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SSE2 (Streaming SIMD Extensions 2) and further x86- or x86-64 streaming SIMD extensions, like SSE3, SSSE3, SSE4 and AMD's announced SSE5, as major enhancement to SSE, provide an instruction set on 128-bit registers, namely on vectors of four floats or two doubles, as well since SSE2 as vectors of 16 bytes, eight words, four double words or two quad words [1]. In 64-bit mode there are 16 xmm registers available, xmm0..xmm15, in 32-bit mode only eight, xmm0..xmm7. SSE is explicitly available through C-Compiler intrinsics [2] or (inline) assembly. Some compiler implicitly use SSE-float and double instructions for floating point data types, others even provide automatic SSE2 vectorization, while processing arrays of various integer types. SSE- and SSE2-intrinsic functions are available in Visual C [3] or Intel-C [4].

Integer Instructions

x86 and x86-64 - SSE2 Instructions, C-Intrinsic reference from Intel Intrinsics Guide

Mnemonic Description C-Intrinsic
bitwise logical return parameter
pand packed and, r := a & b _m128i _mm_and_si128 (_m128i a, _m128i b)
pandn packed and not, r := ~a & b _m128i _mm_andnot_si128 (_m128i a, _m128i b)
por packed or, r := a | b _m128i _mm_or_si128 (_m128i a, _m128i b)
pxor packed xor, r:= a ^ b _m128i _mm_xor_si128 (_m128i a, _m128i b)
quad word shifts return parameter
psrlq packed shift right logical quad _m128i _mm_srl_epi64 (_m128i a, _m128i cnt)
immediate _m128i _mm_srli_epi64 (_m128i a, int cnt)
psllq packed shift left logical quad _m128i _mm_sll_epi64 (_m128i a, _m128i cnt)
immediate _m128i _mm_slli_epi64 (_m128i a, int cnt)
arithmetical return parameter
paddb packed add bytes _m128i _mm_add_epi8 (_m128i a, _m128i b)
psubb packed subtract bytes _m128i _mm_sub_epi8 (_m128i a, _m128i b)
psadbw packed sum of absolute differences
of bytes into a word
_m128i _mm_sad_epu8 (_m128i a, _m128i b)
pmaxsw packed maximum signed words _m128i _mm_max_epi16 (_m128i a, _m128i b)
pmaxub packed maximum unsigned bytes _m128i _mm_max_epu8 (_m128i a, _m128i b)
pminsw packed minimum signed words _m128i _mm_min_epi16 (_m128i a, _m128i b)
pminub packed minimum unsigned bytes _m128i _mm_min_epu8 (_m128i a, _m128i b)
pcmpeqb packed compare equal bytes _m128i _mm_cmpeq_epi8 (_m128i a, _m128i b)
pmullw packed multiply mow signed (unsigned) word _m128i _mm_mullo_epi16 (_m128i a, _m128i b)
pmulhw packed multiply high signed word _m128i _mm_mulhi_epi16 (_m128i a, _m128i b)
