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by mitchs
1097 days ago
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The doubling of J as a means of striding across the array also gives me some concern. While it is cache related, it is cache related for sneaky reasons. J is going to end up, in binary, with a ton of 0s at the end after being doubled over and over. After 6 iterations after each reset, the position within the cache line is guaranteed to be 0. Using the standard 32K of L1 D$ would be 512x64 byte lines. Assuming 8-way associative (decently common as far as I know) means these 512 lines are organized into 64 set indexes, each with 8 lines. So after the next 6 post-reset iterations you are guaranteed to only be hitting the 0th index into the cache, effectively reducing the L1 cache size to 8 lines. (edit: Not the 0th index, but the same index as the base of the array.) |
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