[jira] [Created] (SYSTEMML-1843) Wrong loop update-in-place decisions

["Matthias Boehm (JIRA)" <ji...@apache.org>]

Matthias Boehm created SYSTEMML-1843:
----------------------------------------

             Summary: Wrong loop update-in-place decisions 
                 Key: SYSTEMML-1843
                 URL: https://issues.apache.org/jira/browse/SYSTEMML-1843
             Project: SystemML
          Issue Type: Bug
            Reporter: Matthias Boehm


For special cases, where a matrix is simply updated in a loop, the rewrite for marking updated loop variables as update-in-place mistakenly flags these variables. For example, consider the following script:

{code}
...
for(i in 1:100) {
  q = as.matrix(sum(X * U%*%t(V)))
  print("at iteration "+i);
}
{code}

and the related hop explain output

{code}
FOR (lines 9-13) [in-place=[q]]
------GENERIC (lines 10-12) [recompile=true]
--------(46) TRead X [8026324,2330066,1000,1000,22507155] [0,0,1317 -> 1317MB], CP
--------(48) TRead U [8026324,10,1000,1000,80263240] [0,0,612 -> 612MB], CP
--------(49) TRead V [2330066,10,1000,1000,23300660] [0,0,178 -> 178MB], CP
--------(50) r(t) (49) [10,2330066,1000,1000,23300660] [178,0,178 -> 356MB], CP
--------(51) ba(+*) (48,50) [8026324,2330066,1000,1000,-1] [790,85611347,142683904 -> 228296041MB], SPARK
--------(52) b(*) (46,51) [8026324,2330066,1000,1000,-1] [142685221,0,1317 -> 142686537MB], SPARK
--------(53) ua(+RC) (52) [0,0,-1,-1,-1] [1317,0,0 -> 1317MB], SPARK
--------(54) u(cast_as_matrix) (53) [1,1,1000,1000,-1] [0,0,0 -> 0MB]
--------(55) TWrite q (54) [1,1,1000,1000,-1] [0,0,0 -> 0MB], CP
--------(47) TRead i [0,0,0,0,-1] [0,0,0 -> 0MB], CP
--------(57) b(+) (47) [0,0,-1,-1,-1] [0,0,0 -> 0MB], CP
--------(58) u(print) (57) [-1,-1,-1,-1,-1] [0,0,0 -> 0MB]
{code}

As can be seen above variable q is mistakenly marked as update in place, which causes unnecessary copies and thus can negatively affect performance.



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