Does GCC optimize std::tie used only for readability?

Question

Suppose I have a std::tuple:

std::tuple<int,int,int,int> t = {1,2,3,4};

and I want to use std::tie just for readability purpose like that:

int a, b, c, d; // in real context these names would be meaningful
std::tie(a, b, c, d) = t;

vs. just using t.get<int>(0), etc.

Would a GCC optimize the memory use of this tuple or would it allocate additional space for a, b, c, d variables?

Answer 1

In this case I don't see any reason why not, under the as-if rule the compiler only has to emulate the observable behavior of the program. A quick experiment using godbolt:

#include <tuple>
#include <cstdio>

void func( int x1, int x2,int  x3, int x4)
{
  std::tuple<int,int,int,int> t{x1,x2,x3,x4};

  int a, b, c, d; // in real context these names would be meaningful
  std::tie(a, b, c, d) = t;

  printf( "%d %d %d %d\n", a, b, c, d ) ;
}

shows that gcc does indeed optimize it away:

func(int, int, int, int):
    movl    %ecx, %r8d
    xorl    %eax, %eax
    movl    %edx, %ecx
    movl    %esi, %edx
    movl    %edi, %esi
    movl    $.LC0, %edi
    jmp printf

On the other hand if you used a address of t and printed it out, we now have observable behavior which relies on t existing (see it live):

printf( "%p\n", static_cast<void*>(&t) );

and we can see that gcc no longer optimizes away the t:

movl    %esi, 12(%rsp)
leaq    16(%rsp), %rsi
movd    12(%rsp), %xmm1
movl    %edi, 12(%rsp)
movl    $.LC0, %edi
movd    12(%rsp), %xmm2
movl    %ecx, 12(%rsp)
movd    12(%rsp), %xmm0
movl    %edx, 12(%rsp)
movd    12(%rsp), %xmm3
punpckldq   %xmm2, %xmm1
punpckldq   %xmm3, %xmm0
punpcklqdq  %xmm1, %xmm0

At the end of the day you need to look at what the compiler generates and profile your code, in more complicated cases it may surprise you. Just because the compiler is allowed to do certain optimizations does not mean it will. I have looked at more complicated cases where the compiler does not do what I would expect with std::tuple. godbolt is a very helpful tool here, I can not count how many optimizations assumptions I used to have that were upended by plugging in simple examples into godbolt.

Note, I typically use printf in these examples because iostreams generates a lot of code that gets in the way of the example.