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Subtle differences in efficiency between ++i and i++

Popularity：857 ℃/2024-09-20 20:20:57

In some specific uses, thei++ It may be possible to store the original value in an intermediate amount to be used, see below the assembly code of the program in question (using thegcc ）。

  • i++ Source program:
#include <>

int main(){

    int i = 1;

    printf("%d\n", i++);

    return 0;

}
  • i++ Compilation:
Main:

.LFB0:
    .cfi_startproc
    endbr64
    pushq %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq %rsp, %rbp
    .cfi_def_cfa_register 6
    subq $16, %rsp
    movl $1, -4(%rbp) //commander-in-chief (military)iassign1.
    movl -4(%rbp), %eax //commander-in-chief (military)iThe original value of is transferred to the intermediate variable
    leal 1(%rax), %edx //commander-in-chief (military)%edxassigni+1
    movl %edx, -4(%rbp)//iA change in the value of
    movl %eax, %esi //verticalizationiThe print operation is performed on the value of the
    leaq .LC0(%rip), %rax
    movq %rax, %rdi
    movl $0, %eax
    call printf@PLT
    movl $0, %eax
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc

Look at the same operation again++i

  • ++i of the source program:
#include <>

int main(){

    int i = 1;

    printf("%d\n", ++i);

}
  • ++i Compilation:
Main:

.LFB0:
    .cfi_startproc
    endbr64
    pushq %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq %rsp, %rbp
    .cfi_def_cfa_register 6
    subq $16, %rsp
    movl $1, -4(%rbp) //treat (sb a certain way)iassign。
    addl $1, -4(%rbp) //iThe value of plus one。
    movl -4(%rbp), %eax //treat (sb a certain way)iThe print operation is performed on the value of the。
    movl %eax, %esi
    leaq .LC0(%rip), %rax
    movq %rax, %rdi
    movl $0, %eax
    call printf@PLT
    movl $0, %eax
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
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