Mercurial > hg
view mercurial/bdiff.c @ 42222:57203e0210f8
copies: calculate mergecopies() based on pathcopies()
When copies are stored in changesets, we need a changeset-centric
version of mergecopies() just like we have a changeset-centric version
of pathcopies(). I think the natural way of thinking about
mergecopies() is in terms of pathcopies() from the base to each of the
commits. So if we can rewrite mergecopies() based on two such
pathcopies() calls, we'll get the changeset-centric version for
free. That's what this patch does.
A nice bonus is that it ends up being a lot simpler. mergecopies() has
accumulated a lot of technical debt over time. One good example is the
code for dealing with grafts (the "partial/incomplete/dirty"
stuff). Since pathcopies() already deals with backwards renames and
ping-pong renames, we get that for free.
I've run tests with hard-coded debug logging for "fullcopy" and while
I haven't looked at every difference it produces, all the ones I have
looked at seemed reasonable to me. I'm a little surprised that no more
tests fail when run with '--extra-config-opt
experimental.copies.read-from=compatibility' compared to before this
patch. This patch also fixes the broken cases in test-annotate.t and
test-fastannotate.t. It also enables the part of test-copies.t that
was previously disabled exactly because mergecopies() needed to get a
changeset-centric version.
One drawback of the rewritten code is that we may now make
remotefilelog prefetch more files. We used to prefetch files that were
unique to either side of the merge compared to the other. We now
prefetch files that are unique to either side of the merge compared to
the base. This means that if you added the same file to each side, we
would not prefetch it before, but we would now. Such cases are
probably quite rare, but one likely scenario where they happen is when
moving from a commit to its successor (or the other way around). The
user will probably already have the files in the cache in such cases,
so it's probably not a big deal.
Some timings for calculating mergecopies between two revisions
(revisions shown on each line, all using the common ancestor as base):
In the hg repo:
4.8 4.9: 0.21s -> 0.21s
4.0 4.8: 0.35s -> 0.63s
In and old copy of the mozilla-unified repo:
FIREFOX_BETA_60_BASE^ FIREFOX_BETA_60_BASE: 0.82s -> 0.82s
FIREFOX_NIGHTLY_59_END FIREFOX_BETA_60_BASE: 2.5s -> 2.6s
FIREFOX_BETA_59_END FIREFOX_BETA_60_BASE: 3.9s -> 4.1s
FIREFOX_AURORA_50_BASE FIREFOX_BETA_60_BASE: 31s -> 33s
So it's measurably slower in most cases. The most significant
difference is in the hg repo between revisions 4.0 and 4.8. In that
case it seems to come from the fact that pathcopies() uses
fctx.isintroducedafter() (in _tracefile), while the old mergecopies()
used fctx.linkrev() (in _checkcopies()). That results in a single call
to filectx._adjustlinkrev(), which is responsible for the entire
difference in time (in my repo). So we pay a performance penalty but
we get more correct code (see change in
test-mv-cp-st-diff.t). Deleting the "== f.filenode()" in _tracefile()
recovers the lost performance in the hg repo.
There were are few other optimizations in _checkcopies() that I could
not measure any impact from. One was from the "seen" set. Another was
from a "continue" when the file was not in the destination manifest
(corresponding to "am" in _tracefile).
Also note that merge copies are not calculated when updating with a
clean working copy, which is probably the most common case. I
therefore think the much simpler code is worth the slowdown.
Differential Revision: https://phab.mercurial-scm.org/D6255
author | Martin von Zweigbergk <martinvonz@google.com> |
---|---|
date | Thu, 11 Apr 2019 23:22:54 -0700 |
parents | 763b45bc4483 |
children | d4ba4d51f85f |
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/* bdiff.c - efficient binary diff extension for Mercurial Copyright 2005, 2006 Matt Mackall <mpm@selenic.com> This software may be used and distributed according to the terms of the GNU General Public License, incorporated herein by reference. Based roughly on Python difflib */ #include <limits.h> #include <stdlib.h> #include <string.h> #include "bdiff.h" #include "bitmanipulation.h" #include "compat.h" /* Hash implementation from diffutils */ #define ROL(v, n) ((v) << (n) | (v) >> (sizeof(v) * CHAR_BIT - (n))) #define HASH(h, c) ((c) + ROL(h, 7)) struct pos { int pos, len; }; int bdiff_splitlines(const char *a, ssize_t len, struct bdiff_line **lr) { unsigned hash; int i; const char *p, *b = a; const char *const plast = a + len - 1; struct bdiff_line *l; /* count the lines */ i = 1; /* extra line for sentinel */ for (p = a; p < plast; p++) { if (*p == '\n') { i++; } } if (p == plast) { i++; } *lr = l = (struct bdiff_line *)calloc(i, sizeof(struct bdiff_line)); if (!l) { return -1; } /* build the line array and calculate hashes */ hash = 0; for (p = a; p < plast; p++) { hash = HASH(hash, *p); if (*p == '\n') { l->hash = hash; hash = 0; l->len = p - b + 1; l->l = b; l->n = INT_MAX; l++; b = p + 1; } } if (p == plast) { hash = HASH(hash, *p); l->hash = hash; l->len = p - b + 1; l->l = b; l->n = INT_MAX; l++; } /* set up a sentinel */ l->hash = 0; l->len = 0; l->l = a + len; return i - 1; } static inline int cmp(struct bdiff_line *a, struct bdiff_line *b) { return a->hash != b->hash || a->len != b->len || memcmp(a->l, b->l, a->len); } static int equatelines(struct bdiff_line *a, int an, struct bdiff_line *b, int bn) { int i, j, buckets = 1, t, scale; struct pos *h = NULL; /* build a hash table of the next highest power of 2 */ while (buckets < bn + 1) { buckets *= 2; } /* try to allocate a large hash table to avoid collisions */ for (scale = 4; scale; scale /= 2) { h = (struct pos *)calloc(buckets, scale * sizeof(struct pos)); if (h) { break; } } if (!h) { return 0; } buckets = buckets * scale - 1; /* clear the hash table */ for (i = 0; i <= buckets; i++) { h[i].pos = -1; h[i].len = 0; } /* add lines to the hash table chains */ for (i = 0; i < bn; i++) { /* find the equivalence class */ for (j = b[i].hash & buckets; h[j].pos != -1; j = (j + 1) & buckets) { if (!cmp(b + i, b + h[j].pos)) { break; } } /* add to the head of the equivalence class */ b[i].n = h[j].pos; b[i].e = j; h[j].pos = i; h[j].len++; /* keep track of popularity */ } /* compute popularity threshold */ t = (bn >= 31000) ? bn / 1000 : 1000000 / (bn + 1); /* match items in a to their equivalence class in b */ for (i = 0; i < an; i++) { /* find the equivalence class */ for (j = a[i].hash & buckets; h[j].pos != -1; j = (j + 1) & buckets) { if (!cmp(a + i, b + h[j].pos)) { break; } } a[i].e = j; /* use equivalence class for quick compare */ if (h[j].len <= t) { a[i].n = h[j].pos; /* point to head of match list */ } else { a[i].n = -1; /* too popular */ } } /* discard hash tables */ free(h); return 1; } static int longest_match(struct bdiff_line *a, struct bdiff_line *b, struct pos *pos, int a1, int a2, int b1, int b2, int *omi, int *omj) { int mi = a1, mj = b1, mk = 0, i, j, k, half, bhalf; /* window our search on large regions to better bound worst-case performance. by choosing a window at the end, we reduce skipping overhead on the b chains. */ if (a2 - a1 > 30000) { a1 = a2 - 30000; } half = (a1 + a2 - 1) / 2; bhalf = (b1 + b2 - 1) / 2; for (i = a1; i < a2; i++) { /* skip all lines in b after the current block */ for (j = a[i].n; j >= b2; j = b[j].n) { ; } /* loop through all lines match a[i] in b */ for (; j >= b1; j = b[j].n) { /* does this extend an earlier match? */ for (k = 1; j - k >= b1 && i - k >= a1; k++) { /* reached an earlier match? */ if (pos[j - k].pos == i - k) { k += pos[j - k].len; break; } /* previous line mismatch? */ if (a[i - k].e != b[j - k].e) { break; } } pos[j].pos = i; pos[j].len = k; /* best match so far? we prefer matches closer to the middle to balance recursion */ if (k > mk) { /* a longer match */ mi = i; mj = j; mk = k; } else if (k == mk) { if (i > mi && i <= half && j > b1) { /* same match but closer to half */ mi = i; mj = j; } else if (i == mi && (mj > bhalf || i == a1)) { /* same i but best earlier j */ mj = j; } } } } if (mk) { mi = mi - mk + 1; mj = mj - mk + 1; } /* expand match to include subsequent popular lines */ while (mi + mk < a2 && mj + mk < b2 && a[mi + mk].e == b[mj + mk].e) { mk++; } *omi = mi; *omj = mj; return mk; } static struct bdiff_hunk *recurse(struct bdiff_line *a, struct bdiff_line *b, struct pos *pos, int a1, int a2, int b1, int b2, struct bdiff_hunk *l) { int i, j, k; while (1) { /* find the longest match in this chunk */ k = longest_match(a, b, pos, a1, a2, b1, b2, &i, &j); if (!k) { return l; } /* and recurse on the remaining chunks on either side */ l = recurse(a, b, pos, a1, i, b1, j, l); if (!l) { return NULL; } l->next = (struct bdiff_hunk *)malloc(sizeof(struct bdiff_hunk)); if (!l->next) { return NULL; } l = l->next; l->a1 = i; l->a2 = i + k; l->b1 = j; l->b2 = j + k; l->next = NULL; /* tail-recursion didn't happen, so do equivalent iteration */ a1 = i + k; b1 = j + k; } } int bdiff_diff(struct bdiff_line *a, int an, struct bdiff_line *b, int bn, struct bdiff_hunk *base) { struct bdiff_hunk *curr; struct pos *pos; int t, count = 0; /* allocate and fill arrays */ t = equatelines(a, an, b, bn); pos = (struct pos *)calloc(bn ? bn : 1, sizeof(struct pos)); if (pos && t) { /* generate the matching block list */ curr = recurse(a, b, pos, 0, an, 0, bn, base); if (!curr) { return -1; } /* sentinel end hunk */ curr->next = (struct bdiff_hunk *)malloc(sizeof(struct bdiff_hunk)); if (!curr->next) { return -1; } curr = curr->next; curr->a1 = curr->a2 = an; curr->b1 = curr->b2 = bn; curr->next = NULL; } free(pos); /* normalize the hunk list, try to push each hunk towards the end */ for (curr = base->next; curr; curr = curr->next) { struct bdiff_hunk *next = curr->next; if (!next) { break; } if (curr->a2 == next->a1 || curr->b2 == next->b1) { while (curr->a2 < an && curr->b2 < bn && next->a1 < next->a2 && next->b1 < next->b2 && !cmp(a + curr->a2, b + curr->b2)) { curr->a2++; next->a1++; curr->b2++; next->b1++; } } } for (curr = base->next; curr; curr = curr->next) { count++; } return count; } /* deallocate list of hunks; l may be NULL */ void bdiff_freehunks(struct bdiff_hunk *l) { struct bdiff_hunk *n; for (; l; l = n) { n = l->next; free(l); } }