From cf2dc1c238c6fd5f93c315a3045ccf95459701cd Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Wed, 7 Jul 2021 23:10:15 +0000
Subject: [PATCH 1/5] speed up alt_odb_usable() with many alternates

With many alternates, the duplicate check in alt_odb_usable()
wastes many cycles doing repeated fspathcmp() on every existing
alternate.  Use a khash to speed up lookups by odb->path.

Since the kh_put_* API uses the supplied key without
duplicating it, we also take advantage of it to replace both
xstrdup() and strbuf_release() in link_alt_odb_entry() with
strbuf_detach() to avoid the allocation and copy.

In a test repository with 50K alternates and each of those 50K
alternates having one alternate each (for a total of 100K total
alternates); this speeds up lookup of a non-existent blob from
over 16 minutes to roughly 2.7 seconds on my busy workstation.

Note: all underlying git object directories were small and
unpacked with only loose objects and no packs.  Having to load
packs increases times significantly.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 dir.c          | 10 ++++++++++
 dir.h          |  2 ++
 object-file.c  | 35 ++++++++++++++++++++++-------------
 object-store.h |  7 +++++++
 object.c       |  2 ++
 5 files changed, 43 insertions(+), 13 deletions(-)

diff --git a/dir.c b/dir.c
index ebe5ec046e..20b942d161 100644
--- a/dir.c
+++ b/dir.c
@@ -84,11 +84,21 @@ int fspathcmp(const char *a, const char *b)
 	return ignore_case ? strcasecmp(a, b) : strcmp(a, b);
 }
 
+int fspatheq(const char *a, const char *b)
+{
+	return !fspathcmp(a, b);
+}
+
 int fspathncmp(const char *a, const char *b, size_t count)
 {
 	return ignore_case ? strncasecmp(a, b, count) : strncmp(a, b, count);
 }
 
+unsigned int fspathhash(const char *str)
+{
+	return ignore_case ? strihash(str) : strhash(str);
+}
+
 int git_fnmatch(const struct pathspec_item *item,
 		const char *pattern, const char *string,
 		int prefix)
diff --git a/dir.h b/dir.h
index e3db9b9ec6..2af7bcd7e5 100644
--- a/dir.h
+++ b/dir.h
@@ -489,7 +489,9 @@ int remove_dir_recursively(struct strbuf *path, int flag);
 int remove_path(const char *path);
 
 int fspathcmp(const char *a, const char *b);
+int fspatheq(const char *a, const char *b);
 int fspathncmp(const char *a, const char *b, size_t count);
+unsigned int fspathhash(const char *str);
 
 /*
  * The prefix part of pattern must not contains wildcards.
diff --git a/object-file.c b/object-file.c
index f233b440b2..a13f49b192 100644
--- a/object-file.c
+++ b/object-file.c
@@ -517,9 +517,9 @@ const char *loose_object_path(struct repository *r, struct strbuf *buf,
  */
 static int alt_odb_usable(struct raw_object_store *o,
 			  struct strbuf *path,
-			  const char *normalized_objdir)
+			  const char *normalized_objdir, khiter_t *pos)
 {
-	struct object_directory *odb;
+	int r;
 
 	/* Detect cases where alternate disappeared */
 	if (!is_directory(path->buf)) {
@@ -533,14 +533,20 @@ static int alt_odb_usable(struct raw_object_store *o,
 	 * Prevent the common mistake of listing the same
 	 * thing twice, or object directory itself.
 	 */
-	for (odb = o->odb; odb; odb = odb->next) {
-		if (!fspathcmp(path->buf, odb->path))
-			return 0;
-	}
-	if (!fspathcmp(path->buf, normalized_objdir))
-		return 0;
+	if (!o->odb_by_path) {
+		khiter_t p;
 
-	return 1;
+		o->odb_by_path = kh_init_odb_path_map();
+		assert(!o->odb->next);
+		p = kh_put_odb_path_map(o->odb_by_path, o->odb->path, &r);
+		assert(r == 1); /* never used */
+		kh_value(o->odb_by_path, p) = o->odb;
+	}
+	if (fspatheq(path->buf, normalized_objdir))
+		return 0;
+	*pos = kh_put_odb_path_map(o->odb_by_path, path->buf, &r);
+	/* r: 0 = exists, 1 = never used, 2 = deleted */
+	return r == 0 ? 0 : 1;
 }
 
