1. 07 Mar, 2012 1 commit
  2. 06 Apr, 2010 2 commits
  3. 28 Dec, 2009 1 commit
    • Akinobu Mita's avatar
      x86, core: Optimize hweight32() · 39d997b5
      Akinobu Mita authored
      Optimize hweight32 by using the same technique in hweight64.
      
      The proof of this technique can be found in the commit log for
      f9b41929 ("bitops: hweight()
      speedup").
      
      The userspace benchmark on x86_32 showed 20% speedup with
      bitmap_weight() which uses hweight32 to count bits for each
      unsigned long on 32bit architectures.
      
       int main(void)
       {
      	#define SZ (1024 * 1024 * 512)
      
      	static DECLARE_BITMAP(bitmap, SZ) = {
      	        [0 ... 100] = 1,
      	};
      
      	return bitmap_weight(bitmap, SZ);
       }
      Signed-off-by: 's avatarAkinobu Mita <akinobu.mita@gmail.com>
      Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
      Cc: Linus Torvalds <torvalds@linux-foundation.org>
      LKML-Reference: <1258603932-4590-1-git-send-email-akinobu.mita@gmail.com>
      [ only x86 sets ARCH_HAS_FAST_MULTIPLIER so we do this via the x86 tree]
      Signed-off-by: 's avatarIngo Molnar <mingo@elte.hu>
      39d997b5
  4. 19 Oct, 2007 1 commit
  5. 26 Sep, 2006 1 commit
    • Andi Kleen's avatar
      [PATCH] optimize hweight64 for x86_64 · 0136611c
      Andi Kleen authored
      Based on patch from David Rientjes <rientjes@google.com>, but
      changed by AK.
      
      Optimizes the 64-bit hamming weight for x86_64 processors assuming they
      have fast multiplication.  Uses five fewer bitops than the generic
      hweight64.  Benchmark on one EMT64 showed ~25% speedup with 2^24
      consecutive calls.
      
      Define a new ARCH_HAS_FAST_MULTIPLIER that can be set by other
      architectures that can also multiply fast.
      Signed-off-by: 's avatarAndi Kleen <ak@suse.de>
      0136611c
  6. 26 Mar, 2006 2 commits
    • Akinobu Mita's avatar
      [PATCH] bitops: hweight() speedup · f9b41929
      Akinobu Mita authored
      <linux@horizon.com> wrote:
      
      This is an extremely well-known technique.  You can see a similar version that
      uses a multiply for the last few steps at
      http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel whch
      refers to "Software Optimization Guide for AMD Athlon 64 and Opteron
      Processors"
      http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF
      
      It's section 8.6, "Efficient Implementation of Population-Count Function in
      32-bit Mode", pages 179-180.
      
      It uses the name that I am more familiar with, "popcount" (population count),
      although "Hamming weight" also makes sense.
      
      Anyway, the proof of correctness proceeds as follows:
      
      	b = a - ((a >> 1) & 0x55555555);
      	c = (b & 0x33333333) + ((b >> 2) & 0x33333333);
      	d = (c + (c >> 4)) & 0x0f0f0f0f;
      #if SLOW_MULTIPLY
      	e = d + (d >> 8)
      	f = e + (e >> 16);
      	return f & 63;
      #else
      	/* Useful if multiply takes at most 4 cycles */
      	return (d * 0x01010101) >> 24;
      #endif
      
      The input value a can be thought of as 32 1-bit fields each holding their own
      hamming weight.  Now look at it as 16 2-bit fields.  Each 2-bit field a1..a0
      has the value 2*a1 + a0.  This can be converted into the hamming weight of the
      2-bit field a1+a0 by subtracting a1.
      
      That's what the (a >> 1) & mask subtraction does.  Since there can be no
      borrows, you can just do it all at once.
      
      Enumerating the 4 possible cases:
      
      0b00 = 0  ->  0 - 0 = 0
      0b01 = 1  ->  1 - 0 = 1
      0b10 = 2  ->  2 - 1 = 1
      0b11 = 3  ->  3 - 1 = 2
      
      The next step consists of breaking up b (made of 16 2-bir fields) into
      even and odd halves and adding them into 4-bit fields.  Since the largest
      possible sum is 2+2 = 4, which will not fit into a 4-bit field, the 2-bit
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                "which will not fit into a 2-bit field"
      
      fields have to be masked before they are added.
      
      After this point, the masking can be delayed.  Each 4-bit field holds a
      population count from 0..4, taking at most 3 bits.  These numbers can be added
      without overflowing a 4-bit field, so we can compute c + (c >> 4), and only
      then mask off the unwanted bits.
      
      This produces d, a number of 4 8-bit fields, each in the range 0..8.  From
      this point, we can shift and add d multiple times without overflowing an 8-bit
      field, and only do a final mask at the end.
      
      The number to mask with has to be at least 63 (so that 32 on't be truncated),
      but can also be 128 or 255.  The x86 has a special encoding for signed
      immediate byte values -128..127, so the value of 255 is slower.  On other
      processors, a special "sign extend byte" instruction might be faster.
      
      On a processor with fast integer multiplies (Athlon but not P4), you can
      reduce the final few serially dependent instructions to a single integer
      multiply.  Consider d to be 3 8-bit values d3, d2, d1 and d0, each in the
      range 0..8.  The multiply forms the partial products:
      
      	           d3 d2 d1 d0
      	        d3 d2 d1 d0
      	     d3 d2 d1 d0
      	+ d3 d2 d1 d0
      	----------------------
      	           e3 e2 e1 e0
      
      Where e3 = d3 + d2 + d1 + d0.   e2, e1 and e0 obviously cannot generate
      any carries.
      Signed-off-by: 's avatarAkinobu Mita <mita@miraclelinux.com>
      Signed-off-by: 's avatarAndrew Morton <akpm@osdl.org>
      Signed-off-by: 's avatarLinus Torvalds <torvalds@osdl.org>
      f9b41929
    • Akinobu Mita's avatar
      [PATCH] bitops: generic hweight{64,32,16,8}() · 3b9ed1a5
      Akinobu Mita authored
      This patch introduces the C-language equivalents of the functions below:
      
      unsigned int hweight32(unsigned int w);
      unsigned int hweight16(unsigned int w);
      unsigned int hweight8(unsigned int w);
      unsigned long hweight64(__u64 w);
      
      In include/asm-generic/bitops/hweight.h
      
      This code largely copied from: include/linux/bitops.h
      Signed-off-by: 's avatarAkinobu Mita <mita@miraclelinux.com>
      Signed-off-by: 's avatarAndrew Morton <akpm@osdl.org>
      Signed-off-by: 's avatarLinus Torvalds <torvalds@osdl.org>
      3b9ed1a5