http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57230
--- Comment #8 from Zack Weinberg <zackw at panix dot com> ---
(In reply to Jakub Jelinek from comment #6)
> We have just one strlen pass instance, and even if we optimize the first
> strlen
> there, having strlen pass duplicate constant propagation functionality just
> to handle this weird testcase (storing strlen sizes inside of characters is
> nothing common) doesn't make sense.
I think I gave the wrong impression there; I was trying to exhibit a more
comprehensive *correctness* test for this transformation, using strlen() as a
convenient-to-hand function known in context to return a nonzero value. Lemme
try again.
extern __SIZE_TYPE__ strlen(const char *);
extern int rand(void);
extern void abort(void) __attribute__((noreturn));
#define assert(x) do { if (!(x)) abort(); } while (0)
/* Returns a value that might or might not be zero. */
static inline char maybe_zero() { return rand() & 0x10; }
/* Returns an unknown but definitely-nonzero value. */
static inline char nonzero() { return (rand() & 0x10) | 0x01; }
int main(void)
{
char s[] = "abc\0def";
/* These operations do not change the length of the string. */
s[0] = nonzero(); assert(strlen(s) == 3);
s[1] = nonzero(); assert(strlen(s) == 3);
s[2] = nonzero(); assert(strlen(s) == 3);
s[3] = '\0'; assert(strlen(s) == 3);
s[4] = '\0'; assert(strlen(s) == 3);
s[4] = 'd'; assert(strlen(s) == 3);
/* These operations change the length of the string, to a
value (in principle) computable at compile time. */
s[3] = nonzero(); assert(strlen(s) == 7);
s[5] = '\0'; assert(strlen(s) == 5);
s[2] = '\0'; assert(strlen(s) == 2);
s[2] = 'c'; assert(strlen(s) == 5);
s[5] = nonzero(); assert(strlen(s) == 7);
/* This operation requires runtime recalculation of the
length of the string. */
s[4] = maybe_zero();
assert(strlen(s) == (s[4] ? 7 : 4));
return 0;
}
Of all of that, the most important thing IMHO (given the original bug report)
is to verify that writing '\0' at a nonzero *offset* within a string does not
cause a subsequent strlen() to report the length of the string as zero.
