c++ - GCC -Wuninitialized/-Wmaybe-uninitialized 问题

Question

我在使用 gcc-4.7 (Ubuntu/Linaro 4.7.2-11precise2) 4.7.2 时遇到了一个非常奇怪的问题。我无法在没有警告的情况下编译以下有效代码:

extern void dostuff(void);

int test(int arg1, int arg2)
{
    int ret;

    if (arg1) ret = arg2 ? 1 : 2;

    dostuff();

    if (arg1) return ret;

    return 0;
}

编译选项和输出:

$ gcc-4.7 -o test.o -c -Os test.c -Wall
test.c: In function ‘test’:
test.c:5:6: warning: ‘ret’ may be used uninitialized in this function [-Wmaybe-uninitialized]

但是，以下代码编译时没有警告(尽管汇编效率稍低):

extern void dostuff(void);

int test(int arg1, int arg2)
{
    int ret;

    if (arg1 && arg2) ret = 1;
    if (arg1 && !arg2) ret = 2;

    dostuff();

    if (arg1) return ret;

    return 0;
}

我有点卡住了，正在考虑这是一个编译器错误。有什么想法吗？

Answer 1

这确实是 gcc 中的一个已知问题。
gcc 因报告 incorrect uninitialized variables 而臭名昭著。
不足之处已及时指出，并有克服不足的主动权:
Better Uninitialized Warnings :

The GNU Compiler Collection warns about the use of uninitialized variables with the option -Wuninitialized. However, the current implementation has some perceived shortcomings. On one hand, some users would like more verbose and consistent warnings. On the other hand, some users would like to get as few warnings as possible. The goal of this project is to implement both possibilities while at the same time improving the current capabilities.

该计划旨在提供更好的警告，并引用了与您的案例相似的示例案例。相关部分是:

What an user understands as a false positive may be different for the particular user. Some users are interested in cases that are hidden because of actions of the optimizers combined with the current environment. However, many users aren't, since that case is hidden because it cannot arise in the compiled code. The canonical example is

int x;
if (f ())
     x = 3;
return x;

where 'f' always return non-zero for the current environment, and thus, it may be optimized away. Here, a group of users would like to get an uninitialized warning since 'f' may return zero when compiled elsewhere. Yet, other group of users would consider spurious a warning about a situation that cannot arise in the executable being compiled.