 /*
@@ -566,6 +572,7 @@ static int link_alt_odb_entry(struct repository *r, const char *entry,
 {
 	struct object_directory *ent;
 	struct strbuf pathbuf = STRBUF_INIT;
+	khiter_t pos;
 
 	if (!is_absolute_path(entry) && relative_base) {
 		strbuf_realpath(&pathbuf, relative_base, 1);
@@ -587,23 +594,25 @@ static int link_alt_odb_entry(struct repository *r, const char *entry,
 	while (pathbuf.len && pathbuf.buf[pathbuf.len - 1] == '/')
 		strbuf_setlen(&pathbuf, pathbuf.len - 1);
 
-	if (!alt_odb_usable(r->objects, &pathbuf, normalized_objdir)) {
+	if (!alt_odb_usable(r->objects, &pathbuf, normalized_objdir, &pos)) {
 		strbuf_release(&pathbuf);
 		return -1;
 	}
 
 	CALLOC_ARRAY(ent, 1);
-	ent->path = xstrdup(pathbuf.buf);
+	/* pathbuf.buf is already in r->objects->odb_by_path */
+	ent->path = strbuf_detach(&pathbuf, NULL);
 
 	/* add the alternate entry */
 	*r->objects->odb_tail = ent;
 	r->objects->odb_tail = &(ent->next);
 	ent->next = NULL;
+	assert(r->objects->odb_by_path);
+	kh_value(r->objects->odb_by_path, pos) = ent;
 
 	/* recursively add alternates */
-	read_info_alternates(r, pathbuf.buf, depth + 1);
+	read_info_alternates(r, ent->path, depth + 1);
 
-	strbuf_release(&pathbuf);
 	return 0;
 }
 
diff --git a/object-store.h b/object-store.h
index ec32c23dcb..6077065d90 100644
--- a/object-store.h
+++ b/object-store.h
@@ -7,6 +7,8 @@
 #include "oid-array.h"
 #include "strbuf.h"
 #include "thread-utils.h"
+#include "khash.h"
+#include "dir.h"
 
 struct object_directory {
 	struct object_directory *next;
@@ -30,6 +32,9 @@ struct object_directory {
 	char *path;
 };
 
+KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
+	struct object_directory *, 1, fspathhash, fspatheq);
+
 void prepare_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
 typedef int alt_odb_fn(struct object_directory *, void *);
@@ -116,6 +121,8 @@ struct raw_object_store {
 	 */
 	struct object_directory *odb;
 	struct object_directory **odb_tail;
+	kh_odb_path_map_t *odb_by_path;
+
 	int loaded_alternates;
 
 	/*
diff --git a/object.c b/object.c
index 14188453c5..2b3c075a15 100644
--- a/object.c
+++ b/object.c
@@ -511,6 +511,8 @@ static void free_object_directories(struct raw_object_store *o)
 		free_object_directory(o->odb);
 		o->odb = next;
 	}
+	kh_destroy_odb_path_map(o->odb_by_path);
+	o->odb_by_path = NULL;
 }
 
 void raw_object_store_clear(struct raw_object_store *o)

From 407532f82d3fdfd18d4ec276ddeb359e7c724aa6 Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Wed, 7 Jul 2021 23:10:16 +0000
Subject: [PATCH 2/5] avoid strlen via strbuf_addstr in link_alt_odb_entry

We can save a few milliseconds (across 100K odbs) by using
strbuf_addbuf() instead of strbuf_addstr() by passing `entry' as
a strbuf pointer rather than a "const char *".

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 object-file.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/object-file.c b/object-file.c
index a13f49b192..2dd70ddf3a 100644
--- a/object-file.c
+++ b/object-file.c
@@ -567,18 +567,18 @@ static int alt_odb_usable(struct raw_object_store *o,
 static void read_info_alternates(struct repository *r,
 				 const char *relative_base,
 				 int depth);
-static int link_alt_odb_entry(struct repository *r, const char *entry,
+static int link_alt_odb_entry(struct repository *r, const struct strbuf *entry,
 	const char *relative_base, int depth, const char *normalized_objdir)
 {
 	struct object_directory *ent;
 	struct strbuf pathbuf = STRBUF_INIT;
 	khiter_t pos;
 
