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 linux-c-programming" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
