莫名奇妙的 -Wunused-but-set-variable 误报

polejo

1. static unsigned int
   
2. ip6t_mangle_out(struct sk_buff *skb, const struct net_device *out)
   
3. {
   
4.         unsigned int ret;
   
5.         struct in6_addr saddr, daddr;
   
6.         u_int8_t hop_limit;
   
7.         u_int32_t flowlabel, mark;
   
8. 
   
9. 
   
10.         /* save source/dest address, mark, hoplimit, flowlabel, priority,  */
    
11.         memcpy(&saddr, &ipv6_hdr(skb)->saddr, sizeof(saddr));
    
12.         memcpy(&daddr, &ipv6_hdr(skb)->daddr, sizeof(daddr));
    
13.         mark = skb->mark;
    
14.         hop_limit = ipv6_hdr(skb)->hop_limit;
    
15. 
    
16.         /* flowlabel and prio (includes version, which shouldn't change either */
    
17.         flowlabel = *((u_int32_t *)ipv6_hdr(skb));
    
18. 
    
19.         ret = ip6t_do_table(skb, NF_INET_LOCAL_OUT, NULL, out,
    
20.                             dev_net(out)->ipv6.ip6table_mangle);
    
21. 
    
22.         if (ret != NF_DROP && ret != NF_STOLEN &&
    
23.             (memcmp(&ipv6_hdr(skb)->saddr, &saddr, sizeof(saddr)) ||
    
24.              memcmp(&ipv6_hdr(skb)->daddr, &daddr, sizeof(daddr)) ||
    
25.              skb->mark != mark ||
    
26.              ipv6_hdr(skb)->hop_limit != hop_limit))
    
27.                 return ip6_route_me_harder(skb) == 0 ? ret : NF_DROP;
    
28. 
    
29.         return ret;
    
30. }

复制代码

编译结果
  CC [M]  net/ipv6/netfilter/ip6table_mangle.o
net/ipv6/netfilter/ip6table_mangle.c: In function 'ip6t_mangle_out':
net/ipv6/netfilter/ip6table_mangle.c:39:12: warning: variable 'flowlabel' set but not used [-Wunused-but-set-variable]
  u_int32_t flowlabel, mark;

明明flowlabel已经use了，为什么编译还会提示这个，实际上我的内核编译时有非常多这个告警，都属于这种情况？莫名其妙啊


########@@@@@@########## KBUILD_CFLAGS = -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -I. -DBCMDRIVER -Dlinux -O2  -ffreestanding -fno-dwarf2-cfi-asm -mabi=apcs-gnu      -msoft-float -Uarm -ggdb   -fomit-frame-pointer
########@@@@@@########## HOSTCFLAGS = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer
########@@@@@@########## CFLAGS =
########@@@@@@########## EXTRA_CFLAGS =