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From: | Eric Blake |
Subject: | Re: [Qemu-devel] [PATCH 5/6] json: Eliminate lexer state IN_ERROR |
Date: | Tue, 28 Aug 2018 10:01:56 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 |
On 08/27/2018 11:40 PM, Markus Armbruster wrote:
typedef enum json_token_type { - JSON_MIN = 100, - JSON_LCURLY = JSON_MIN, + JSON_ERROR = 0, /* must be zero, see json_lexer[] */ + /* Gap for lexer states */ + JSON_LCURLY = 100, + JSON_MIN = JSON_LCURLY,In an earlier version of this type of cleanup, you swapped the IN_ and JSON_ values and eliminated the gap, to make the overall table more compact (no storage wasted on any of the states in the gap between the two). https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg01178.html Is it still worth trying to minimize the gap between the two sequences, even if you now no longer swap them in order?You caught me :) Eliminating the gap actually enlarges the table.
Rather, switching the order enlarges the table.
I first got confused, then measured the size change backwards to confirm my confused ideas. When I looked at the patch again, I realized my mistake, and silently dropped this part of the change.
The size of the table is determined by the fact that we must initialize entry 0 (whether we spell it IN_ERROR or JSON_ERROR), then pay attention to the largest value assigned. Re-reading json_lexer[], you are only initializing IN_* states, and not JSON_* states; swapping JSON_* to come first enlarged the table because you now have a bunch of additional rows in the table that are all 0-initialized to JSON_ERROR transitions.
So at the end of the day, leaving IN_* to be first, and putting JSON_* second, makes sense.
The question remains, then, if a fixed-size gap (by making JSON_MIN be exactly 100) is any smarter than a contiguous layout (by making JSON_MIN be IN_START_INTERP + 1). I can't see any strong reason for preferring one form over the other, so keeping the gap doesn't hurt.
-- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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