On Wed, Mar 05, 2025 at 09:50:27PM +0000, David Laight wrote: > On Wed, 5 Mar 2025 21:56:22 +0200 > Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote: > > On Thu, Mar 06, 2025 at 02:17:18AM +0900, Vincent Mailhol wrote: > > > On 06/03/2025 at 00:48, Andy Shevchenko wrote: > > > > On Wed, Mar 05, 2025 at 11:48:10PM +0900, Vincent Mailhol wrote: > > > >> On 05/03/2025 at 23:33, Andy Shevchenko wrote: > > > >>> On Wed, Mar 05, 2025 at 10:00:16PM +0900, Vincent Mailhol via B4 Relay wrote: ... > > > >>>> +#define BIT_U8(b) (BIT_INPUT_CHECK(u8, b) + (unsigned int)BIT(b)) > > > >>>> +#define BIT_U16(b) (BIT_INPUT_CHECK(u16, b) + (unsigned int)BIT(b)) > > > >>> > > > >>> Why not u8 and u16? This inconsistency needs to be well justified. > > > >> > > > >> Because of the C integer promotion rules, if casted to u8 or u16, the > > > >> expression will immediately become a signed integer as soon as it is get > > > >> used. For example, if casted to u8 > > > >> > > > >> BIT_U8(0) + BIT_U8(1) > > > >> > > > >> would be a signed integer. And that may surprise people. > > > > > > > > Yes, but wouldn't be better to put it more explicitly like > > > > > > > > #define BIT_U8(b) (BIT_INPUT_CHECK(u8, b) + (u8)BIT(b) + 0 + UL(0)) // + ULL(0) ? > > > > > > OK, the final result would be unsigned. But, I do not follow how this is > > > more explicit. > > > > > > Also, why doing: > > > > > > (u8)BIT(b) + 0 + UL(0) > > > > > > and not just: > > > > > > (u8)BIT(b) + UL(0) > > > > > > ? > > > > > > What is that intermediary '+ 0' for? > > > > > > I am sorry, but I am having a hard time understanding how casting to u8 > > > and then doing an addition with an unsigned long is more explicit than > > > directly doing a cast to the desired type. > > > > Reading this again, I think we don't need it at all. u8, aka unsigned char, > > will be promoted to int, but it will be int with a value < 256, can't be signed > > as far as I understand this correctly. > > The value can't be negative, but the type will be a signed one. Yes, that's what I mentioned above: "int with the value < 256". > Anything comparing types (and there are a few) will treat it as signed. > It really is bad practise to even pretend you can have an expression > (rather that a variable) that has a type smaller than 'int'. > It wouldn't surprise me if even an 'a = b' assignment promotes 'b' to int. We have tons of code with u8/u16, what you are proposing here is like "let's get rid of those types and replace all of them by int/unsigned int". We have ISAs that are byte-oriented despite being 32- or 64-bit platforms. > So it is even questionable whether BIT8() and BIT16() should even exist at all. The point is to check the boundaries and not in the returned value per se. > There can be reasons to return 'unsigned int' rather than 'unsigned long'. > But with the type definitions that Linux uses (and can't really be changed) > you can have BIT32() that is 'unsigned int' and BIT64() that is 'unsigned long > long'. These are then the same on 32bit and 64bit. -- With Best Regards, Andy Shevchenko
