On Mon, Mar 12, 2018 at 13:32 Lawrence Crowl <Lawrence@crowl.org> wrote:
On 3/12/18, Myria <myriachan@gmail.com> wrote:
> The severity of the current situation is that I generally avoid signed
> integers if I intend to do any arithmetic on them whatsoever, lest the
> compiler decide to make demons come out of my nose.

So why not specify the option to turn on trapping?

> And even then, I'm not safe:
>
> std::uint16_t x = 0xFFFF;
> x *= x;  // undefined behavior on most modern platforms

How?  The C++ standard defines unsigned arithmetic as
modular arithmetic.

But that's the catch: it's double secret signed arithmetic.  The promotion rules of C, inherited by C++, state that on any arithmetic operation, integer types of rank less than int promote to int.  This promotion is regardless of signedness.

On a "typical modern platform", std::uint16_t is unsigned short.  That is of lesser rank than signed int, so it promotes to signed int on any arithmetic operation, resulting in the following:

int promoted_x = x;
x = static_cast<std::uint16_t>(promoted_x * promoted_x);

65535 * 65535 overflows a signed int on a typical 32-bit int platform, which is undefined behavior.


More importantly, what happens to your program when x*x < x?

The code that led me to finding this was a 16-bit variant of the FNV hash function, so it worked properly after the correct casts were added to allow the wrap.


> My code has to do silly things like this in order to safeguard against
> such potential compiler abuses:
>
> typedef decltype(std::uint16_t() + 0u) promoted_uint16;

How does this typedef help?

Arithmetic between any unsigned type and unsigned int results in a type of at least the first type's size that cannot be promoted to a signed type in arithmetic with other unsigned types.

It's like uint16_fast_t, except that it guarantees that all operations performed will be well-defined to wrap, with just the inconvenience of potentially being larger than the actual intended type.


> I would be happy if an option like -fwrapv were supported everywhere,
> but Visual Studio doesn't have such an option, and Microsoft has
> already denied requests for such an option to be implemented.

What about -ftrapv?

If I were working on something where signed int overflow were a problem, then sure, in debug builds.  In release builds, I wouldn't use that for performance reasons (except where it's mostly free, like on MIPS).

Melissa