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

Linus Torvalds
2008-01-17T01:06:04Z

On Wed, 16 Jan 2008, Mathieu Desnoyers wrote:
>
> > + int num = !cs->base_num;
> > + cycle_t offset = (now - cs->base[!num].cycle_base_last);
>
> !0 is not necessarily 1.

Incorrect.

!0 _is_ necessarily 1. It's how all C logical operators work. If you find
a compiler that turns !x into anything but 0/1, you found a compiler for
another language than C.

It's true that any non-zero value counts as "true", but the that does not
mean that a logical operator can return any non-zero value for true. As a
return value of the logical operations in C, true is *always* 1.

So !, ||, &&, when used as values, will *always* return either 0 or 1 (but
when used as part of a conditional, the compiler will often optimize out
unnecessary stuff, so the CPU may not actually ever see a 0/1 value, if
the value itself was never used, only branched upon).

So doing "!cs->base_num" to turn 0->1 and 1->0 is perfectly fine.

That's not to say it's necessarily the *best* way.

If you *know* that you started with 0/1 in the first place, the best way
to flip it tends to be to do (1-x) (or possibly (x^1)).

And if you can't guarantee that, !x is probably better than x ? 0 : 1,
but you might also decide to use ((x+1)&1) for example.

And obviously, the compiler may sometimes surprise you, and if *it* also
knows it's always 0/1 (for something like the source being a single-bit
bitfield for example), it may end up doing something else than you coded
that is equivalent. And the particular choice of operation the compiler
chooses may well depend on the code _around_ that sequence.

(One reason to potentially prefer (1-x) over (x^1) is that it's often
easier to combine a subtraction with other operations, while an xor seldom
combines with anything around it)

Linus