dotprod_mmx_assist.s

FEC Optimized viterbi code

# SIMD MMX dot product
# Equivalent to the following C code:
# long dotprod(signed short *a,signed short *b,int cnt)
# {
#	long sum = 0; 
#	cnt *= 4; 
#	while(cnt--)
#		sum += *a++ + *b++;
#	return sum;
# }
# a and b should also be 64-bit aligned, or speed will suffer greatly
# Copyright 1999, Phil Karn KA9Q
# May be used under the terms of the GNU Lesser General Public License (LGPL)
	
	.text
	.global dotprod_mmx_assist
	.type dotprod_mmx_assist,@function
dotprod_mmx_assist:
	pushl %ebp
	movl %esp,%ebp
	pushl %esi
	pushl %edi
	pushl %ecx
	pushl %ebx
	movl 8(%ebp),%esi	# a
	movl 12(%ebp),%edi	# b
	movl 16(%ebp),%ecx	# cnt
	pxor %mm0,%mm0		# clear running sum (in two 32-bit halves)
	
# MMX dot product loop unrolled 4 times, crunching 16 terms per loop
	.align 16
.Loop1:	subl $4,%ecx
	jl   .Loop1Done
	
	movq (%esi),%mm1	# mm1 = a[3],a[2],a[1],a[0]
 	pmaddwd (%edi),%mm1	# mm1 = b[3]*a[3]+b[2]*a[2],b[1]*a[1]+b[0]*a[0]
	paddd %mm1,%mm0
	
	movq 8(%esi),%mm1
	pmaddwd 8(%edi),%mm1
	paddd %mm1,%mm0

	movq 16(%esi),%mm1
	pmaddwd 16(%edi),%mm1
	paddd %mm1,%mm0

	movq 24(%esi),%mm1
	addl $32,%esi	
	pmaddwd 24(%edi),%mm1
	addl $32,%edi	
	paddd %mm1,%mm0

	jmp .Loop1
.Loop1Done:
	
	addl $4,%ecx	
	
# MMX dot product loop, not unrolled, crunching 4 terms per loop
# This could be redone as Duff's Device on the unrolled loop above
.Loop2:	subl $1,%ecx
	jl   .Loop2Done
	
	movq (%esi),%mm1
	addl $8,%esi
	pmaddwd (%edi),%mm1
	addl $8,%edi
	paddd %mm1,%mm0
	jmp .Loop2
.Loop2Done:
	
	movd %mm0,%ebx		# right-hand word to ebx
	punpckhdq %mm0,%mm0	# left-hand word to right side of %mm0
	movd %mm0,%eax
	addl %ebx,%eax		# running sum now in %eax
	emms			# done with MMX
	
	popl %ebx
	popl %ecx
	popl %edi
	popl %esi
	movl %ebp,%esp
	popl %ebp
	ret