Parallel Prefix
s3,4
c3
P0
P1
P2
P3
s1,2
c1
s5,6
c5
s7,8
c7
s1,4
c1,2
s5,8
c5,6
c...
c1,4
s1,8
c1,4
c1,2
c1,4
c1,6
c...
y1
y1,2
x1
x3
x2
x4
x5
x7
x6
x8
y1,3
y1,4
y1,5
y1,6
y1,7
y1,8
Pi does binary add, receives sum of items to its left, fwds to left subtree, adds sum of left subtree, sends to root of right subtree
