The most surprising UB-based wrong optimization to me, ever, was that the
following program would crash, instead of looping infinitely, when compiled
by certain versions of Clang:

int main()
{
    for (;;) {
    }
}

Of course, I understand the reason now (and aware that it is now "fixed"),
and I am really trying hard to justify this optimization when teaching
people about undefined behaviour. Unfortunately, I have not found a case
where a compiler can do loop fusion, which I suppose was a reason for the
forward progress guarantee. (On the contrary, I have found that loop
fission is more common, supposedly beneficial due to cache locality and
vectorization.)

I am wondering what are the real-world benefits of the forward progress
guarantee today.... They probably existed, but are they still there? (If
not, should we ...?)

Best regards,

-- 
Yongwei Wu
URL: http://wyw.dcweb.cn/