--- a/mercurial/tags.py
+++ b/mercurial/tags.py
@@ -6,16 +6,29 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2, incorporated herein by reference.
 
-# Currently this module only deals with reading tags.  Soon it will grow
-# support for caching tag info.  Eventually, it could take care of
-# updating (adding/removing/moving) tags too.
+# Currently this module only deals with reading and caching tags.
+# Eventually, it could take care of updating (adding/removing/moving)
+# tags too.
 
-from node import bin, hex
+import os
+from node import nullid, bin, hex, short
 from i18n import _
 import encoding
 import error
 
-def findglobaltags(ui, repo, alltags, tagtypes):
+def _debugalways(ui, *msg):
+    ui.write(*msg)
+
+def _debugconditional(ui, *msg):
+    ui.debug(*msg)
+
+def _debugnever(ui, *msg):
+    pass
+
+_debug = _debugalways
+_debug = _debugnever
+
+def findglobaltags1(ui, repo, alltags, tagtypes):
     '''Find global tags in repo by reading .hgtags from every head that
     has a distinct version of it.  Updates the dicts alltags, tagtypes
     in place: alltags maps tag name to (node, hist) pair (see _readtags()
@@ -44,6 +57,36 @@
             ui, repo, fctx.data().splitlines(), fctx)
         _updatetags(filetags, "global", alltags, tagtypes)
 
+def findglobaltags2(ui, repo, alltags, tagtypes):
+    '''Same as findglobaltags1(), but with caching.'''
+    (heads, tagfnode, shouldwrite) = _readtagcache(ui, repo)
+
+    _debug(ui, "reading tags from %d head(s): %s\n"
+           % (len(heads), map(short, reversed(heads))))
+    seen = set()                    # set of fnode
+    fctx = None
+    for head in reversed(heads):        # oldest to newest
+        assert head in repo.changelog.nodemap, \
+               "tag cache returned bogus head %s" % short(head)
+
+        fnode = tagfnode.get(head)
+        if fnode and fnode not in seen:
+            seen.add(fnode)
+            if not fctx:
+                fctx = repo.filectx('.hgtags', fileid=fnode)
+            else:
+                fctx = fctx.filectx(fnode)
+
+            filetags = _readtags(ui, repo, fctx.data().splitlines(), fctx)
+            _updatetags(filetags, 'global', alltags, tagtypes)
+
+    # and update the cache (if necessary)
+    if shouldwrite:
+        _writetagcache(ui, repo, heads, tagfnode)
+
+# Set this to findglobaltags1 to disable tag caching.
+findglobaltags = findglobaltags2
+
 def readlocaltags(ui, repo, alltags, tagtypes):
     '''Read local tags in repo.  Update alltags and tagtypes.'''
     try:
@@ -120,3 +163,148 @@
         alltags[name] = anode, ahist
         tagtypes[name] = tagtype
 