pmulhuw packed multiply high unsigned word _m128i _mm_mulhi_epu16 (_m128i a, _m128i b)
pmaddwd packed multiply words and add doublewords _m128 _mm_madd_epi16 (_m128i a, _m128i b)
unpack, shuffle return parameter
punpcklbw unpack and interleave low bytes
gGhHfFeE:dDcCbBaA :=
xxxxxxxx:GHFEDCBA #
_m128i _mm_unpacklo_epi8 (_m128i A, _m128i a)
punpckhbw unpack and interleave high bytes
gGhHfFeE:dDcCbBaA :=
GHFEDCBA:xxxxxxxx #
_m128i _mm_unpackhi_epi8 (_m128i A, _m128i a)
punpcklwd unpack and interleave low words
dDcC:bBaA := xxxx:DCBA#xxxx:dcba
_m128i _mm_unpacklo_epi16 (_m128i A, _m128i a)
punpckhwd unpack and interleave high words
dDcC:bBaA := DCBA:xxxx#dcba:xxxx
_m128i _mm_unpackhi_epi16 (_m128i A, _m128i a)
punpckldq unpack and interleave low doublewords
bB:aA := xx:BA # xx:ba
_m128i _mm_unpacklo_epi32 (_m128i A, _m128i a)
punpckhdq unpack and interleave high doublewords
bB:aA := BA:xx # ba:xx
_m128i _mm_unpackhi_epi32 (_m128i A, _m128i a)
punpcklqdq unpack and interleave low quadwords
a:A := x:A # x:a
_m128i _mm_unpacklo_epi64 (_m128i A, _m128i a)
punpckhqdq unpack and interleave high quadwords
a:A := A:x # a:x
_m128i _mm_unpackhi_epi64 (_m128i A, _m128i a)
pshuflw packed shuffle low words _m128i _mm_shufflelo_epi16 (_m128i a, int imm)
pshufhw packed shuffle high words _m128i _mm_shufflehi_epi16 (_m128i a, int imm)
pshufd packed shuffle doublewords _m128i _mm_shuffle_epi32 (_m128i a, int imm)
load, store, moves return parameter
movdqa move aligned double quadword
xmm := *p
_m128i _mm_load_si128 (_m128i const *p)
movdqu move unaligned double quadword
xmm := *p
_m128i _mm_loadu_si128 (_m128i const*p)
movdqa move aligned double quadword
*p := xmm
void _mm_store_si128 (_m128i *p, _m128i a)
movdqu move unaligned double quadword
*p := xmm
void _mm_storeu_si128 (_m128i *p, _m128i a)
movq move quadword, xmm := gp64 _m128i _mm_cvtsi64_si128 (_int64 a)
movq move quadword, gp64 := xmm _int64 _mm_cvtsi128_si64 (_m128i a)
movd move double word or quadword
xmm := gp64
_m128i _mm_cvtsi64x_si128 (_int64 value)
movd move doubleword, xmm := gp32 _m128i _mm_cvtsi32_si128 (int a)
movd move doubleword, gp32 := xmm int _mm_cvtsi128_si32 (_m128i a)
pextrw extract packed word, gp16 := xmm[i] int _mm_extract_epi16 (_m128i a, int imm)
pinsrw packed insert word, xmm[i] := gp16 _m128i _mm_insert_epi16 (_m128i a, int b, int imm)
pmovmskb packed move mask byte,
gp32 := 16 sign-bits(xmm)
int _mm_movemask_epi (_m128i a)
cache support return parameter
prefetch void _mm_prefetch (char const* p , int i)


