Module 17: Loops
Lecture 33: Data Dependence in Parallel Loops
Consider:
dopar i = 2, 20
X[i] = Y[i] + 1
Z[i] = X[i-1] + X[i] + X[i+1]
enddopar
I
S2
S3
2
3
4
X[2]=Y[2]+1
X[3]=Y[3]+1
X[4]=Y[4]+1
Z[2]=X[1]+X[2]+X[3]
Z[3]=X[2]+X[3]+X[4]
Z[4]=X[3]+X[4]+X[5]
Within each iteration
Across the iteration anti dependence from X[i] to X[i-1] and X[i+1]
Dosingle Loop
Single assignment rules excludes output dependence
There can not be anti dependence since each variable is defined only once
Any access conflict must resolve in favour of definition first followed by use, therefore, only flow dependencies
Consider:
dosingle i = 2, 20
X[i] = Y[i] + 1
Z[i] = X[i-1] + X[i] + X[i+1]
enddosingle
Def
Use
Dependence
Distance
X[i]
X[i-1]
X[i]
X[i+1]
flow
flow
flow
1
0
-1
