Difference at assembly level of printf from cstdio and iostream - c++

Question

So this question is just out of curiosity.
I have some tiny program:
#include <some_header>
void print(){ printf("abc"); } // don't care about main, I'm not gonna run it
Then I compiled it to assembly, with once some_header=>iostream and another time some_header=>cstdio with gcc.godbolt.org (6.1 for x86_64) with -O3 -pedantic -std=c++14. Look at this:
.LC0:
.string "abc"
print(): (iostream) or (both included)
movl $.LC0, %edi
xorl %eax, %eax
jmp printf
subq $8, %rsp
movl std::__ioinit, %edi
call std::ios_base::Init::Init()
movl $__dso_handle, %edx
movl std::__ioinit, %esi
movl std::ios_base::Init::~Init(), %edi
addq $8, %rsp
jmp __cxa_atexit
print(): (cstdio)
movl $.LC0, %edi
xorl %eax, %eax
jmp printf
There's a significant difference between them, and they're identical for the first three lines, so why do iostream need such amount of code to clean up or what are those lines just doing? OR just to say that godbolt is unreliable upon this task?
Also it seems that standard doesn't guarantee that printf is accessible from iostream, should this be relied upon?

Answer 1

Your print function compiles to a pretty much the same assembly code in both cases.
The additional lines you see are to initialise and de-initialise iostream library. You may see that clearly if you remove the optimisation flag -O3.
Here is a complete listing with iostream included and optimisation switched off.
std::piecewise_construct:
.zero 1
.LC0:
.string "abc"
print():
pushq %rbp
movq %rsp, %rbp
movl $.LC0, %edi
movl $0, %eax
call printf
nop
popq %rbp
ret
__static_initialization_and_destruction_0(int, int):
pushq %rbp
movq %rsp, %rbp
subq $16, %rsp
movl %edi, -4(%rbp)
movl %esi, -8(%rbp)
cmpl $1, -4(%rbp)
jne .L4
cmpl $65535, -8(%rbp)
jne .L4
movl std::__ioinit, %edi
call std::ios_base::Init::Init()
movl $__dso_handle, %edx
movl std::__ioinit, %esi
movl std::ios_base::Init::~Init(), %edi
call __cxa_atexit
.L4:
nop
leave
ret
pushq %rbp
movq %rsp, %rbp
movl $65535, %esi
movl $1, %edi
call __static_initialization_and_destruction_0(int, int)
popq %rbp
ret