统69%cpu 0.004总计
% time ./a.out 42 %时间./a.out 42
1564058296 1564058296
./a.out 42 0.00s user 0.00s system 56% cpu 0.006 total ./a.out 42 0.00s用户0.00s系统56%cpu 0.006总计
% g++ -O2 test.cc %g ++ -O2 test.cc
% ./a.out 42 #infinte recursion %./a.out 42#无限递归
^C ^ 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.
So is it a bug or a feature of clang++? 那是clang ++的错误还是功能?
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). 虽然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,并且在其DNA中嵌入了一些C ++ 11行为(可以这么说) 。
With C++11 the C++ standard became thread aware, and that means that now there are some specific thread behavior. 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。 In particular 1.10/24 states:
特别是1.10 / 24指出:
The implementation may assume that any thread will eventually do one of the following:
该实现可以假定任何线程最终都将执行以下操作之一:
— terminate,
-终止,
— make a call to a library I/O function,
—调用库I / O函数,
— access or modify a volatile object, or
—访问或修改易失性对象,或
— perform a synchronization operation or an atomic operation.
—执行同步操作或原子操作。
[Note: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven.
[注:这旨在允许编译器转换,例如删除空循环,即使无法证明终止也是如此。 — end note ]
—尾注]
And that is precisely what clang++ is doing when optimizing. 而这正是clang ++在优化时所做的。 It sees that the function has no side effects and removes it even if it does not terminate.
可以看到该函数没有副作用, 即使没有终止也将其删除。
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