[Bug tree-optimization/49872] Missed optimization: Could coalesce neighboring memsets

["sgunderson at bigfoot dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49872

--- Comment #2 from sgunderson at bigfoot dot com 2011-07-28 10:09:51 UTC ---
I'm not sure if I've seen exactly this construction in real-world code, but
I've certainly seen examples of the hybrid I sketched out (looking at one was
what motivated me to file the bug), ie. something like:

struct S {
    int f[1024];
    int g;
};

void func(struct S* s)
{
    memset(s->f, 0, sizeof(s->f));
    s->g = 0;
}

which I would argue should be rewritten to

void func(struct S* s)
{
    memset(s->f, 0, sizeof(s->f) + sizeof(s->g));
}

I'd argue that programmers should not be doing this kind of optimization
themselves, since it's very prone to break when changing the structure,
especially as alignment etc. comes into play.