-	if (!is_absolute_path(entry) && relative_base) {
+	if (!is_absolute_path(entry->buf) && relative_base) {
 		strbuf_realpath(&pathbuf, relative_base, 1);
 		strbuf_addch(&pathbuf, '/');
 	}
-	strbuf_addstr(&pathbuf, entry);
+	strbuf_addbuf(&pathbuf, entry);
 
 	if (strbuf_normalize_path(&pathbuf) < 0 && relative_base) {
 		error(_("unable to normalize alternate object path: %s"),
@@ -669,7 +669,7 @@ static void link_alt_odb_entries(struct repository *r, const char *alt,
 		alt = parse_alt_odb_entry(alt, sep, &entry);
 		if (!entry.len)
 			continue;
-		link_alt_odb_entry(r, entry.buf,
+		link_alt_odb_entry(r, &entry,
 				   relative_base, depth, objdirbuf.buf);
 	}
 	strbuf_release(&entry);

From 33f379eee63a0529f85079857598ac6325d3aec5 Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Wed, 7 Jul 2021 23:10:17 +0000
Subject: [PATCH 3/5] make object_directory.loose_objects_subdir_seen a bitmap

There's no point in using 8 bits per-directory when 1 bit
will do.  This saves us 224 bytes per object directory, which
ends up being 22MB when dealing with 100K alternates.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 object-file.c  | 11 ++++++++---
 object-store.h |  2 +-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/object-file.c b/object-file.c
index 2dd70ddf3a..91ded8c22a 100644
--- a/object-file.c
+++ b/object-file.c
@@ -2461,12 +2461,17 @@ struct oid_array *odb_loose_cache(struct object_directory *odb,
 {
 	int subdir_nr = oid->hash[0];
 	struct strbuf buf = STRBUF_INIT;
+	size_t word_bits = bitsizeof(odb->loose_objects_subdir_seen[0]);
+	size_t word_index = subdir_nr / word_bits;
+	size_t mask = 1 << (subdir_nr % word_bits);
+	uint32_t *bitmap;
 
 	if (subdir_nr < 0 ||
-	    subdir_nr >= ARRAY_SIZE(odb->loose_objects_subdir_seen))
+	    subdir_nr >= bitsizeof(odb->loose_objects_subdir_seen))
 		BUG("subdir_nr out of range");
 
-	if (odb->loose_objects_subdir_seen[subdir_nr])
+	bitmap = &odb->loose_objects_subdir_seen[word_index];
+	if (*bitmap & mask)
 		return &odb->loose_objects_cache[subdir_nr];
 
 	strbuf_addstr(&buf, odb->path);
@@ -2474,7 +2479,7 @@ struct oid_array *odb_loose_cache(struct object_directory *odb,
 				    append_loose_object,
 				    NULL, NULL,
 				    &odb->loose_objects_cache[subdir_nr]);
-	odb->loose_objects_subdir_seen[subdir_nr] = 1;
+	*bitmap |= mask;
 	strbuf_release(&buf);
 	return &odb->loose_objects_cache[subdir_nr];
 }
diff --git a/object-store.h b/object-store.h
index 6077065d90..ab6d469970 100644
--- a/object-store.h
+++ b/object-store.h
@@ -22,7 +22,7 @@ struct object_directory {
 	 *
 	 * Be sure to call odb_load_loose_cache() before using.
 	 */
-	char loose_objects_subdir_seen[256];
+	uint32_t loose_objects_subdir_seen[8]; /* 256 bits */
 	struct oid_array loose_objects_cache[256];
 
 	/*

From 90e07f0a342df836a33b92e179eb105243dba88d Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Wed, 7 Jul 2021 23:10:18 +0000
Subject: [PATCH 4/5] oidcpy_with_padding: constify `src' arg

As with `oidcpy', the source struct will not be modified and
this will allow an upcoming const-correct caller to use it.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 hash.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hash.h b/hash.h
index 9c6df4d952..27a180248f 100644
--- a/hash.h
+++ b/hash.h
@@ -265,7 +265,7 @@ static inline void oidcpy(struct object_id *dst, const struct object_id *src)
 
 /* Like oidcpy() but zero-pads the unused bytes in dst's hash array. */
 static inline void oidcpy_with_padding(struct object_id *dst,
-				       struct object_id *src)
+				       const struct object_id *src)
 {
 	size_t hashsz;
 

From 92d8ed8ac101d62183d51f280b90efb1de1bda5c Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Wed, 7 Jul 2021 23:10:19 +0000
Subject: [PATCH 5/5] oidtree: a crit-bit tree for odb_loose_cache

This saves 8K per `struct object_directory', meaning it saves
around 800MB in my case involving 100K alternates (half or more
of those alternates are unlikely to hold loose objects).

