【编译】gcc -O2 编译flag里面做了什么

本文只从结果进行表述，详细原因请参考：记一次gcc -O2大幅度增加binary size的问题_电影旅行敲代码的博客-CSDN博客

用live555进行了一个测试，正常代码逻辑为以下
<1> -> <2> -> <3>

不设置-O   flag，调用堆栈为<1> -> <2> -> <3>
    编译出的文件size
    $ ll live555/libBasicUsageEnvironment.so.1 
    -rwxrwxr-x 1 dljx dljx 119256 Apr 14 17:36 live555/libBasicUsageEnvironment.so.1
调用堆栈：
#1  0x00007ffff30c8a69 in AlarmHandler::handleTimeout (this=0x7fffe4001230) at BasicTaskScheduler0.cpp:34
#2  0x00007ffff30caae4 in DelayQueue::handleAlarm (this=0x66f1f8) at DelayQueue.cpp:187
#3  0x00007ffff30c9a8c in BasicTaskScheduler::SingleStep (this=0x66f1f0, maxDelayTime=0) at BasicTaskScheduler.cpp:212
#4  0x00007ffff30c82c6 in BasicTaskScheduler0::doEventLoop (this=0x66f1f0, watchVariable=0x66f1d0 "") at BasicTaskScheduler0.cpp:80

设置-O2 flag，调用堆栈为<1> -> <3>
    编译出的文件size还变大
    $ ll live555/libBasicUsageEnvironment.so.1 
    -rwxrwxr-x 1 dljx dljx 131824 Apr 14 17:23 live555/libBasicUsageEnvironment.so.1
调用堆栈：
#1  0x00007ffff30c9d8b in AlarmHandler::handleTimeout (this=0x7fffe4001230) at BasicTaskScheduler0.cpp:34
#2  0x00007ffff30ca5dd in BasicTaskScheduler::SingleStep (this=0x66f1f0, maxDelayTime=<optimized out>) at BasicTaskScheduler.cpp:212
#3  0x00007ffff30c96dc in BasicTaskScheduler0::doEventLoop (this=0x66f1f0, watchVariable=0x66f1d0 "") at BasicTaskScheduler0.cpp:80

代码块如下：

<1>

void BasicTaskScheduler::SingleStep(unsigned maxDelayTime) {
 ....
  // Also handle any delayed event that may have come due.
  fDelayQueue.handleAlarm();    // call <2>
}

<2>

void DelayQueue::handleAlarm() {
  if (head()->fDeltaTimeRemaining != DELAY_ZERO) synchronize();

  if (head()->fDeltaTimeRemaining == DELAY_ZERO) {
    // This event is due to be handled:
    DelayQueueEntry* toRemove = head();
    removeEntry(toRemove); // do this first, in case handler accesses queue

    toRemove->handleTimeout();  // call <3>
  }
}

<3>

 virtual void AlarmHandler::handleTimeout() {
    (*fProc)(fClientData);
    DelayQueueEntry::handleTimeout();
  }

最后

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