Some vc2005 tested, sample SSE2 bitboard routines:

One Step Only

Since 128-bit xmm registers may treated as vector of 16 bytes, shifting techniques such as one step in all eight directions can be done more efficiently with respect to wraps from a- to the h-file or vice versa. It is recommend to write a own SSE2-wrapper class with overloaded operators in C++ to encapsulate a vector of two bitboards.

  northwest    north   northeast
  noWe         nort         noEa
          +7    +8    +9
              \  |  /
  west    -1 <-  0 -> +1    east
              /  |  \
          -9    -8    -7
  soWe         sout         soEa
  southwest    south   southeast

Veritcal steps as usual with 64-byte shift a rank each:

__m128i nortOne(__m128i b) {
   b = _mm_slli_epi64 (b, 8);
   return b;

__m128i soutOne(__m128i b) {
   b = _mm_srli_epi64 (b, 8);
   return b;

Unfortunately there is no byte-wise shift in the SSE2-instruction set (as well as MMX), but using byte-wise parallel add avoids using the wrap masks, which need otherwise load from memory or computed. Applying the wraps mask takes two instructions.

__m128i butNotA(__m128i b) {
   b = _mm_srli_epi64 (b, 1);
   b = _mm_add_epi8   (b, b);
   return b;

__m128i butNotH(__m128i b) {
   b = _mm_add_epi8   (b, b);
   b = _mm_srli_epi64 (b, 1);
   return b;

This is how the east direction are computed based on parallel byte-wise add. Either one or two SSE2-instructions:

__m128i eastOne(__m128i b) {
   b = _mm_add_epi8   (b, b);
   return b;

__m128i noEaOne (__m128i b) {
   b = _mm_add_epi8   (b, b);
   b = _mm_slli_epi64 (b, 8);
   return b;

__m128i soEaOne (__m128i b) {
   b = _mm_add_epi8   (b, b);
   b = _mm_srli_epi64 (b, 8);
   return b;

West directions need a leading not A-file and take three instructions each:

__m128i westOne(__m128i b) {
   b = _mm_srli_epi64 (b, 1);
   b = _mm_add_epi8   (b, b);
   b = _mm_srli_epi64 (b, 1);
   return b;

__m128i soWeOne (__m128i b) {
   b = _mm_srli_epi64 (b, 1);
   b = _mm_add_epi8   (b, b);
   b = _mm_srli_epi64 (b, 9);
   return b;

__m128i noWeOne (__m128i b) {
   b = _mm_srli_epi64 (b, 1);
   b = _mm_add_epi8   (b, b);
   b = _mm_slli_epi64 (b, 7);
   return b;

East Attacks

SIMD-wise Fill by Subtraction with byte- or rank-wise arithmetic takes only a few instructions with SSE2:

__m128i eastAttacks (__m128i occ, __m128i rooks) {
   __m128i tmp;
   occ  = _mm_or_si128 (occ, rooks);  //  make rooks member of occupied
   tmp  = _mm_xor_si128(occ, rooks);  // occ - rooks
   tmp  = _mm_sub_epi8 (tmp, rooks);  // occ - 2*rooks
   return _mm_xor_si128(occ, tmp);    // occ ^ (occ - 2*rooks)

SSE2 dot product

The dot product [5] of a vector of bits by a weight vector of bytes might be used in determining mobility for evaluation purposes. The vector of bits is a bitboard of all squares attacked by one (or multiple) piece(s), while the the weight vector considers the "importance" of squares, like center control, or even square controls near the opponent king, e.g. by providing 64 weight vectors for each king square.

bb·weights =  bbi weightsi = bb1 weights1 + ... + bb64 weights64

The 64-bit times 64-byte dot product implements a kind of weighted population count similar than following loop approach, but completely unrolled and branchless:

int dotProduct(U64 bb, BYTE weights[])
   U64 bit  = 1;
   int accu = 0;
   for (int sq=0; sq < 64; sq++, bit += bit) {
      if ( bb & bit) accu += weights[sq];
      // accu += weights[sq] & -((  bb & bit) == bit); // branchless 1
      // accu += weights[sq] & -(( ~bb & bit) == 0);   // branchless 2
   return accu;

The SSE2 routine expands a bitboard as a vector of 64 bits into 64-bytes inside four 128-bit xmm registers, and performs the multiplication with the byte-vector by bitwise 'and', before it finally adds all bytes together. The bitboard is therefor manifolded eight times with a sequence of seven unpack and interleave instructions to remain the same expanded byte-order of the bits from the bitboards, before to mask and compare them with a vector of bytes with one appropriate bit set each.

The dot product is designed for unsigned weights in the 0..63 range, so that vertical bytewise adds of the four weights can not overflow. Nevertheless, three PADDUSB - packed add unsigned byte with saturation instructions (_mm_adds_epu8) are used to limit the maximum four byte sum to 255 to make the routine more "robust" for cases with average weights greater than 63. The horizontal add of both quad words of the 128-bit xmmregister is performed by the PSADBW - packed sum of absolute differences of bytes into a word instruction (_mm_sad_epu8) with zero, while the final add of the two resulting word sums in the high and low quad word of the xmm register is done with general purpose registers.

