From: Phillip Wood Date: Wed, 16 Jun 2021 08:57:49 GMT Subject: Re: [PATCH 0/2] RFC: implement new zdiff3 conflict style Message-ID: <255df678-9a31-bba2-f023-c7d98e5ffc15@gmail.com> In-Reply-To: On 15/06/2021 20:35, Elijah Newren wrote: > On Tue, Jun 15, 2021 at 2:43 AM Jeff King wrote: >> >> On Tue, Jun 15, 2021 at 05:16:08AM +0000, Elijah Newren via GitGitGadget wrote: >> >>> Implement a zealous diff3, or "zdiff3". This new mode is identical to >>> ordinary diff3 except that it allows compaction of common lines between the >>> two sides of history, if those common lines occur at the beginning or end of >>> a conflict hunk. >>> >>> This is just RFC, because I need to add tests. Also, while I've remerged >>> every merge, revert, or duly marked cherry-pick from both git.git and >>> linux.git with this patch using the new zdiff3 mode, that only shows it >>> doesn't segfault. (Though I also reran 10% of the linux remerges with zdiff3 >>> under valgrind without issues.) I looked through some differences between >>> --remerge-diff with diff3 and --remerge-diff with zdiff3, but those are >>> essentially diffs of a diff of a diff, which I found hard to read. I'd like >>> to look through more examples, and use it for a while before submitting the >>> patches without the RFC tag. >> >> I did something similar (but I wasn't smart enough to try your >> remerge-diff, and just re-ran a bunch of merges). > > What I did was great for testing if there were funny merges that might > cause segfaults or such with zdiff3, but not so clever for viewing the > direct output from zdiff3. Using remerge-diff in this way does not > show the [z]diff3 output directly, but a diff of that output against > what was ultimately recorded in the merge, forcing me to attempt to > recreate the original in my head. > > (And, of course, I made it even worse by taking the remerge-diff > output with diff3, and the remerge-diff output with zdiff3, and then > diffing those, resulting in yet another layer of diffs that I'd have > to undo in my head to attempt to construct the original.) > >> Skimming over the results, I didn't see anything that looked incorrect. >> Many of them are pretty non-exciting, though. A common case seems to be >> ones like 01a2a03c56 (Merge branch 'jc/diff-filter-negation', >> 2013-09-09), where two sides both add functions in the same place, and >> the common lines are just the closing "}" followed by a blank line. >> >> Removing those shared lines actually makes things less readable, IMHO, >> but I don't think it's the wrong thing to do. The usual "merge" zealous >> minimization likewise produces the same unreadability. If we want to >> address that, I think the best way would be by teaching the minimization >> some heuristics about which lines are trivial. >> >> Here's another interesting one. In 0c52457b7c (Merge branch >> 'nd/daemon-informative-errors-typofix', 2014-01-10), the diff3 looks >> like: >> >> <<<<<<< ours >> if (starts_with(arg, "--informative-errors")) { >> ||||||| base >> if (!prefixcmp(arg, "--informative-errors")) { >> ======= >> if (!strcmp(arg, "--informative-errors")) { >> >>>>>>> theirs >> informative_errors = 1; >> continue; >> } >> <<<<<<< ours >> if (starts_with(arg, "--no-informative-errors")) { >> ||||||| base >> if (!prefixcmp(arg, "--no-informative-errors")) { >> ======= >> if (!strcmp(arg, "--no-informative-errors")) { >> >>>>>>> theirs >> >> A little clunky, but it's easy-ish to see what's going on. With zdiff3, >> the context between the two hunks is rolled into a single hunk: >> >> <<<<<<< ours >> if (starts_with(arg, "--informative-errors")) { >> informative_errors = 1; >> continue; >> } >> if (starts_with(arg, "--no-informative-errors")) { >> ||||||| base >> if (!prefixcmp(arg, "--informative-errors")) { >> ======= >> if (!strcmp(arg, "--informative-errors")) { >> informative_errors = 1; >> continue; >> } >> if (!strcmp(arg, "--no-informative-errors")) { >> >>>>>>> theirs >> >> which seems worse. I haven't dug/thought carefully enough into your >> change yet to know if this is expected, or if there's a bug. XDL_MERGE_ZEALOUS coalesces adjacent conflicts that are separated by fewer than four lines. Unfortunately the existing code in xdl_merge_two_conflicts() only coalesces 'ours' and 'theirs', not 'base'. Applying diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c index b1dc9df7ea..5f76957169 100644 --- a/xdiff/xmerge.c +++ b/xdiff/xmerge.c @@ -455,6 +455,7 @@ static int lines_contain_alnum(xdfenv_t *xe, int i, int chg) static void xdl_merge_two_conflicts(xdmerge_t *m) { xdmerge_t *next_m = m->next; + m->chg0 = next_m->i0 + next_m->chg0 - m->i0; m->chg1 = next_m->i1 + next_m->chg1 - m->i1; m->chg2 = next_m->i2 + next_m->chg2 - m->i2; m->next = next_m->next; and running git checkout 0c52457b7c^ && bin-wrappers/git -c merge.conflictstyle=zdiff3 merge 0c52457b7c^2 gives <<<<<<< HEAD if (starts_with(arg, "--informative-errors")) { informative_errors = 1; continue; } if (starts_with(arg, "--no-informative-errors")) { ||||||| 2f93541d88 if (!prefixcmp(arg, "--informative-errors")) { informative_errors = 1; continue; } if (!prefixcmp(arg, "--no-informative-errors")) { ======= if (!strcmp(arg, "--informative-errors")) { informative_errors = 1; continue; } if (!strcmp(arg, "--no-informative-errors")) { >>>>>>> 0c52457b7c^2 Which I think is correct. Whether combining single line conflicts in this way is useful is a different question (and is independent of your patch). I can see that with larger conflicts it is worth it but here we end up with conflicts where 60% of the lines are from the base version. One the other hand there are fewer conflicts to resolve - I'm not sure which I prefer. Best Wishes Phillip > Yeah, I don't know for sure either but that does look buggy to me. > Thanks for digging up these examples. I'm a bit overdrawn on time for > this, so I'll pick it back up in a week or two and investigate this > case further. >