This is implemented in two parts: a generic, allocation-free
`cbtree' and the `oidtree' wrapper on top of it.  The latter
provides allocation using alloc_state as a memory pool to
improve locality and reduce free(3) overhead.

Unlike oid-array, the crit-bit tree does not require sorting.
Performance is bound by the key length, for oidtree that is
fixed at sizeof(struct object_id).  There's no need to have
256 oidtrees to mitigate the O(n log n) overhead like we did
with oid-array.

Being a prefix trie, it is natively suited for expanding short
object IDs via prefix-limited iteration in
`find_short_object_filename'.

On my busy workstation, p4205 performance seems to be roughly
unchanged (+/-8%).  Startup with 100K total alternates with no
loose objects seems around 10-20% faster on a hot cache.
(800MB in memory savings means more memory for the kernel FS
cache).

The generic cbtree implementation does impose some extra
overhead for oidtree in that it uses memcmp(3) on
"struct object_id" so it wastes cycles comparing 12 extra bytes
on SHA-1 repositories.  I've not yet explored reducing this
overhead, but I expect there are many places in our code base
where we'd want to investigate this.

More information on crit-bit trees: https://cr.yp.to/critbit.html

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Makefile                |   3 +
 cbtree.c                | 167 ++++++++++++++++++++++++++++++++++++++++
 cbtree.h                |  56 ++++++++++++++
 object-file.c           |  23 +++---
 object-name.c           |  28 +++----
 object-store.h          |   5 +-
 oidtree.c               | 104 +++++++++++++++++++++++++
 oidtree.h               |  22 ++++++
 t/helper/test-oidtree.c |  49 ++++++++++++
 t/helper/test-tool.c    |   1 +
 t/helper/test-tool.h    |   1 +
 t/t0069-oidtree.sh      |  49 ++++++++++++
 12 files changed, 478 insertions(+), 30 deletions(-)
 create mode 100644 cbtree.c
 create mode 100644 cbtree.h
 create mode 100644 oidtree.c
 create mode 100644 oidtree.h
 create mode 100644 t/helper/test-oidtree.c
 create mode 100755 t/t0069-oidtree.sh

