On Fri, Jan 27, 2012 at 08:29:13PM +0000, Allan, Bruce W wrote: > For a simple boolean test of whether, for example, bit 5 is set in the variable 'flags', which of the following methods is preferred over the other? > > bool result = (flags & (1 << 5)) ? true : false; > > or > > bool result = !!(flags & (1 << 5)); > > I've seen examples of both in the Linux kernel. I guess this is rather a matter of taste. Personally I compare performance of different solutions if in doubt. S result = !!(flags & (1 << 5)); o I added both ways (plus a third one I've just made up) into a simple program, compiled it using 'gcc -g test.c' and looked at the disassembly ('objdump -S a.out'): | int main(void) | { | 80483be: 55 push %ebp | 80483b5: 89 e5 mov %esp,%ebp | 80483b7: 83 ec 10 sub $0x10,%esp | | char flags = 0xff; | 80483ba: c6 45 ff ff movb $0xff,-0x1(%ebp) | int result; | | result = (flags & (1 << 5)) ? 1 : 0; | 80483be: 0f be 45 ff movsbl -0x1(%ebp),%eax | 80483c2: 83 e0 20 and $0x20,%eax | 80483c5: 85 c0 test %eax,%eax | 80483c7: 0f 95 c0 setne %al | 80483ca: 0f b6 c0 movzbl %al,%eax | 80483cd: 89 45 f8 mov %eax,-0x8(%ebp) | result = !!(flags & (1 << 5)); | 80483d0: 0f be 45 ff movsbl -0x1(%ebp),%eax | 80483d4: 83 e0 20 and $0x20,%eax | 80483d7: 85 c0 test %eax,%eax | 80483d9: 0f 95 c0 setne %al | 80483dc: 0f b6 c0 movzbl %al,%eax | 80483df: 89 45 f8 mov %eax,-0x8(%ebp) | | result = (flags >> 5) & 0x1; | 80483e2: 0f b6 45 ff movzbl -0x1(%ebp),%eax | 80483e6: c0 f8 05 sar $0x5,%al | 80483e9: 0f be c0 movsbl %al,%eax | 80483ec: 83 e0 01 and $0x1,%eax | 80483ef: 89 45 f8 mov %eax,-0x8(%ebp) | | return 0; | 80483f2: b8 00 00 00 00 mov $0x0,%eax | } As you see, both of your choices generate identical code. Looking at the code size, mine is the winner! :) HTH, Phil -- To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html