Sorry, I screwed up the bit-twiddling while messing with various options. I was trying to get size == 32 to work; that should have been:
> tmp &= (2UL << ((size-1) % BITS_PER_LONG)) - 1; /* Mask last word */ And you're right that LAST_WORD_MASK is a good wrapper. Vasrious working solutions include: #define LAST_WORD_MASK(bits) ((2UL << (bits-1) % BITS_PER_LONG) - 1) #define LAST_WORD_MASK(bits) ~(~0UL << bits % BITS_PER_LONG) #define LAST_WORD_MASK(bits) (~0UL >> -bits % BITS_PER_LONG) I'm not sure which generates the nicest code. It's 4 instructions each way, with the last being 1 byte smaller: 0000000000000000 <lwm1>: 0: 8d 4f ff lea -0x1(%rdi),%ecx 3: b8 02 00 00 00 mov $0x2,%eax 8: 48 d3 e0 shl %cl,%rax b: 48 83 e8 01 sub $0x1,%rax f: c3 retq 0000000000000010 <lwm2>: 10: 48 c7 c0 ff ff ff ff mov $0xffffffffffffffff,%rax 17: 89 f9 mov %edi,%ecx 19: 48 d3 e0 shl %cl,%rax 1c: 48 f7 d0 not %rax 1f: c3 retq 0000000000000020 <lwm3>: 20: 89 f9 mov %edi,%ecx 22: f7 d9 neg %ecx 24: 48 c7 c0 ff ff ff ff mov $0xffffffffffffffff,%rax 2b: 48 d3 e8 shr %cl,%rax 2e: c3 retq > Also, I think it is best to handle size==0 appropriately, meaning that > one cannot dereference addr in any way (and certainly not addr[-1]). Ah, okay; l I figured that was a safe case to omit. But your solution is nicer than mine overall. It may be that omitting the mask *is* safe, but it's a lot of wading through callers to prove it. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
