Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Steven Rostedt
2008-01-17T00:36:55Z

On Wed, 16 Jan 2008, Steven Rostedt wrote:
> On Wed, 16 Jan 2008, Mathieu Desnoyers wrote:
> >
> > !0 is not necessarily 1. This is why I use cpu_synth->index ? 0 : 1 in
>
> How about simply "cpu_synth->index ^ 1"? Seems the best choice if you ask
> me, if all you are doing is changing it from 1 to zero and back to 1.
>

FYI:

rostedt@bilbo:~/c$ cat flipme.c
int flip1 (int x)
{
return !x;
}

int flip2 (int x)
{
return x ? 0 : 1;
}

int flip3(int x)
{
return x ^ 1;
}
rostedt@bilbo:~/c$ gcc -O2 -c flipme.c
rostedt@bilbo:~/c$ objdump -d flipme.o

flipme.o: file format elf32-i386

Disassembly of section .text:

00000000 <flip1>:
0: 55 push %ebp
1: 31 c0 xor %eax,%eax
3: 89 e5 mov %esp,%ebp
5: 83 7d 08 00 cmpl $0x0,0x8(%ebp)
9: 5d pop %ebp
a: 0f 94 c0 sete %al
d: c3 ret
e: 66 90 xchg %ax,%ax

00000010 <flip2>:
10: 55 push %ebp
11: 31 c0 xor %eax,%eax
13: 89 e5 mov %esp,%ebp
15: 83 7d 08 00 cmpl $0x0,0x8(%ebp)
19: 5d pop %ebp
1a: 0f 94 c0 sete %al
1d: c3 ret
1e: 66 90 xchg %ax,%ax

00000020 <flip3>:
20: 55 push %ebp
21: 89 e5 mov %esp,%ebp
23: 8b 45 08 mov 0x8(%ebp),%eax
26: 5d pop %ebp
27: 83 f0 01 xor $0x1,%eax
2a: c3 ret

So, if you know for sure that x is only 1 or 0, then using x ^ 1 to invert
it, seems the most efficient.

-- Steve