clang infinite tail recursion optimization

Question

#include  

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

int main(int argc,char * argv[]){ 
     if(argc != 2){ 
         return 1; 
     } 
     std::co         


        

Answer 1

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).

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:

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 ]

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.