Agner`s CPU blog

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Matrix multiplication
Author: Agner Date: 2016-07-29 07:51
Anonymous wrote:
Many algorithms use matrix multiplication, which can greatly benefit from instruction-level parallelism, but the current instructions sets are simply disappointing [...] it's simply not possible to express it nicely with the instructions given by x86 or x64.
I wonder if matrix multiplication is so important that it would be worthwhile to implement special instructions that facilitate matrix multiplication. A complete matrix multiply instruction would be far too complex, but instructions for doing the necessary permutations would be realistic.

Assume that we have a NxM matrix A and a MxP matrix B and we want to calculate the NxP product AB. The matrixes are stored in row-major order. We can calculate the matrix product in the following way. Copy the first column of A into P identical columns to make a NxP matrix with all columns identical. Copy the first row of B into N identical rows to make a NxP matrix with all rows identical. Multiply the two NxP matrixes, element by element. Do the same with the second column of A and the second row of B, and so on. Now we have M matrixes of size NxP. The result is the sum of these M matrixes.

If we have a universal permute instruction then we can copy the rows or columns without problems. However, the complexity of a full permute instruction grows with the vector length squared so this may be too expensive for very long vectors. Instead, we could have a repeat-block instruction which repeats the first sub-vector into a long vector, and a repeat-within-block instruction that copies the first element of each subvector into all elements of their subvector block. These instructions can be used for repeating the rows and columns, respectively. If the vector register is not long enough to hold the entire matrix, then split the matrix into multiple registers, each holding an integral number of rows.

This will still be somewhat complicated to implement, but much cheaper than a universal permute instruction.

 
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