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SubjectRe: [PATCH 1/2] perf tools: Fix next_pow2_l()
Em Fri, Dec 13, 2013 at 10:53:35AM +0200, Adrian Hunter escreveu:

> My implementation of next_pow2_l() was incorrect. e.g.

> perf record -m4296015872 uname
> rounding mmap pages size to 17592186044416 bytes (4294967296 pages)
> Invalid argument for --mmap_pages/-m

> Notice that the next power-of-2 value 4294967296 is less than the
> option value 4296015872.

> Change to using __builtin_clzl() and prevent the shift being equal to
> the width of the operand. Also __builtin_clzl(x) is undefined if x is
> 0, so adjust the condition to preclude that possibility. Now:

Can we try to look first if there is an implementation for what we need
being used in the kernel sources, in include/ and then try to use, if
possible, exactly like done in kernel code?

For instance, include/linux/log2.h has roundup_pow_of_two() that seems
to be what you need, lemme check...

roundup_pow_of_two(4296015872)=8589934592

So, it yields what we need, I'll try to cook up a patch that makes us
use it, as I did in my dwarves tools in this changeset:

https://git.kernel.org/cgit/devel/pahole/pahole.git/commit/?id=e31fda3063e3c88ca0b93a9fb5e6d6e32478e36b

- Arnaldo

> perf record -m4296015872 uname
> rounding mmap pages size to 35184372088832 bytes (8589934592 pages)
> Invalid argument for --mmap_pages/-m
>
> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> ---
> tools/perf/util/util.h | 15 ++++++++-------
> 1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/tools/perf/util/util.h b/tools/perf/util/util.h
> index a1eea3e..ae609fe 100644
> --- a/tools/perf/util/util.h
> +++ b/tools/perf/util/util.h
> @@ -284,13 +284,14 @@ static inline unsigned next_pow2(unsigned x)
>
> static inline unsigned long next_pow2_l(unsigned long x)
> {
> -#if BITS_PER_LONG == 64
> - if (x <= (1UL << 31))
> - return next_pow2(x);
> - return (unsigned long)next_pow2(x >> 32) << 32;
> -#else
> - return next_pow2(x);
> -#endif
> + int leading_zeros;
> +
> + if (x < 2)
> + return 1;
> + leading_zeros = __builtin_clzl(x - 1);
> + if (!leading_zeros)
> + return 0;
> + return 1UL << (BITS_PER_LONG - leading_zeros);
> }
>
> size_t hex_width(u64 v);
> --
> 1.7.11.7


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