Tony Finch – blog

It seems selog tickles a live-code analysis bug in gcc. Here's a fairly minimal test case:

$ cat gccbug.c

struct sel {
        const char  *name;
        unsigned int enabled;
};

int selog_on(struct sel *);

#define selog_on(sel) \
        (sel.enabled == 0 \
                ? selog_on(&sel) \
                : sel.enabled != 42)

void dostuff(int *);

void gccbug1(void) {
        struct sel sel = { "test", 0 };
        if(selog_on(sel)) {
                dostuff(0);
        }
}

void gccbug2(void) {
        struct sel sel = { "test", 0 };
        if(selog_on(sel)) {
                int i;
                dostuff(&i);
        }
}


$ gcc -O -Wunreachable-code -c gccbug.c
gccbug.c: In function `gccbug2':
gccbug.c:25: warning: will never be executed
$ gcc --version
gcc --version
gcc (GCC) 3.4.4 [FreeBSD] 20050518

$ gcc -O -Wunreachable-code -c gccbug.c
gccbug.c: In function ‘gccbug2’:
gccbug.c:24: warning: will never be executed
gccbug.c: In function ‘gccbug1’:
gccbug.c:17: warning: will never be executed
$ gcc -O -Werror -Wunreachable-code -c gccbug.c
cc1: warnings being treated as errors
gccbug.c: In function ‘gccbug2’:
gccbug.c:24: warning: will never be executed
$ gcc --version
gcc (GCC) 4.1.2 20070115 (prerelease) (SUSE Linux)

$ gcc -O -Wunreachable-code -c gccbug.c
gccbug.c: In function 'gccbug2':
gccbug.c:24: warning: will never be executed
gccbug.c: In function 'gccbug1':
gccbug.c:17: warning: will never be executed
$ gcc -O -Werror -Wunreachable-code -c gccbug.c
cc1: warnings being treated as errors
gccbug.c: In function 'gccbug2':
gccbug.c:24: warning: will never be executed
$ gcc --version
gcc (GCC) 4.2.1 20070719  [FreeBSD]