带/不带-O2的g ++编译器

Question

我有以下源代码由g ++ 4.8.2编译：

g++ --std=c++11 testx.cpp -pthread  -lrt  -O2 -o testx.exe
g++ --std=c++11 testx.cpp -pthread  -lrt  -o testy.exe

layout.h：

#pragma once
typedef struct DBInfo_
{
    volatile int seqno ;
    char groupname[32] ;
    char action[32] ;
    int booklayer ;
} DBInfo ;
#define DBARRAYSIZE 100
static DBInfo  *conf;
#define STATE_FILE "/strategy2/test.shared"

testx.cpp：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/file.h>
#include <ctype.h>
#include <pthread.h>
#include <sys/stat.h>
#include <assert.h>
#include "layout.h"

int g_DBSharedMemIdx = 0 ;
volatile int iGlbErrorSeqLock = 0 ;

void create_shmem(void)
{
    int shm_fd;
    if((shm_fd = shm_open(STATE_FILE, (O_CREAT | O_EXCL | O_RDWR),
                       (S_IREAD | S_IWRITE))) > 0 ) {
        printf("O_CREAT | O_EXCL | O_RDWR \n"); /* We are the first instance */
    }
    else if((shm_fd = shm_open(STATE_FILE, (O_CREAT | O_RDWR),
                        (S_IREAD | S_IWRITE))) < 0)
    {
        printf("Could not create shm object. \n");
        exit( 0 ) ;
    }
    int iTotalByte = sizeof(DBInfo)*DBARRAYSIZE ;
    ftruncate(shm_fd, iTotalByte );
    conf =  (DBInfo*) mmap(0, iTotalByte , (PROT_READ | PROT_WRITE), MAP_SHARED, shm_fd, 0) ;
    if(conf  == MAP_FAILED)
    {
        printf(" mmap error ....\n")  ;
        exit( 0 ) ;
    }
    g_DBSharedMemIdx = 0 ;
    (conf+g_DBSharedMemIdx)->seqno = 0 ;
    strcpy( (conf+g_DBSharedMemIdx)->groupname,"" ) ;
    strcpy( (conf+g_DBSharedMemIdx)->action,"" ) ;
    (conf+g_DBSharedMemIdx)->booklayer = 0 ;    
}//create_shmem

int getDBInfo(DBInfo& pDBInfo)
{
    if( pDBInfo.seqno == (conf+g_DBSharedMemIdx)->seqno)
        return 0 ;
    volatile int ilocalseqno1 = 0 , ilocalseqno2 = 0 ;
    volatile int icnt=0;
    while( 1 ){
        icnt++ ;
        if( icnt >= 10000 ){
            iGlbErrorSeqLock = 1 ;
            break ;
        }
        ilocalseqno1 = (conf+g_DBSharedMemIdx)->seqno ;
        if( ilocalseqno1 % 2 ){
            printf("***************************************************************************** \n");
            continue;
        }
        strcpy( pDBInfo.action,(conf+g_DBSharedMemIdx)->action ) ;
        pDBInfo.booklayer = (conf+g_DBSharedMemIdx)->booklayer ;

        ilocalseqno2 = (conf+g_DBSharedMemIdx)->seqno ;
        if( ilocalseqno1 != ilocalseqno2 ){
            printf("***************************************************************************** \n");
            continue;
        }
        pDBInfo.seqno = ilocalseqno2 ;
        break ;
    } //while
    if( iGlbErrorSeqLock == 1 )
        return -1 ;
    return 1 ;
}//getDBInfo


void *ThreadONE(void *param)
{
    printf("TestThread...(%p)\n",param) ;
    pthread_detach(pthread_self());
    while( 1 ){
        (conf+g_DBSharedMemIdx)->seqno++ ;
        strcpy( (conf+g_DBSharedMemIdx)->groupname,"group1" ) ;
        strcpy( (conf+g_DBSharedMemIdx)->action,"RUN" ) ;
        (conf+g_DBSharedMemIdx)->booklayer = 3 ;
        if( (conf+g_DBSharedMemIdx)->seqno % 2 == 1 )
            (conf+g_DBSharedMemIdx)->seqno++ ;
        usleep( 1 ) ;
    }//while 
}//ThreadONE

void *ThreadTWO(void *param)
{
    DBInfo pDBInfo ;
    printf("TestThread...(%p)\n",param) ;
    pthread_detach(pthread_self());
    int icnt = 0 ;
    while( 1 ){
        int iret = getDBInfo(pDBInfo);
        if( iret < 0 ){
            printf("iGlbErrorSeqLock happened \n") ;
            assert( iGlbErrorSeqLock == 100 );
        }else if(iret == 1){
            icnt++ ;
            //printf("icnt=(%d)\n",icnt);
        }
        usleep( 1 ) ;
    }//while 
}//ThreadTWO

int main(int argc, char** argv)
{
    create_shmem();

    sleep( 1 ) ;
    pthread_t tid ;
    pthread_create(&tid , NULL, ThreadONE, (void*)(long)3);
    pthread_create(&tid , NULL, ThreadTWO, (void*)(long)3);
    while( 1 )
        sleep( 5 ) ;
} //main

testy.exe（在没有-O2的情况下编译）运行每秒会看到“**************”2次，

testx.exe（用-O2编译）运行会看到“***********”不太容易。

我有这个测试，因为在我的原始资源中，有时会调用函数 如果用-O2编译，像getDBInfo将被困在无限循环中， 没有-O2就可以了，所以我很好奇编译器在做什么 getDBInfo函数，根据它应该怎么做。