[GitHub] [systemds] mboehm7 commented on pull request #1159: [WIP] Advanced CSE

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mboehm7 commented on pull request #1159:
URL: https://github.com/apache/systemds/pull/1159#issuecomment-761688647


   Thanks for the initial document. The individual DAGs are delineated by conditional control flow and hence, there is only limited potential for doing CSE on sub-sequences of program blocks (and thus, DAGs). Doing CSE across control flow (e.g., across branches and loops) is possible but is not the first step for extensions. 
   
   I would recommend to focus on a single DAG with commutative operations like `A + 7` and `7 + A`, which are currently only indirectly eliminated (for plus and add, but not for `max(A,B)` vs `max(B,A)`). There are approaches: (1) either introduce canonicalization rewrites that systematically order inputs (e.g., by hop ID) or (2) extend the actual CSE rewrite for consider such permutations. I would recommend approach (1) which would be best added to  `RewriteAlgebraicSimplificationDynamic` as this canonicalization rewrite would need to check for matrix sizes too as we only support broadcasting (e.g., matrix vector elementwise operations) on the right hand side. 
   
   The scope of operations could be: OpOpN - MIN, MAX, PLUS, as well as OpOp2 - AND, OR, XOR, COV, EQUAL, NOTEQUAL, MIN, MAX, PLUS, MULT, and potentially other comparisons but they additionally require modifications of the comparison operator on reordering. 


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