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Hash : 7c42945e
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Date : 2019-11-13T22:41:38
parser: fix quadratic pointer chasing In the AST, lists (e.g. the list of statements in a file) are kept in singly-linked lists -- each AST node has a `next` pointer available for this purpose. Previously, a node was added to the list by starting from the head, chasing to the last, and appending. So creating a list of length N would take ~N^2/2 pointer dereferences. Now, we always (temporarily) keep the last as well, so appending is O(1) instead of O(N). Given a keymap xkb_keymap { xkb_keycodes { minimum = 8; minimum = 8; minimum = 8; minimum = 8; minimum = 8; [... repeated N times ...] }; xkb_types {}; xkb_compat {}; xkb_symbols {}; }; The compilation times are N | Before | After --------|----------|------- 10,000 | 0.407s | 0.006s 20,000 | 1.851s | 0.015s 30,000 | 5.737s | 0.021s 40,000 | 12.759s | 0.023s 50,000 | 21.489s | 0.035s 60,000 | 40.473s | 0.041s 70,000 | 53.336s | 0.039s 80,000 | 72.485s | 0.044s 90,000 | 94.703s | 0.048s 100,000 | 118.390s | 0.057s Another option is to ditch the linked lists and use arrays instead. I got it to work, but its more involved and allocation heavy so turns out to be worse without further optimizations. Signed-off-by: Ran Benita <ran@unusedvar.com>
