> On Tue, 21 Feb 2017, Steffen Nurpmeso wrote:
> ...
> > But mind you, it is true that i still think it is funny that this
> > happened on a BSD system, the origin of bcopy(3). To me memcpy(3) never
> > has been anything but an optimization for cases where you know it is
> > save, so that the tests, the move to the end to start there etc., can be
> > avoided. This was at least nine (9) cycles iirc on the above CPUs that
> > can be saved, and that almost sufficient to copy a small string! Ciao.
>
> The catch is that that the standard *doesn't* say "memcpy() copies the
> buffer when they don't overlap or when the target is before the source".
> It says "memcpy() copies the buffer if they don't overlap; the behavior is
> not defined (i.e., *anything* can happen) if they overlap in any way".
>
> Compilers now are saying "well, undefined behavior includes doing nothing
> or acting like the arguments were totally different; I will assume that
> this code path *cannot be reached* with arguments that have undefined
> behavior". For example, it would be a totally legal optimization for a
> compiler to transform this:
> memcpy(p, p+1, n);
> to
> if (n)
> p[0] = p[1];
>
> The memcpy() behavior is only defined if there's no overlap, and overlap
> occurs if n>1, ergo the behavior is only defined if n is zero or one.
> Indeed, the compiler could then optimize the code *before* this memcpy()
> call on the assumption that _that_ code could only leave n set to zero or
> one!
>
>
> If your code triggers the memcpy() log+abort, then the code only works
> elsewhere because the compiler isn't smart enough. Yet.
How about undefined behaviour such as the following:
if (backwards) {
fprintf(stderr, " ----------\n");
fprintf(stderr, " / \\\n");
fprintf(stderr, " / REST \\\n");
fprintf(stderr, " / IN \\\n");
fprintf(stderr, " / PEACE \\\n");
fprintf(stderr, " / \\\n");
fprintf(stderr, " | Steffen |\n");
fprintf(stderr, " | 0 Au |\n");
fprintf(stderr, " | killed by a |\n");
fprintf(stderr, " | backward |\n");
fprintf(stderr, " | memcpy |\n");
fprintf(stderr, " | 2017 |\n");
fprintf(stderr, " *| * * * | *\n");
fprintf(stderr, " _________)/\\\\_//(\\/(/\\)/\\//\\/|_)_______\n");
reboot(RB_NOSYNC);
abort();
}