diff --git a/Makefile b/Makefile
index c3565fc0f8..a1525978fb 100644
--- a/Makefile
+++ b/Makefile
@@ -722,6 +722,7 @@ TEST_BUILTINS_OBJS += test-mergesort.o
 TEST_BUILTINS_OBJS += test-mktemp.o
 TEST_BUILTINS_OBJS += test-oid-array.o
 TEST_BUILTINS_OBJS += test-oidmap.o
+TEST_BUILTINS_OBJS += test-oidtree.o
 TEST_BUILTINS_OBJS += test-online-cpus.o
 TEST_BUILTINS_OBJS += test-parse-options.o
 TEST_BUILTINS_OBJS += test-parse-pathspec-file.o
@@ -845,6 +846,7 @@ LIB_OBJS += branch.o
 LIB_OBJS += bulk-checkin.o
 LIB_OBJS += bundle.o
 LIB_OBJS += cache-tree.o
+LIB_OBJS += cbtree.o
 LIB_OBJS += chdir-notify.o
 LIB_OBJS += checkout.o
 LIB_OBJS += chunk-format.o
@@ -940,6 +942,7 @@ LIB_OBJS += object.o
 LIB_OBJS += oid-array.o
 LIB_OBJS += oidmap.o
 LIB_OBJS += oidset.o
+LIB_OBJS += oidtree.o
 LIB_OBJS += pack-bitmap-write.o
 LIB_OBJS += pack-bitmap.o
 LIB_OBJS += pack-check.o
diff --git a/cbtree.c b/cbtree.c
new file mode 100644
index 0000000000..b0c65d810f
--- /dev/null
+++ b/cbtree.c
@@ -0,0 +1,167 @@
+/*
+ * crit-bit tree implementation, does no allocations internally
+ * For more information on crit-bit trees: https://cr.yp.to/critbit.html
+ * Based on Adam Langley's adaptation of Dan Bernstein's public domain code
+ * git clone https://github.com/agl/critbit.git
+ */
+#include "cbtree.h"
+
+static struct cb_node *cb_node_of(const void *p)
+{
+	return (struct cb_node *)((uintptr_t)p - 1);
+}
+
+/* locate the best match, does not do a final comparision */
+static struct cb_node *cb_internal_best_match(struct cb_node *p,
+					const uint8_t *k, size_t klen)
+{
+	while (1 & (uintptr_t)p) {
+		struct cb_node *q = cb_node_of(p);
+		uint8_t c = q->byte < klen ? k[q->byte] : 0;
+		size_t direction = (1 + (q->otherbits | c)) >> 8;
+
+		p = q->child[direction];
+	}
+	return p;
+}
+
+/* returns NULL if successful, existing cb_node if duplicate */
+struct cb_node *cb_insert(struct cb_tree *t, struct cb_node *node, size_t klen)
+{
+	size_t newbyte, newotherbits;
+	uint8_t c;
+	int newdirection;
+	struct cb_node **wherep, *p;
+
+	assert(!((uintptr_t)node & 1)); /* allocations must be aligned */
+
+	if (!t->root) {		/* insert into empty tree */
+		t->root = node;
+		return NULL;	/* success */
+	}
+
+	/* see if a node already exists */
+	p = cb_internal_best_match(t->root, node->k, klen);
+
+	/* find first differing byte */
+	for (newbyte = 0; newbyte < klen; newbyte++) {
+		if (p->k[newbyte] != node->k[newbyte])
+			goto different_byte_found;
+	}
+	return p;	/* element exists, let user deal with it */
+
+different_byte_found:
+	newotherbits = p->k[newbyte] ^ node->k[newbyte];
+	newotherbits |= newotherbits >> 1;
+	newotherbits |= newotherbits >> 2;
+	newotherbits |= newotherbits >> 4;
+	newotherbits = (newotherbits & ~(newotherbits >> 1)) ^ 255;
+	c = p->k[newbyte];
+	newdirection = (1 + (newotherbits | c)) >> 8;
+
+	node->byte = newbyte;
+	node->otherbits = newotherbits;
+	node->child[1 - newdirection] = node;
+
+	/* find a place to insert it */
+	wherep = &t->root;
+	for (;;) {
+		struct cb_node *q;
+		size_t direction;
+
+		p = *wherep;
+		if (!(1 & (uintptr_t)p))
+			break;
+		q = cb_node_of(p);
+		if (q->byte > newbyte)
+			break;
+		if (q->byte == newbyte && q->otherbits > newotherbits)
+			break;
+		c = q->byte < klen ? node->k[q->byte] : 0;
+		direction = (1 + (q->otherbits | c)) >> 8;
+		wherep = q->child + direction;
+	}
+
+	node->child[newdirection] = *wherep;
+	*wherep = (struct cb_node *)(1 + (uintptr_t)node);
+
+	return NULL; /* success */
+}
+
+struct cb_node *cb_lookup(struct cb_tree *t, const uint8_t *k, size_t klen)
+{
+	struct cb_node *p = cb_internal_best_match(t->root, k, klen);
+
+	return p && !memcmp(p->k, k, klen) ? p : NULL;
+}
+
+struct cb_node *cb_unlink(struct cb_tree *t, const uint8_t *k, size_t klen)
+{
