What does this x86-64 addq instruction mean, which only have one operand? (From CSAPP book 3rd Edition)

Question

In the following instructions, how does the addq work? It only has one operand, the book claims that it increments %rdx, but %rdx is not in this instruction. I am so confused...

This is from the book Computer Systems A Programmers Perspective, 3rd Edition.

Answer 1

As @Jester pointed out in the comment. It is indeed an error. I actually typed in the program and compiled it using gcc on linux. Below is the results.

C program: badcnt.c

/*
 * badcnt.c - An improperly synchronized counter program
 */
#include <stdlib.h>
#include <pthread.h>
#include <stdio.h>

void *thread(void *vargp);  /* Thread routine prototype */

/* Global shared variable */
volatile int cnt = 0; /* Counter */

int main(int argc, char **argv)
{
  int niters;
  pthread_t tid1, tid2;

  /* Check input argument */
  if (argc != 2) {
    printf("usage: %s <niters>\n", argv[0]);
    exit(0);
  }
  niters = atoi(argv[1]);

  /* Create threads and wait for them to finish */
  pthread_create(&tid1, NULL, thread, &niters);
  pthread_create(&tid2, NULL, thread, &niters);
  pthread_join(tid1, NULL);
  pthread_join(tid2, NULL);

  /* Check result */
  if (cnt != (2 * niters))
    printf("BOOM! cnt=%d\n", cnt);
  else
    printf("OK cnt=%d\n", cnt);
  exit(0);
}

/* Thread routine */
void *thread(void *vargp)
{
  int i, niters = *((int *)vargp);

  for (i = 0; i < niters; i++)
    cnt++;

  return NULL;
}

Compile using gcc 6.3.0

$ gcc -pthread -Og -S badcnt.c

Below is the contents in badcnt.s

        .file   "badcnt.c"
        .text
        .globl  thread
        .type   thread, @function
thread:
.LFB20:
        .cfi_startproc
        movl    (%rdi), %ecx
        movl    $0, %edx
        jmp     .L2
.L3:
        movl    cnt(%rip), %eax
        addl    $1, %eax
        movl    %eax, cnt(%rip)
        addl    $1, %edx
.L2:
        cmpl    %ecx, %edx
        jl      .L3
        movl    $0, %eax
        ret
        .cfi_endproc
.LFE20:
        .size   thread, .-thread
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "usage: %s <niters>\n"
.LC1:
        .string "BOOM! cnt=%d\n"
.LC2:
        .string "OK cnt=%d\n"
        .text
        .globl  main
        .type   main, @function
main:
.LFB19:
        .cfi_startproc
        subq    $40, %rsp
        .cfi_def_cfa_offset 48
        cmpl    $2, %edi
        je      .L5
        movq    (%rsi), %rsi
        movl    $.LC0, %edi
        movl    $0, %eax
        call    printf
        movl    $0, %edi
        call    exit
.L5:
        movq    8(%rsi), %rdi
        movl    $10, %edx
        movl    $0, %esi
        call    strtol
        movl    %eax, 28(%rsp)
        leaq    28(%rsp), %rcx
        movl    $thread, %edx
        movl    $0, %esi
        leaq    16(%rsp), %rdi
        call    pthread_create
        leaq    28(%rsp), %rcx
        movl    $thread, %edx
        movl    $0, %esi
        leaq    8(%rsp), %rdi
        call    pthread_create
        movl    $0, %esi
        movq    16(%rsp), %rdi
        call    pthread_join
        movl    $0, %esi
        movq    8(%rsp), %rdi
        call    pthread_join
        movl    28(%rsp), %eax
        addl    %eax, %eax
        movl    cnt(%rip), %edx
        cmpl    %edx, %eax
        je      .L6
        movl    cnt(%rip), %esi
        movl    $.LC1, %edi
        movl    $0, %eax
        call    printf
.L7:
        movl    $0, %edi
        call    exit
.L6:
        movl    cnt(%rip), %esi
        movl    $.LC2, %edi
        movl    $0, %eax
        call    printf
        jmp     .L7
        .cfi_endproc
.LFE19:
        .size   main, .-main
        .globl  cnt
        .bss
        .align 4
        .type   cnt, @object
        .size   cnt, 4
cnt:
        .zero   4
        .ident  "GCC: (GNU) 6.3.0"
        .section        .note.GNU-stack,"",@progbits

So it confirms the mistake of the book.