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b55b5c7280
The row-major matrix-vector multiplication code uses a threshold to check if processing 8 rows at a time would thrash the cache. This change introduces two modifications to this logic. 1. A smaller threshold for ARM and ARM64 devices. The value of this threshold was determined empirically using a Pixel2 phone, by benchmarking a large number of matrix-vector products in the range [1..4096]x[1..4096] and measuring performance separately on small and little cores with frequency pinning. On big (out-of-order) cores, this change has little to no impact. But on the small (in-order) cores, the matrix-vector products are up to 700% faster. Especially on large matrices. The motivation for this change was some internal code at Google which was using hand-written NEON for implementing similar functionality, processing the matrix one row at a time, which exhibited substantially better performance than Eigen. With the current change, Eigen handily beats that code. 2. Make the logic for choosing number of simultaneous rows apply unifiormly to 8, 4 and 2 rows instead of just 8 rows. Since the default threshold for non-ARM devices is essentially unchanged (32000 -> 32 * 1024), this change has no impact on non-ARM performance. This was verified by running the same set of benchmarks on a Xeon desktop.