[Bug tree-optimization/81772] Compile-time snprintf optimization
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81772 Andrew Pinski changed: What|Removed |Added Severity|normal |enhancement
[Bug tree-optimization/81772] Compile-time snprintf optimization
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81772 Martin Sebor changed: What|Removed |Added See Also||https://gcc.gnu.org/bugzill ||a/show_bug.cgi?id=88814 --- Comment #4 from Martin Sebor --- I forgot about this request. An alternate solution to the strncat trick is to use memccpy (bug 88814). I'm hoping to look into one or the other for GCC 10.
[Bug tree-optimization/81772] Compile-time snprintf optimization
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81772 Martin Sebor changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2017-08-09 Ever confirmed|0 |1 --- Comment #3 from Martin Sebor --- Thanks for the strncat suggestion. That seems quite doable. I'll try to find some time to look into it. I explored some of the other ideas when I implemented the GCC 7 sprintf optimization (-fprintf-return-value) and warnings (-Wformat-overflow and -Wformat-truncation) but they didn't seem applicable in enough situations (when all or at least some of the arguments were constant) to justify either the additional complexity or the space overhead.
[Bug tree-optimization/81772] Compile-time snprintf optimization
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81772
--- Comment #2 from Daniel Fruzynski ---
You probably could optimize snprintf(buf, size, "%s", str) into this:
if (size > 0)
{
buf[0] = 0;
strncat(buf, str, size - 1);
}
Other optimizations would require to generate more code aa you wrote. Some of
them probably could be applied, e.g. transforming first line to second:
sprintf(buf, "%s %d", "123", arg);
memcpy(buf, "123 ", 4); sprintf(buf+4, "%d", arg);
Dedicated string concatenation function with multiple args also would be
helpful to perform following transformation:
sprintf(buf, "Content-Type: %s/%s\n", str1, str2);
multistrcat(buf, "Content-Type: ", str1, "/", str2, "\n");
It is also possible to parse format string and create sprintf with series of
commands which will create the same result. Of course this will increase code
size like you wrote, so user would have to explicitly enable this optimization.
There are many factors there which must be taken into account, e.g. number of
format arguments, use of advanced formatting (e.g. setting field width or
precision), etc.
[Bug tree-optimization/81772] Compile-time snprintf optimization
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81772 Martin Sebor changed: What|Removed |Added CC||msebor at gcc dot gnu.org --- Comment #1 from Martin Sebor --- GCC 7 does implement some additional sprintf and snprintf optimizations but it doesn't go quite as far as you'd like it to. The trouble is that there is no suitable alternative function to call when the arguments aren't constant. I suppose in the non-constant case and for small enough positive n, snprintf(d, n, "%s", s) could be transformed into something like tmp = strlen (s); if (n <= tmp) tmp = n - 1; memcpy (d, s, tmp); d[tmp] = '\0'; trading off space (more code) for speed. But whether or not this would be a win would depend on the snprintf implementation. It seems to me that a better place to implement this optimization is in snprintf itself.
