On Mon, Jul 06, 2015 at 03:54:35PM -0700, Junio C Hamano wrote:
> Mike Hommey <mh@xxxxxxxxxxxx> writes:
>
> > One of the first things parse_from does is unconditionally throw away
> > the tree for the given branch, and then the "from" tree is loaded. So
> > when the "from" commit is the current head of the branch, that make
> > fast-import do more work than necessary.
>
> If it is very common that the next commit the input stream wants to
> create is often on top of the commit that was just created, and if
> it is very common that the input stream producer knows what the
> commit object name of the commit that was just created, then
> optimising for that case does not sound too bad. It really depends
> on two factors:
>
> - How likely is it that other people make the same mistake?
Looks like the answer is: very. Assuming my quick glance at the code is
not mistaken, the same mistake is made in at least git-p4.py (yes, the
one that comes with git), felipec's git-remote-hg, and hg-fast-export,
and that's 100% of the sample I looked at.
I won't claim to know what fast-import is doing, not having looked at
more than the parse_from* functions and the commit message for 4cabf858,
but it seems plausible this also skips making tree deltas for those
trees.
> - How bad the damage to parse_from() would be if we wanted to
> optimize for this case?
I /think/ it would look like this (untested), which doesn't seem too
damaging:
diff --git a/fast-import.c b/fast-import.c
index e78ca10..cb232e0 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -2588,14 +2588,12 @@ static int parse_from(struct branch *b)
{
const char *from;
struct branch *s;
+ unsigned char sha1[20];
if (!skip_prefix(command_buf.buf, "from ", &from))
return 0;
- if (b->branch_tree.tree) {
- release_tree_content_recursive(b->branch_tree.tree);
- b->branch_tree.tree = NULL;
- }
+ hashcpy(sha1, b->branch_tree.versions[1].sha1);
s = lookup_branch(from);
if (b == s)
@@ -2626,6 +2624,11 @@ static int parse_from(struct branch *b)
else
die("Invalid ref name or SHA1 expression: %s", from);
+ if (b->branch_tree.tree && hashcmp(sha1, b->branch_tree.versions[1].sha1)) {
+ release_tree_content_recursive(b->branch_tree.tree);
+ b->branch_tree.tree = NULL;
+ }
+
read_next_command();
return 1;
}
Mike
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