On Tue, Feb 24, 2015 at 3:58 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> I have a situation where I want to do something that boils down to:
>
> if (test_bit(BIT_A, &p->flags) {
> ...
> } else if (test_bit(BIT_B, &p->flags) {
> ...
> } else if (test_bit(BIT_C, &p->flags) {
> ...
> } else if (test_bit(BIT_D, &p->flags) {
> ...
> } else if (test_bit(BIT_E, &p->flags) {
> }
>
> Note that all bits are in the same unsigned long and I don't necessarily
> expect the contents of p->flags to change whilst I'm looking at it.
>
> Since the address parameter for test_bit() contains a 'volatile' keyword, the
> compiler will emit a separate load (or bit test) for each condition when in
> fact the most efficient way is almost certainly one load and a bunch of
> bitwise-and or bit-test-in-register instructions.
>
> Is it worth adding a __test_bit() that doesn't have the volatile? Or should I
> just rewrite it manually as?
>
> unsigned long flags = p->flags;
> if (test_bit(BIT_A, &flags) {
> ...
> } else if (test_bit(BIT_B, &flags) {
> ...
>
> Or even:
>
> unsigned long flags = p->flags;
> if ((1 << BIT_A) & flags) {
> ...
> } else if ((1 << BIT_B) & flags) {
> ...
How about using READ_ONCE() and BIT()?
unsigned long flags = READ_ONCE(p->flags_;
if (BIT(BIT_A) & flags) {
...
} else ...
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
--
To unsubscribe from this list: send the line "unsubscribe linux-arch" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
