Richard Smith <richardsmith_at_[hidden]> writes:

| On Fri, Oct 25, 2013 at 1:10 PM, Jeffrey Yasskin <jyasskin_at_[hidden]> wrote:
|
| On Fri, Oct 25, 2013 at 12:50 PM, Jens Maurer <Jens.Maurer_at_[hidden]> wrote:
| > On 10/25/2013 09:36 PM, John Regehr wrote:
| >>> What reason do you have to believe that crypto is using any signed
| >>> arithmetic? I would not.
| >>
| >> Here's an example that's at least slightly interesting, from the latest
| >> version of LibTomCrypt:
| >>
| >> kappa[i] =
| >> (key[pos ] << 24) ^
| >> (key[pos + 1] << 16) ^
| >> (key[pos + 2] << 8) ^
| >> (key[pos + 3] );
| >>
| >> key is a pointer to unsigned char. Of course, the array element becomes
| >> signed after promotion. The shift by 24 then executes an undefined
| >> behavior whenever the shifted value is >127.
| >>
| >> So the interesting thing is that the developer is basically doing things
| >> right and getting hosed by the arithmetic conversions.
| >
| > If I'm reading 5p10 correctly, this should help (and is consistently
| > expressing intent):
| >
| > kappa[i] =
| > (key[pos ] << 24u) ^
| > (key[pos + 1] << 16u) ^
| > (key[pos + 2] << 8u) ^
| > (key[pos + 3] );
| >
| > Jens
|
| Nope: [expr.shift]p1 says, "The type of the result is that of the
| promoted left operand."
|
|
| I believe John is correct. 5.8/1 says we perform integral promotions on the
| operands, not the usual arithmetic conversions. Clang, g++, and EDG agree --
| the type of (unsigned char)0 << 0u is int.

Yes, John correctly pointed out the integral promotion, and yes the type
of the expression you show is 'int' -- and it has been so for decades.

|
| However, I think p2 saves our intrepid developer in C++14: "Otherwise,
| if E1 has a signed type and non-negative value, and E1 × 2^E2 is
| representable in the corresponding unsigned type of the result type,
| then that value, converted to the result type, is the
| resulting value;"
|
|
| We gave this defined behavior as a DR, so I view this code has having de facto
| defined behavior in C++11 and C++98 too. But it's UB in C.
|
| In other words, we've already fixed this one (for some value of "fixed").