On Thu, Apr 28, 2016 at 7:58 PM, Rich Felker <dalias@xxxxxxxx> wrote:
On Thu, Apr 28, 2016 at 07:51:06PM +0200, Geert Uytterhoeven wrote:
On Thu, Apr 28, 2016 at 6:48 PM, George Spelvin <linux@xxxxxxxxxxx> wrote:
Another few comments:
1. Would ARCH_HAS_FAST_FFS involve fewer changes than CPU_NO_EFFICIENT_FFS?
No, as you want to _disable_ ARCH_HAS_FAST_FFS / _enable_
CPU_NO_EFFICIENT_FFS as soon as you're enabling support for a
CPU that doesn't support it.
Logical OR is easier in both the Kconfig and C preprocessor languages
than logical NAND.
E.g. in Kconfig, a CPU core not supporting it can just select
CPU_NO_EFFICIENT_FFS.
How does a CPU lack an efficient ffs/ctz anyway? There are all sorts
of ways to implement it without a native insn, some of which are
almost or just as fast as the native insn on cpus that have the
latter. On anything with a fast multiply, the de Bruijn sequence
approach is near-optimal, and otherwise one of the binary-search type
approaches (possibly branchless) can be used. If the compiler doesn't
generate an appropriate one for __builtin_ctz, that's arguably a
compiler bug.
m68k-linux-gcc 4.6.3 generates:
jsr __ctzsi2
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-m68k" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
