From: Mark Rutland > Sent: 24 May 2021 13:29 > > On Mon, May 24, 2021 at 12:46:26PM +0100, Christoph Hellwig wrote: > > On Thu, May 20, 2021 at 03:43:37AM -0700, Joe Richey wrote: > > > This patch series changes all UAPI uses of BIT() to just be open-coded. > > > However, there really should be a check for this in checkpatch.pl > > > Currently, the script actually _encourages_ users to use the BIT macro > > > even if adding things to UAPI. > > > > Yes. In fact it should warn about BIT() in general. It is totally > > pointless obsfucation that doesn't even save any typing at all. > > That's not quite true; the point is that if you use BIT() consistently, > then even when you refer to bits 32 to 63 you won't accidentally > introduce shifts of more than the width of int, and the definition will > work equally well for assembly and C, which isn't true if you use `1UL` > in the definition. > > With that in mind it's shorter and clearer than its functional > equivalent: > > BIT(x) > (UL(1) << (x)) > > So IMO it's preferable to use BIT() generally, or _BITUL() in uapi > headers. And then, suddenly the compiler warns about truncation of the high bits when ~BIT(x) is used to mask a 32bit value on 64bit systems. Once the C standard committee had decided to change from K&R's 'sign preserving' integer promotions to 'value preserving' you always lose somewhere. Personally I prefer hex constants - I can't count bits at all. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
