gcc optimization of struct assignment - gcc

Question

Here is my example code:
struct AAA {
union{
struct{
int a;
int b;
};
long A;
};
union{
struct{
short c;
char d;
char e;
};
int B;
};
} __attribute__((packed));
void fun1(struct AAA *aaa){
aaa->a = 1;
aaa->b = 2;
aaa->c = 3;
aaa->d = 4;
aaa->e = 5;
}
void fun2(struct AAA *aaa){
aaa->A = (2L<<32)+1;
aaa->B = (5 << 24) + (4<<16) + 3;
}
When I compile it to asm code using gcc 5.4.0, I got:
fun1:
.LFB0:
.cfi_startproc
movl $3, %eax
movl $1, (%rdi)
movl $2, 4(%rdi)
movw %ax, 8(%rdi)
movb $4, 10(%rdi)
movb $5, 11(%rdi)
ret
.cfi_endproc
.LFE0:
.size fun1, .-fun1
.section .text.unlikely
.LCOLDE0:
.text
.LHOTE0:
.section .text.unlikely
.LCOLDB1:
.text
.LHOTB1:
.p2align 4,,15
.globl fun2
.type fun2, #function
fun2:
.LFB1:
.cfi_startproc
movabsq $8589934593, %rax
movl $84148227, 8(%rdi)
movq %rax, (%rdi)
ret
.cfi_endproc
And when I compile it with gcc 7.3.0, I got
fun1:
.LFB0:
.cfi_startproc
movabsq $8589934593, %rax
movl $84148227, 8(%rdi)
movq %rax, (%rdi)
ret
.cfi_endproc
.LFE0:
.size fun1, .-fun1
.p2align 4,,15
.globl fun2
.type fun2, #function
fun2:
.LFB1:
.cfi_startproc
movabsq $8589934593, %rax
movl $84148227, 8(%rdi)
movq %rax, (%rdi)
ret
.cfi_endproc
Both using -O3 option. The difference is obvious. Newer versions of gcc optimizes fun1 just like fun2.
Is fun2 really faster than fun1 when generating by gcc 5.4.0?
I have some old projects and they are compiled using even older versions of gcc (4.x), and I found many similar code like my example. If I want to do optimization, is it a good idea to change fun1 to fun2? I can't update gcc for now.

Answer 1

Assuming these programs are running on modern CPU architectures, the difference would be in the measure of nanoseconds.
Unless your code consists mostly of these assignments, and you really need to squeeze out the tiny bit of performance, I would keep it the same as fun1 for better readability and maintainability.