#include <emmintrin.h>
#define XMM_ALIGN __declspec(align(16))

/* for average weights < 64 */
int dotProduct64(U64 bb, BYTE weights[] /* XMM_ALIGN */)
   static const U64 XMM_ALIGN sbitmask[2] = {
   __m128i x0, x1, x2, x3, bm;
   __m128i* pW = (__m128i*) weights;
   bm = _mm_load_si128 ( (__m128i*) sbitmask);
   x0 = _mm_cvtsi64x_si128(bb);        // 0000000000000000:8040201008040201
   // extend bits to bytes
   x0 = _mm_unpacklo_epi8  (x0, x0);   // 8080404020201010:0808040402020101
   x2 = _mm_unpackhi_epi16 (x0, x0);   // 8080808040404040:2020202010101010
   x0 = _mm_unpacklo_epi16 (x0, x0);   // 0808080804040404:0202020201010101
   x1 = _mm_unpackhi_epi32 (x0, x0);   // 0808080808080808:0404040404040404
   x0 = _mm_unpacklo_epi32 (x0, x0);   // 0202020202020202:0101010101010101
   x3 = _mm_unpackhi_epi32 (x2, x2);   // 8080808080808080:4040404040404040
   x2 = _mm_unpacklo_epi32 (x2, x2);   // 2020202020202020:1010101010101010
   x0 = _mm_and_si128      (x0, bm);
   x1 = _mm_and_si128      (x1, bm);
   x2 = _mm_and_si128      (x2, bm);
   x3 = _mm_and_si128      (x3, bm);
   x0 = _mm_cmpeq_epi8     (x0, bm);
   x1 = _mm_cmpeq_epi8     (x1, bm);
   x2 = _mm_cmpeq_epi8     (x2, bm);
   x3 = _mm_cmpeq_epi8     (x3, bm);
   // multiply by "and" with -1 or 0
   x0 = _mm_and_si128      (x0, pW[0]);
   x1 = _mm_and_si128      (x1, pW[1]);
   x2 = _mm_and_si128      (x2, pW[2]);
   x3 = _mm_and_si128      (x3, pW[3]);
   // add all bytes (with saturation)
   x0 = _mm_adds_epu8      (x0, x1);
   x0 = _mm_adds_epu8      (x0, x2);
   x0 = _mm_adds_epu8      (x0, x3);
   x0 = _mm_sad_epu8       (x0, _mm_setzero_si128 ());
   return _mm_cvtsi128_si32(x0)
        + _mm_extract_epi16(x0, 4);

Rotated Dot Product

A little bit cheaper is to expand the bitboard to a vector or 90 degree rotated {0,255} bytes, which requires a rotated weight vector as well [6].

/* for average weights < 64 */
int dotProduct64(U64 bb, BYTE weightsRot90[] /* XMM_ALIGN */)
   static const U64 CACHE_ALIGN masks[8] = {
      C64(0x0101010101010101), C64(0x0202020202020202),
      C64(0x0404040404040404), C64(0x0808080808080808),
      C64(0x1010101010101010), C64(0x2020202020202020),
      C64(0x4040404040404040), C64(0x8080808080808080),
   __m128i x0, x1, x2, x3, zr; U32 cnt;
   __m128i * pM = (__m128i*) masks;
   __m128i * pW = (__m128i*) weightsRot90;
   x0 = _mm_cvtsi64x_si128 (bb);
   x0 = _mm_unpacklo_epi64 (x0, x0);
   zr = _mm_setzero_si128  ();
   x3 = _mm_andnot_si128   (x0, pM[3]);
   x2 = _mm_andnot_si128   (x0, pM[2]);
   x1 = _mm_andnot_si128   (x0, pM[1]);
   x0 = _mm_andnot_si128   (x0, pM[0]);
   x3 = _mm_cmpeq_epi8     (x3, zr);
   x2 = _mm_cmpeq_epi8     (x2, zr);
   x1 = _mm_cmpeq_epi8     (x1, zr);
   x0 = _mm_cmpeq_epi8     (x0, zr);
   // multiply by "and" with -1 or 0
   x3 = _mm_and_si128      (x3, pW[3]);
   x2 = _mm_and_si128      (x2, pW[2]);
   x1 = _mm_and_si128      (x1, pW[1]);
   x0 = _mm_and_si128      (x0, pW[0]);
   // add all bytes (with saturation)
   x3 = _mm_adds_epu8      (x3, x2);
   x0 = _mm_adds_epu8      (x0, x1);
   x0 = _mm_adds_epu8      (x0, x3);
   x0 = _mm_sad_epu8       (x0, zr);
   return _mm_cvtsi128_si32(x0)
        + _mm_extract_epi16(x0, 4);

