linux-kernel - Re: [PATCH] Optimize int_sqrt for small values for faster idle

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Message-ID: <201601290658.K1apCD7G%fengguang.wu@intel.com>
Date:	Fri, 29 Jan 2016 06:04:39 +0800
From:	kbuild test robot <lkp@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	kbuild-all@...org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, davidlohr.bueso@...com,
	rafael.j.wysocki@...el.com, lenb@...nel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] Optimize int_sqrt for small values for faster idle

Hi Andi,

[auto build test WARNING on v4.5-rc1]
[also build test WARNING on next-20160128]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Andi-Kleen/Optimize-int_sqrt-for-small-values-for-faster-idle/20160129-054629
config: x86_64-randconfig-x015-01270835 (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

Note: it may well be a FALSE warning. FWIW you are at least aware of it now.
http://gcc.gnu.org/wiki/Better_Uninitialized_Warnings

All warnings (new ones prefixed by >>):

   lib/int_sqrt.c: In function 'int_sqrt':
>> lib/int_sqrt.c:25:6: warning: 'm' may be used uninitialized in this function [-Wmaybe-uninitialized]
      if (m <= 0xff)
         ^

vim +/m +25 lib/int_sqrt.c

     9	#include <linux/export.h>
    10	
    11	/**
    12	 * int_sqrt - rough approximation to sqrt
    13	 * @x: integer of which to calculate the sqrt
    14	 *
    15	 * A very rough approximation to the sqrt() function.
    16	 */
    17	unsigned long int_sqrt(unsigned long x)
    18	{
    19		unsigned long b, m, y = 0;
    20	
    21		if (x <= 1)
    22			return x;
    23	
    24		if (x <= 0xffff) {
  > 25			if (m <= 0xff)
    26				m = 1UL << (8 - 2);
    27			else
    28				m = 1UL << (16 - 2);
    29		} else if (x <= 0xffffffff)
    30			m = 1UL << (32 - 2);
    31		else
    32			m = 1UL << (BITS_PER_LONG - 2);
    33		while (m != 0) {

---
0-DAY kernel test infrastructure                Open Source Technology Center
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