PCLMULLQLQDQ

What the fuck Intel?

.globl gfmul
gfmul:
#xmm0 holds operand a (128 bits)
#xmm1 holds operand b (128 bits)
#rdi holds the pointer to output (128 bits)

movdqa %xmm0, %xmm3
pclmulqdq $0, %xmm1, %xmm3 # xmm3 holds a0*b0
movdqa %xmm0, %xmm4
pclmulqdq $16, %xmm1, %xmm4 #xmm4 holds a0*b1
movdqa %xmm0, %xmm5
pclmulqdq $1, %xmm1, %xmm5 # xmm5 holds a1*b0
movdqa %xmm0, %xmm6
pclmulqdq $17, %xmm1, %xmm6 # xmm6 holds a1*b1
pxor %xmm5, %xmm4 # xmm4 holds a0*b1 + a1*b0
movdqa %xmm4, %xmm5
psrldq $8, %xmm4
pslldq $8, %xmm5
pxor %xmm5, %xmm3
pxor %xmm4, %xmm6 # <xmm6:xmm3> holds the result of
# the carry-less multiplication of xmm0 by xmm1
# shift the result by one bit position to the left cope for the fact
# that bits are reversed
movdqa %xmm3, %xmm7
movdqa %xmm6, %xmm8
pslld $1, %xmm3
pslld $1, %xmm6
psrld $31, %xmm7
psrld $31, %xmm8
movdqa %xmm7, %xmm9
pslldq $4, %xmm8
pslldq $4, %xmm7
psrldq $12, %xmm9
por %xmm7, %xmm3
por %xmm8, %xmm6
por %xmm9, %xmm6
#first phase of the reduction
movdqa %xmm3, %xmm7
movdqa %xmm3, %xmm8
movdqa %xmm3, %xmm9
pslld $31, %xmm7 # packed right shifting << 31
pslld $30, %xmm8 # packed right shifting shift << 30
pslld $25, %xmm9 # packed right shifting shift << 25
pxor %xmm8, %xmm7 # xor the shifted versions
pxor %xmm9, %xmm7

movdqa %xmm7, %xmm8
pslldq $12, %xmm7
psrldq $4, %xmm8
pxor %xmm7, %xmm3 # first phase of the reduction complete
movdqa %xmm3,%xmm2 # second phase of the reduction
movdqa %xmm3,%xmm4
movdqa %xmm3,%xmm5
psrld $1, %xmm2 # packed left shifting >> 1
psrld $2, %xmm4 # packed left shifting >> 2
psrld $7, %xmm5 # packed left shifting >> 7
pxor %xmm4, %xmm2 # xor the shifted versions
pxor %xmm5, %xmm2
pxor %xmm8, %xmm2
pxor %xmm2, %xmm3
pxor %xmm3, %xmm6 # the result is in xmm6
movdqu %xmm6, (%rdi) # store the result
ret

Picklemullkillkdick

x86 is like Common Lisp. Everything was added at different times for different reasons and the names show this.

>>1
The worst part is the P at the beginning, it's utterly meaningless, since just about every instruction a CPU does is ``performing" some kind of operation. We don't have PMUL, PADD, PJMP, or PCALL instructions, even though we most certainly do ``perform" multiplications, additions, jumps, and function calls. Other than that, we should just drop the data size suffixes and let the assembler figure them out. CLMUL is a perfectly workable instruction mnemonic.

>>5

CLMUL

Common Lisp multiply?

Perform carry-less multiply from lower quad and lower quad to double quad?

I liked it better when there was just mul and imul.

>>5
P is for Packed, meaning it operates on the MMX/XMM registers.

The whole expansion is PackedCarryLessMULtiplyLowQuadwordLowQuadwordDoubleQuadword