SSE2 Population Count

Following proposal of a SWAR-Popcount combined with a dot product might be quite competitive on recent x86-64 processors with a throughput of up to three simd-instructions per cycle [7] [8] . It determines the cardinalities of eight bitboards, multiplies them with a corresponding weight, a signed 16-bit word, to finally add all together as integer. However, Wojciech Muła's SSSE3 PopCnt would save some more cycles, even more with doubled or fourfold register widths using AVX2 or AVX-512.

 * popCountWeight8
 * @author Gerd Isenberg
 * @param bb vector of eight bitboards
 *        weight vector of eight short weights
 * @return sum(0,7) popcnt(bb[i]) * weight[i]
int popCountWeight8(const U64 bb[8], const short weight[8]) {
   static const U64 XMM_ALIGN masks[6] = {
      C64(0x5555555555555555), C64(0x5555555555555555),
      C64(0x3333333333333333), C64(0x3333333333333333),
      C64(0x0f0f0f0f0f0f0f0f), C64(0x0f0f0f0f0f0f0f0f)
   const __m128i* pM = (const __m128i*) masks;
   const __m128i* pb = (const __m128i*) bb;
   __m128i v = pb[0], w = pb[1], x = pb[2], y = pb[3];
   v = _mm_sub_epi8(v, _mm_and_si128(_mm_srli_epi64(v, 1), pM[0]));
   w = _mm_sub_epi8(w, _mm_and_si128(_mm_srli_epi64(w, 1), pM[0]));
   x = _mm_sub_epi8(x, _mm_and_si128(_mm_srli_epi64(x, 1), pM[0]));
   y = _mm_sub_epi8(y, _mm_and_si128(_mm_srli_epi64(y, 1), pM[0]));

   v = _mm_add_epi8(_mm_and_si128(v, pM[1]), _mm_and_si128(_mm_srli_epi64(v, 2), pM[1]));
   w = _mm_add_epi8(_mm_and_si128(w, pM[1]), _mm_and_si128(_mm_srli_epi64(w, 2), pM[1]));
   x = _mm_add_epi8(_mm_and_si128(x, pM[1]), _mm_and_si128(_mm_srli_epi64(x, 2), pM[1]));
   y = _mm_add_epi8(_mm_and_si128(y, pM[1]), _mm_and_si128(_mm_srli_epi64(y, 2), pM[1]));

   v = _mm_and_si128(_mm_add_epi8 (v, _mm_srli_epi64(v, 4)), pM[2]);
   w = _mm_and_si128(_mm_add_epi8 (w, _mm_srli_epi64(w, 4)), pM[2]);
   x = _mm_and_si128(_mm_add_epi8 (x, _mm_srli_epi64(x, 4)), pM[2]);
   y = _mm_and_si128(_mm_add_epi8 (y, _mm_srli_epi64(y, 4)), pM[2]);

   __m128i z = _mm_setzero_si128();
   v = _mm_packs_epi16(_mm_sad_epu8(v, z), _mm_sad_epu8(w, z));
   x = _mm_packs_epi16(_mm_sad_epu8(x, z), _mm_sad_epu8(y, z));
   const __m128i* pW = (const __m128i*) weight;
   v = _mm_madd_epi16 (_mm_packs_epi16(v, x), pW[0]);
   v = _mm_add_epi32  (v, _mm_srli_si128(v, 4));
   v = _mm_add_epi32  (v, _mm_srli_si128(v, 8));
   return _mm_cvtsi128_si32(v);

