Matthias Boehm created SYSTEMML1761:

Summary: Sparsityexploiting weighted squared loss w/o weights
Key: SYSTEMML1761
URL: https://issues.apache.org/jira/browse/SYSTEMML1761
Project: SystemML
Issue Type: Task
Reporter: Matthias Boehm
There are existing rewrites and fused operators for weighted squared loss (wsloss). However,
for the wsloss type {{NONE}}, i.e., without weights {{sum((X(U%*%t(V)))^2)}}, the implementation
is not sparsityexploiting leading huge (unnecessary) computation overhead. As it turns out
this expression can be rewritten into a sparsityexploiting form as follows:
{code}
sum ((X  U %*% t(V)) ^ 2)
> sum(X^2)  sum(2 * (X * (U%*%t(V)))) + sum((U%*%t(V))^2)
> sum(X^2)  sum(2 * (X * (U%*%t(V)))) + sum ((t(U) %*% U) * (t(V) %*% V))
{code}
This task aims to change the blocklevel wsloss NONE implementation to exploit this logical
rewrite by computing {{sum(X^2)  sum(2 * (X * (U%*%t(V))))}} in a sparsityexploiting pass
over nonzeros in X and a subsequent correct for {{+ sum ((t(U) %*% U) * (t(V) %*% V))}} via
two tsmm operations.

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