can explain when it's beneficial use parallel add/subtract arm instructions asx and/or sax? in situation/algorithm 1 need exhange halfwords, add , subtract upper/lower halfwords? below explanation of each:
asx
- exchange halfwords of
rm, add top halfwords , subtract bottom halfwords.sax
- exchange halfwords of
rm, subtract top halfwords , add bottom halfwords.
like russ schultz said in comment useful audio encoded l+r , l-r channels. used like:
ldr r1, [r0] ; r1 = (l-r):(l+r) shasx r1, r1, r1 ; r1 = ((l-r)+(l+r))/2:((l+r)-(l-r))/2 ; = (2l/2):(2r/2) ; = l:r ror r1, r1, #16 ; r1 = r:l str r1, [r0] the swapping of third operand's top , bottom halfwords necessary 2 different components can added/subtracted each other. without exchange you'd ((l-r)+(l-r))/2:((l+r)-(l+r))/2 = (2(l-r)/2):(0/2) = (l-r):0.
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