SSE2-Wrapper in C++

C++ quite efficiently allows to wrap all the intrinsics and to write classes with appropriate operators overloaded - the proposal here overloads + and - operators for byte- or rank-wise arithmetic, shifts work on 64-bit entities as often used in mentioned SSE2-routines or Kogge-Stone fill-stuff. A base class for the memory layout and two derivations provide to implement routines with SSE2 or general purpose instructions - or any other available SIMD-architecture like AltiVec. For instance a quad-bitboard attack-getter:

// T is either XMM or GPR
template <class T> inline
void eastAttacks(QBB& t, const QBB& s, U64 occ) {
   T* pt = (T*)&t;
   T r0([0]);
   T r1([2]);
   T o0(occ, occ);
   T o1  = o0 | r1;
   o0    = o0 | r0;
   pt[0] = o0 ^ ((o0 ^ r0) - r0);
   pt[1] = o1 ^ ((o1 ^ r1) - r1);

A proposal for a class skeleton:

class DBB
   friend class XMM;
   friend class GPR;
   ... more constructors
      __m128i x; // this intrinsice type is wrapped here
      u64 b[2];

// intrinsic sse2 xmm wrapper
class XMM : public DBB
   XMM(U64 b) {x = _mm_cvtsi64x_si128(b);}
   XMM(__m128i a){x = a;}
   XMM(U64 high, U64 low) ...
   ... more constructors

   XMM& operator>>=(int sh) {x = _mm_srli_epi64(x, sh); return *this;}
   XMM& operator<<=(int sh) {x = _mm_slli_epi64(x, sh); return *this;}
   XMM& operator&=(const XMM &a) {x = _mm_and_si128(x, a.x); return *this;}
   XMM& operator|=(const XMM &a) {x = _mm_or_si128 (x, a.x); return *this;}
   XMM& operator^=(const XMM &a) {x = _mm_xor_si128(x, a.x); return *this;}

   // byte- or rankwise arithmetic
   XMM& operator+=(const XMM &a) {x = _mm_add_epi8(x, a.x); return *this;}
   XMM& operator-=(const XMM &a) {x = _mm_sub_epi8(x, a.x); return *this;}

   friend XMM operator>>(const XMM &a, int sh) {return XMM(_mm_srli_epi64(a.x, sh));}
   friend XMM operator<<(const XMM &a, int sh) {return XMM(_mm_slli_epi64(a.x, sh));}
   friend XMM operator& (const XMM &a, const XMM &b) {return XMM(_mm_and_si128(a.x, b.x));}
   friend XMM operator| (const XMM &a, const XMM &b) {return XMM(_mm_or_si128(a.x, b.x));}
   friend XMM operator^ (const XMM &a, const XMM &b) {return XMM(_mm_xor_si128(a.x, b.x));}
   friend XMM operator+ (const XMM &a, const XMM &b) {return XMM(_mm_add_epi8(a.x, b.x));}
   friend XMM operator- (const XMM &a, const XMM &b) {return XMM(_mm_sub_epi8(a.x, b.x));}

// pure C wrapper
class GPR : public DBB

See also


Forum Posts

External Links

Calling conventions for different C++ compilers and operating systems (pdf) by Agner Fog
lineup: Hellmut Hattler, Peter Wolbrandt, Jan Fride Wolbrandt, Ingo Bischof


  1. AMD64 Architecture Programmer’s Manual Volume 4: 128-Bit and 256-Bit Media Instructions (pdf), has detailed explanations on all SSE2 128-Bit Media Instructions
  2. Integer Intrinsics Using Streaming SIMD Extensions 2 Visual C++ Developer Center - Run-Time Library Reference
  3. Instruction Reference Visual C++ Developer Center - Run-Time Library Reference
  4. Intel Intrinsics Guide
  5. Dot Product - from Wolfram MathWorld
  6. SSE2 bit[64] * byte[64] dot product by Gerd Isenberg, CCC, July 17, 2004
  7. Intel 64 and IA32 Architectures Optimization Reference Manual (pdf) Appendix C Instruction Latencies
  8. Software Optimization Guide for AMD Family 10h and 12h Processors (pdf) Appendix C Instruction Latencies

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