+
+# The tag cache only stores info about heads, not the tag contents
+# from each head.  I.e. it doesn't try to squeeze out the maximum
+# performance, but is simpler has a better chance of actually
+# working correctly.  And this gives the biggest performance win: it
+# avoids looking up .hgtags in the manifest for every head, and it
+# can avoid calling heads() at all if there have been no changes to
+# the repo.
+
+def _readtagcache(ui, repo):
+    '''Read the tag cache and return a tuple (heads, fnodes,
+    shouldwrite).  heads is the list of all heads currently in the
+    repository (ordered from tip to oldest) and fnodes is a mapping from
+    head to .hgtags filenode.  Caller is responsible for reading tag
+    info from each head.'''
+
+    try:
+        cachefile = repo.opener('tags.cache', 'r')
+        _debug(ui, 'reading tag cache from %s\n' % cachefile.name)
+    except IOError:
+        cachefile = None
+
+    # The cache file consists of lines like
+    #   <headrev> <headnode> [<tagnode>]
+    # where <headrev> and <headnode> redundantly identify a repository
+    # head from the time the cache was written, and <tagnode> is the
+    # filenode of .hgtags on that head.  Heads with no .hgtags file will
+    # have no <tagnode>.  The cache is ordered from tip to oldest (which
+    # is part of why <headrev> is there: a quick visual check is all
+    # that's required to ensure correct order).
+    # 
+    # This information is enough to let us avoid the most expensive part
+    # of finding global tags, which is looking up <tagnode> in the
+    # manifest for each head.
+    cacherevs = []                      # list of headrev
+    cacheheads = []                     # list of headnode
+    cachefnode = {}                     # map headnode to filenode
+    if cachefile:
+        for line in cachefile:
+            line = line.rstrip().split()
+            cacherevs.append(int(line[0]))
+            headnode = bin(line[1])
+            cacheheads.append(headnode)
+            if len(line) == 3:
+                fnode = bin(line[2])
+                cachefnode[headnode] = fnode
+
+        cachefile.close()
+
+    tipnode = repo.changelog.tip()
+    tiprev = len(repo.changelog) - 1
+
+    # Case 1 (common): tip is the same, so nothing has changed.
+    # (Unchanged tip trivially means no changesets have been added.
+    # But, thanks to localrepository.destroyed(), it also means none
+    # have been destroyed by strip or rollback.)
+    if cacheheads and cacheheads[0] == tipnode and cacherevs[0] == tiprev:
+        _debug(ui, "tag cache: tip unchanged\n")
+        return (cacheheads, cachefnode, False)
+        
+    repoheads = repo.heads()
+
+    # Case 2 (uncommon): empty repo; get out quickly and don't bother
+    # writing an empty cache.
+    if repoheads == [nullid]:
+        return ([], {}, False)
+
+    # Case 3 (uncommon): cache file missing or empty.
+    if not cacheheads:
+        _debug(ui, 'tag cache: cache file missing or empty\n')
+
+    # Case 4 (uncommon): tip rev decreased.  This should only happen
+    # when we're called from localrepository.destroyed().  Refresh the
+    # cache so future invocations will not see disappeared heads in the
+    # cache.
+    elif cacheheads and tiprev < cacherevs[0]:
+        _debug(ui,
+               'tag cache: tip rev decremented (from %d to %d), '
+               'so we must be destroying nodes\n'
+               % (cacherevs[0], tiprev))
+
+    # Case 5 (common): tip has changed, so we've added/replaced heads.
+    else:
+        _debug(ui,
+               'tag cache: tip has changed (%d:%s); must find new heads\n'
+               % (tiprev, short(tipnode)))
+
+    # Luckily, the code to handle cases 3, 4, 5 is the same.  So the
+    # above if/elif/else can disappear once we're confident this thing
+    # actually works and we don't need the debug output.
+
+    # N.B. in case 4 (nodes destroyed), "new head" really means "newly
+    # exposed".
+    newheads = [head
+                for head in repoheads
+                if head not in set(cacheheads)]
+    _debug(ui, 'tag cache: found %d head(s) not in cache: %s\n'
+           % (len(newheads), map(short, newheads)))
+
+    # Now we have to lookup the .hgtags filenode for every new head.
+    # This is the most expensive part of finding tags, so performance
+    # depends primarily on the size of newheads.  Worst case: no cache
+    # file, so newheads == repoheads.
+    for head in newheads:
+        cctx = repo[head]
+        try:
+            fnode = cctx.filenode('.hgtags')
+            cachefnode[head] = fnode
+        except error.LookupError:
+            # no .hgtags file on this head
+            pass
+
+    # Caller has to iterate over all heads, but can use the filenodes in
+    # cachefnode to get to each .hgtags revision quickly.
+    return (repoheads, cachefnode, True)
+
+def _writetagcache(ui, repo, heads, tagfnode):
+
+    cachefile = repo.opener('tags.cache', 'w', atomictemp=True)
+    _debug(ui, 'writing cache file %s\n' % cachefile.name)
+
+    realheads = repo.heads()            # for sanity checks below
+    for head in heads:
+        # temporary sanity checks; these can probably be removed
+        # once this code has been in crew for a few weeks
+        assert head in repo.changelog.nodemap, \
+               'trying to write non-existent node %s to tag cache' % short(head)
+        assert head in realheads, \
+               'trying to write non-head %s to tag cache' % short(head)
+        assert head != nullid, \
+               'trying to write nullid to tag cache'
+
+        # This can't fail because of the first assert above.  When/if we
+        # remove that assert, we might want to catch LookupError here
+        # and downgrade it to a warning.
+        rev = repo.changelog.rev(head)
+
+        fnode = tagfnode.get(head)
+        if fnode:
+            cachefile.write('%d %s %s\n' % (rev, hex(head), hex(fnode)))
+        else:
+            cachefile.write('%d %s\n' % (rev, hex(head)))
+
+    cachefile.rename()
+    cachefile.close()