Using UseZtso I see this (with additional block comments):

  0x00007df6a3da2142: addw t4,t5,s2
  0x00007df6a3da2146: c.addi zero,0
  0x00007df6a3da2148: c.addi zero,0
  0x00007df6a3da214a: c.addi zero,0
  0x00007df6a3da214c: c.addi zero,0
  0x00007df6a3da214e: c.addi zero,0
 ;; B8: # out( B8 B9 ) <- in( B7 B8 ) Loop( B8-B8 inner main of N168 strip mined multiversion_fast) Freq: 1.00022e+06
 ;; membar_acquire
 ;; membar_acquire
 ;; membar_acquire
 ;; membar_release
  0x00007df6a3da2150: c.sw a5,124(a2)


It seem like the membars spits out a nop, but only in some cases.

This was my test program, I only see it the doit C2 compiled method AFIACT:
{code:title=C2java|borderStyle=solid}
public class C2 {
  static volatile int a = 1;
  static volatile int b = 1;

  public static void main(String argz[]) {
      for (int i = 0; i < 40000; i++) {
        dothat();
    }
  }
  
  public static void dothat() {
    for (int i = 0; i < 40000; i++) {
     doit();
        b = i%2;
        a = i%2;
    }
  }

  public static void doit() {
      int va = a;
      int vb = b;
      if (va == 1) b = 2;
      if (vb == 1) a = 2;
  }
}
{code}