What is the fastest way to transpose the bits in an 8x8 block on bits?

Question

I\'m not sure the exact term for what I\'m trying to do. I have an 8x8 block of bits stored in 8 bytes, each byte stores one row. When

Answer 1

If you wanted an optimized solution you would use the SSE extensions in x86. You'd need to use 4 of these SIMD opcodes. MOVQ - move 8 bytes PSLLW - packed shift left logical words PMOVMSKB - packed move mask byte And 2 regular x86 opcodes LEA - load effective address MOV - move

byte[] m = byte[8]; //input
byte[] o = byte[8]; //output
LEA ecx, [o]
// ecx = the address of the output array/matrix
MOVQ xmm0, [m]
// xmm0 = 0|0|0|0|0|0|0|0|m[7]|m[6]|m[5]|m[4]|m[3]|m[2]|m[1]|m[0]
PMOVMSKB eax, xmm0
// eax = m[7][7]...m[0][7] the high bit of each byte
MOV [ecx+7], al
// o[7] is now the last column
PSLLW xmm0, 1
// shift 1 bit to the left
PMOVMSKB eax, xmm0
MOV [ecx+6], al
PSLLW xmm0, 1
PMOVMSKB eax, xmm0
MOV [ecx+5], al
PSLLW xmm0, 1
PMOVMSKB eax, xmm0
MOV [ecx+4], al
PSLLW xmm0, 1
PMOVMSKB eax, xmm0
MOV [ecx+3], al
PSLLW xmm0, 1
PMOVMSKB eax, xmm0
MOV [ecx+2], al
PSLLW xmm0, 1
PMOVMSKB eax, xmm0
MOV [ecx+1], al
PSLLW xmm0, 1
PMOVMSKB eax, xmm0
MOV [ecx], al

25 x86 opcodes/instructions as opposed to the stacked for...loop solution with 64 iterations. Sorry the notation is not the ATT style syntax that c/c++ compilers accept.