Compiler stops optimizing unused string away when adding characters - c++

Question

I am curious why the following piece of code:
#include <string>
int main()
{
std::string a = "ABCDEFGHIJKLMNO";
}
when compiled with -O3 yields the following code:
main: # #main
xor eax, eax
ret
(I perfectly understand that there is no need for the unused a so the compiler can entirely omit it from the generated code)
However the following program:
#include <string>
int main()
{
std::string a = "ABCDEFGHIJKLMNOP"; // <-- !!! One Extra P
}
yields:
main: # #main
push rbx
sub rsp, 48
lea rbx, [rsp + 32]
mov qword ptr [rsp + 16], rbx
mov qword ptr [rsp + 8], 16
lea rdi, [rsp + 16]
lea rsi, [rsp + 8]
xor edx, edx
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_create(unsigned long&, unsigned long)
mov qword ptr [rsp + 16], rax
mov rcx, qword ptr [rsp + 8]
mov qword ptr [rsp + 32], rcx
movups xmm0, xmmword ptr [rip + .L.str]
movups xmmword ptr [rax], xmm0
mov qword ptr [rsp + 24], rcx
mov rax, qword ptr [rsp + 16]
mov byte ptr [rax + rcx], 0
mov rdi, qword ptr [rsp + 16]
cmp rdi, rbx
je .LBB0_3
call operator delete(void*)
.LBB0_3:
xor eax, eax
add rsp, 48
pop rbx
ret
mov rdi, rax
call _Unwind_Resume
.L.str:
.asciz "ABCDEFGHIJKLMNOP"
when compiled with the same -O3. I don't understand why it does not recognize that the a is still unused, regardless that the string is one byte longer.
This question is relevant to gcc 9.1 and clang 8.0, (online: https://gcc.godbolt.org/z/p1Z8Ns) because other compilers in my observation either entirely drop the unused variable (ellcc) or generate code for it regardless the length of the string.

Answer 1

This is due to the small string optimization. When the string data is less than or equal 16 characters, including the null terminator, it is stored in a buffer local to the std::string object itself. Otherwise, it allocates memory on the heap and stores the data over there.
The first string "ABCDEFGHIJKLMNO" plus the null terminator is exactly of size 16. Adding "P" makes it exceed the buffer, hence new is being called internally, inevitably leading to a system call. The compiler can optimize something away if it's possible to ensure that there are no side effects. A system call probably makes it impossible to do this - by constrast, changing a buffer local to the object under construction allows for such a side effect analysis.
Tracing the local buffer in libstdc++, version 9.1, reveals these parts of bits/basic_string.h:
template<typename _CharT, typename _Traits, typename _Alloc>
class basic_string
{
// ...
enum { _S_local_capacity = 15 / sizeof(_CharT) };
union
{
_CharT _M_local_buf[_S_local_capacity + 1];
size_type _M_allocated_capacity;
};
// ...
};
which lets you spot the local buffer size _S_local_capacity and the local buffer itself (_M_local_buf). When the constructor triggers basic_string::_M_construct being called, you have in bits/basic_string.tcc:
void _M_construct(_InIterator __beg, _InIterator __end, ...)
{
size_type __len = 0;
size_type __capacity = size_type(_S_local_capacity);
while (__beg != __end && __len < __capacity)
{
_M_data()[__len++] = *__beg;
++__beg;
}
where the local buffer is filled with its content. Right after this part, we get to the branch where the local capacity is exhausted - new storage is allocated (through the allocate in M_create), the local buffer is copied into the new storage and filled with the rest of the initializing argument:
while (__beg != __end)
{
if (__len == __capacity)
{
// Allocate more space.
__capacity = __len + 1;
pointer __another = _M_create(__capacity, __len);
this->_S_copy(__another, _M_data(), __len);
_M_dispose();
_M_data(__another);
_M_capacity(__capacity);
}
_M_data()[__len++] = *__beg;
++__beg;
}
As a side note, small string optimization is quite a topic on its own. To get a feeling for how tweaking individual bits can make a difference at large scale, I'd recommend this talk. It also mentions how the std::string implementation that ships with gcc (libstdc++) works and changed during the past to match newer versions of the standard.

Answer 2

I was surprised the compiler saw through a std::string constructor/destructor pair until I saw your second example. It didn't. What you're seeing here is small string optimization and corresponding optimizations from the compiler around that.
Small string optimizations are when the std::string object itself is big enough to hold the contents of the string, a size and possibly a discriminating bit used to indicate whether the string is operating in small or big string mode. In such a case, no dynamic allocations occur and the string is stored in the std::string object itself.
Compilers are really bad at eliding unneeded allocations and deallocations, they are treated almost as if having side effects and are thus impossible to elide. When you go over the small string optimization threshold, dynamic allocations occur and the result is what you see.
As an example
void foo() {
delete new int;
}
is the simplest, dumbest allocation/deallocation pair possible, yet gcc emits this assembly even under O3
sub rsp, 8
mov edi, 4
call operator new(unsigned long)
mov esi, 4
add rsp, 8
mov rdi, rax
jmp operator delete(void*, unsigned long)

Answer 3

While the accepted answer is valid, since C++14 it's actually the case that new and delete calls can be optimized away. See this arcane wording on cppreference:
New-expressions are allowed to elide ... allocations made through replaceable allocation functions. In case of elision, the storage may be provided by the compiler without making the call to an allocation function (this also permits optimizing out unused new-expression).
...
Note that this optimization is only permitted when new-expressions are
used, not any other methods to call a replaceable allocation function:
delete[] new int[10]; can be optimized out, but operator
delete(operator new(10)); cannot.
This actually allows compilers to completely drop your local std::string even if it's very long. In fact - clang++ with libc++ already does this (GodBolt), since libc++ uses built-ins __new and __delete in its implementation of std::string - that's "storage provided by the compiler". Thus, we get:
main():
xor eax, eax
ret
with basically any-length unused string.
GCC doesn't do but I've recently opened bug reports about this; see this SO answer for links.