Re: Option to make unsigned->signed conversion always well-defined?

[Pedro Pedruzzi <pedro dot pedruzzi at gmail dot com>]

Em 05-10-2011 17:11, Ulf Magnusson escreveu:
> Hi,
> 
> I've been experimenting with different methods for emulating the
> signed overflow of an 8-bit CPU.

You would like to check whether a 8-bit signed addition will overflow or
not, given the two operands. Is that correct?

As you used the word `emulating', I am assuming that your function will
not run by the mentioned CPU.

Does this 8-bit CPU use two's complement representation?

> The method I've found that seems to
> generate the most efficient code on both ARM and x86 is
> 
> bool overflow(unsigned int a, unsigned int b) {
>     const unsigned int sum = (int8_t)a + (int8_t)b;
>     return (int8_t)sum != sum;
> }
> 
> (The real function would probably be 'inline', of course. Regs are
> stored in overlong variables, hence 'unsigned int'.)
> 
> Looking at the spec, it unfortunately seems the behavior of this
> function is undefined, as it relies on signed int addition wrapping,
> and that (int8_t)sum truncates bits. Is there some way to make this
> guaranteed safe with GCC without resorting to inline asm? Locally
> enabling -fwrap takes care of the addition, but that still leaves the
> conversion.

I believe the cast from unsigned int to int8_t is implementation-defined
for values that can't be represented in int8_t (e.g. 0xff). A kind of
`undefined behavior' as well.

I tried:

bool overflow(unsigned int a, unsigned int b) {
    const unsigned int sum = a + b;
    return ((a & 0x80) == (b & 0x80)) && ((a & 0x80) != (sum & 0x80));
}

But it is not as efficient as yours.

-- 
Pedro Pedruzzi