Messages in this thread | | | Subject | Re: [PATCH] checkpatch: Add --strict preference for #defines using BIT(foo) | From | Joe Perches <> | Date | Mon, 10 Nov 2014 15:53:31 -0800 |
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On Mon, 2014-11-10 at 15:36 -0800, Andrew Morton wrote: > On Fri, 07 Nov 2014 13:15:39 -0800 Joe Perches <joe@perches.com> wrote: > > > Using BIT(foo) and BIT_ULL(bar) is more common now. > > Suggest using these macros over #defines with 1<<value. > > urgh. I'm counting eightish implementations of BIT(), an unknown > number of which are actually being used. Many use 1<<n, some use > 1UL<<N, another uses 1ULL<<n. I'm a bit reluctant to recommend that > anyone should use BIT() until it has has some vigorous scrubbing :( > > Is it actually an improvement? If I see > > #define X (1U << 7) > > then I know exactly what it does. Whereas when I see > > #define X BIT(7) > > I know neither the size or the signedness of X so I have to go look it > up.
I'm not sure the signedness or type of X matters much as the non-64bit comparisons are done by promotion to at least int or unsigned int anyway.
The BIT macro makes sure a single bit is set.
The 'good' one is in bitops.h. It also has #define BIT_ULL
include/linux/bitops.h:#define BIT(nr) (1UL << (nr)) include/linux/bitops.h-#define BIT_ULL(nr) (1ULL << (nr))
The ones in tools/ are independent and should not be changed. Excluding tools/, the others should probably be removed
$ git grep -E "define\s+BIT\b" arch/arm/mach-davinci/sleep.S:#define BIT(nr) (1 << (nr)) drivers/staging/rtl8188eu/include/rtl8188e_spec.h:#define BIT(x) (1 << (x)) drivers/staging/rtl8188eu/include/wifi.h:#define BIT(x) (1 << (x)) drivers/staging/rtl8192e/rtl8192e/rtl_core.h:#define BIT(_i) (1<<(_i)) drivers/staging/rtl8712/osdep_service.h: #define BIT(x) (1 << (x)) drivers/staging/rtl8712/wifi.h:#define BIT(x) (1 << (x))
> I have no strong feelings either way, but I'm wondering what might have > inspired this change?
David Miller commented on a netdev patch where 1<<foo was being used in a #define and suggested using BIT.
http://article.gmane.org/gmane.linux.network/337535/match=bit
Using BIT _is_ more common in recent patches too.
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