On Sat, Jan 8, 2022 at 6:34 PM Liam Beguin <liambeguin@xxxxxxxxx> wrote: > On Wed, Dec 22, 2021 at 11:32:24PM +0200, Andy Shevchenko wrote: > > On Wed, Dec 22, 2021 at 9:59 PM Liam Beguin <liambeguin@xxxxxxxxx> wrote: > > > On Wed, Dec 22, 2021 at 08:56:12PM +0200, Andy Shevchenko wrote: > > > > On Wed, Dec 22, 2021 at 8:38 PM Liam Beguin <liambeguin@xxxxxxxxx> wrote: > > > > > On Wed, Dec 22, 2021 at 02:29:04PM +0200, Andy Shevchenko wrote: > > > > > > On Wed, Dec 22, 2021 at 5:47 AM Liam Beguin <liambeguin@xxxxxxxxx> wrote: ... > > > > > > > - tmp = 1 << *val2; > > > > > > > > > > > > At some point this should be BIT() > > > > > > > > Forgot to add, If it's 64-bit, then BIT_ULL(). > > > > > > > > > I'm not against changing this, but (to me at least) 1 << *val2 seems > > > > > more explicit as we're not working with bitfields. No? > > > > > > > > You may add a comment. You may use int_pow(), but it will be suboptimal. > > > > > > > > > > Rule of thumb (in accordance with C standard), always use unsigned > > > > > > value as left operand of the _left_ shift. > > > > > > > > > > Right, that makes sense! In practice though, since we'll most likely > > > > > never use higher bits of *val2 with IIO_VAL_FRACTIONAL_LOG2, would it be > > > > > enough to simply typecast? > > > > > > > > > > tmp = 1 << (unsigned int)*val2; > > > > > > > > No, it's about the _left_ operand. > > > > I haven't checked if tmp is 64-bit, then even that would be still wrong. > > > > > > Okay so your recommendation is to not use a left shift? > > > > No, I recommend not to use int type as a _leftside_ operand. > > BIT() / BIT_ULL() does a left shift anyway. > > Oh, got it. Sorry for misreading your message. > would something like this be good enough? > > s64 tmp; > u64 tmp2; > tmp2 = 1 << *val2; This still has a UB according to the C standard. That's why BIT()/BIT_ULL() is preferable to use since they don't have such issues. You may open code it, of course (since I remember you wished to show that this is not a bit, but a number). > tmp = tmp2; > How can I validate this? By understanding the C standard? I dunno, actually. GCC will generate correct code, it's just a special warning you may get when supplying a parameter (Linux kernel doesn't use that one even on W=2 IIRC). -Wshift-overflow=2 > > > I can look into that but given how unlikely it is to fall into those bad > > > cases, I'd rather keep things as they are. Would that be okay? > > > > > Also, I don't think using BIT() or BIT_ULL() would address this as they > > > both do the same shift, with no extra checks. > > > > They do slightly different versions of it. They use an unsigned int type. > > > > Open coded or not, it's up to you. Just convert to unsigned int. -- With Best Regards, Andy Shevchenko
