[GitHub] [systemds] mboehm7 commented on pull request #1385: [SYSTEMDS-3123] Rewrite cbind 0 Matrix Multiplication

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mboehm7 commented on pull request #1385:
URL: https://github.com/apache/systemds/pull/1385#issuecomment-915533612


   Thanks for staring the discussion, a few additional points to consider (besides separating it from the compression-related changed):
   * we need to check for know dims, otherwise the rewrite also triggers if hi.getDim2() is unknown (-1), and as we don't consider the other way around, it would never come back during recompilation
   * The overall benefit comes from reduced allocation in the cbind output (while incurring more floating point operations in the matrix multiply in some kernels that avoid branches in the inner loop). Accordingly, we cannot just look at `hi.getDim1() > hi.getDim2() * 2`. Instead given a m-x-n matrix X and n-x-k matrix Y, we have to compare the following: (a): m*k + m*(k+1) versus (b) n*k + m*(k+1), which reduces to comparing m and n, not m and (k+1), isn't it?


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