From: David Laight <David.Laight@ACULAB.COM>
To: 'Andrew Morton' <akpm@linux-foundation.org>,
Jani Nikula <jani.nikula@intel.com>
Cc: "David Gow" <davidgow@google.com>,
"intel-gfx@lists.freedesktop.org"
<intel-gfx@lists.freedesktop.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"Christian König" <christian.koenig@amd.com>
Subject: Re: [Intel-gfx] [PATCH 0/4] log2: make is_power_of_2() more generic
Date: Thu, 30 Mar 2023 21:53:03 +0000 [thread overview]
Message-ID: <549987e4967d45159573901d330c96a0@AcuMS.aculab.com> (raw)
In-Reply-To: <20230330125041.83b0f39fa3a4ec1a42dfd95f@linux-foundation.org>
From: Andrew Morton
> Sent: 30 March 2023 20:51
>
> On Thu, 30 Mar 2023 13:42:39 +0300 Jani Nikula <jani.nikula@intel.com> wrote:
>
> > is_power_of_2() only works for types <= sizeof(unsigned long) and it's
> > also not a constant expression. There are a number of places in kernel
> > where is_power_of_2() is called on u64, which fails on 32-bit
> > builds. Try to remedy that. While at it, make it a constant expression
> > when possible.
>
> Yes, the current `is_power_of_2(unsigned long n)' isn't very general.
>
> But wouldn't all these issues be addressed by simply doing
>
> #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
>
> ?
>
> (With suitable tweaks to avoid evaluating `n' more than once)
I think you need to use the 'horrid tricks' from min() to get
a constant expression from constant inputs.
For non-constants this looks ok (see https://godbolt.org/z/G73MTr9jn)
David
static inline int lisp2(unsigned long n)
{
return n && !(n & (n - 1));
}
static inline int llisp2(unsigned long long lln)
{
#if 0 // I think this looks worse, esp. for gcc on x86
return lln && !(lln & (lln - 1));
#else
unsigned long n = lln;
if (lln >= 1ull << 32) {
if (n)
return 0;
n = lln >> 32;
}
return lisp2(n);
#endif
}
#define isp2(n) (sizeof ((n)+0) == sizeof (long) ? lisp2(n) : llisp2(n))
int is12(unsigned long long i)
{
return isp2(i);
}
-
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next prev parent reply other threads:[~2023-03-30 21:59 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-30 10:42 [Intel-gfx] [PATCH 0/4] log2: make is_power_of_2() more generic Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 1/4] log2: add helper __IS_POWER_OF_2() Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 2/4] log2: have is_power_of_2() support bigger types than unsigned long Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 3/4] log2: allow use of is_power_of_2() in constant expressions Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 4/4] drm/i915/reg: use is_power_of_2() from log2.h Jani Nikula
2023-03-30 10:59 ` [Intel-gfx] [PATCH 0/4] log2: make is_power_of_2() more generic Christian König
2023-03-30 11:10 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for " Patchwork
2023-03-30 11:10 ` [Intel-gfx] ✗ Fi.CI.SPARSE: " Patchwork
2023-03-30 11:23 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork
2023-03-30 19:50 ` [Intel-gfx] [PATCH 0/4] " Andrew Morton
2023-03-30 21:53 ` David Laight [this message]
2023-03-30 22:18 ` Andrew Morton
2023-03-31 7:33 ` David Laight
2023-03-31 8:31 ` Jani Nikula
2023-04-05 15:27 ` Steven Price
2024-04-12 10:01 ` Jani Nikula
2023-03-31 5:58 ` [Intel-gfx] ✓ Fi.CI.IGT: success for " Patchwork
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