+	struct cb_node **wherep = &t->root;
+	struct cb_node **whereq = NULL;
+	struct cb_node *q = NULL;
+	size_t direction = 0;
+	uint8_t c;
+	struct cb_node *p = t->root;
+
+	if (!p) return NULL;	/* empty tree, nothing to delete */
+
+	/* traverse to find best match, keeping link to parent */
+	while (1 & (uintptr_t)p) {
+		whereq = wherep;
+		q = cb_node_of(p);
+		c = q->byte < klen ? k[q->byte] : 0;
+		direction = (1 + (q->otherbits | c)) >> 8;
+		wherep = q->child + direction;
+		p = *wherep;
+	}
+
+	if (memcmp(p->k, k, klen))
+		return NULL;		/* no match, nothing unlinked */
+
+	/* found an exact match */
+	if (whereq)	/* update parent */
+		*whereq = q->child[1 - direction];
+	else
+		t->root = NULL;
+	return p;
+}
+
+static enum cb_next cb_descend(struct cb_node *p, cb_iter fn, void *arg)
+{
+	if (1 & (uintptr_t)p) {
+		struct cb_node *q = cb_node_of(p);
+		enum cb_next n = cb_descend(q->child[0], fn, arg);
+
+		return n == CB_BREAK ? n : cb_descend(q->child[1], fn, arg);
+	} else {
+		return fn(p, arg);
+	}
+}
+
+void cb_each(struct cb_tree *t, const uint8_t *kpfx, size_t klen,
+			cb_iter fn, void *arg)
+{
+	struct cb_node *p = t->root;
+	struct cb_node *top = p;
+	size_t i = 0;
+
+	if (!p) return; /* empty tree */
+
+	/* Walk tree, maintaining top pointer */
+	while (1 & (uintptr_t)p) {
+		struct cb_node *q = cb_node_of(p);
+		uint8_t c = q->byte < klen ? kpfx[q->byte] : 0;
+		size_t direction = (1 + (q->otherbits | c)) >> 8;
+
+		p = q->child[direction];
+		if (q->byte < klen)
+			top = p;
+	}
+
+	for (i = 0; i < klen; i++) {
+		if (p->k[i] != kpfx[i])
+			return; /* "best" match failed */
+	}
+	cb_descend(top, fn, arg);
+}
diff --git a/cbtree.h b/cbtree.h
new file mode 100644
index 0000000000..fe4587087e
--- /dev/null
+++ b/cbtree.h
@@ -0,0 +1,56 @@
+/*
+ * crit-bit tree implementation, does no allocations internally
+ * For more information on crit-bit trees: https://cr.yp.to/critbit.html
+ * Based on Adam Langley's adaptation of Dan Bernstein's public domain code
+ * git clone https://github.com/agl/critbit.git
+ *
+ * This is adapted to store arbitrary data (not just NUL-terminated C strings
+ * and allocates no memory internally.  The user needs to allocate
+ * "struct cb_node" and fill cb_node.k[] with arbitrary match data
+ * for memcmp.
+ * If "klen" is variable, then it should be embedded into "c_node.k[]"
+ * Recursion is bound by the maximum value of "klen" used.
+ */
+#ifndef CBTREE_H
+#define CBTREE_H
+
+#include "git-compat-util.h"
+
+struct cb_node;
+struct cb_node {
+	struct cb_node *child[2];
+	/*
+	 * n.b. uint32_t for `byte' is excessive for OIDs,
+	 * we may consider shorter variants if nothing else gets stored.
+	 */
+	uint32_t byte;
+	uint8_t otherbits;
+	uint8_t k[FLEX_ARRAY]; /* arbitrary data */
+};
+
+struct cb_tree {
+	struct cb_node *root;
+};
+
+enum cb_next {
+	CB_CONTINUE = 0,
+	CB_BREAK = 1
+};
+
+#define CBTREE_INIT { .root = NULL }
+
+static inline void cb_init(struct cb_tree *t)
+{
+	t->root = NULL;
+}
+
+struct cb_node *cb_lookup(struct cb_tree *, const uint8_t *k, size_t klen);
+struct cb_node *cb_insert(struct cb_tree *, struct cb_node *, size_t klen);
+struct cb_node *cb_unlink(struct cb_tree *t, const uint8_t *k, size_t klen);
+
+typedef enum cb_next (*cb_iter)(struct cb_node *, void *arg);
+
+void cb_each(struct cb_tree *, const uint8_t *kpfx, size_t klen,
+		cb_iter, void *arg);
+
+#endif /* CBTREE_H */
diff --git a/object-file.c b/object-file.c
index 91ded8c22a..35f3e7e9bb 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1173,7 +1173,7 @@ static int quick_has_loose(struct repository *r,
 
 	prepare_alt_odb(r);
 	for (odb = r->objects->odb; odb; odb = odb->next) {
-		if (oid_array_lookup(odb_loose_cache(odb, oid), oid) >= 0)
+		if (oidtree_contains(odb_loose_cache(odb, oid), oid))
 			return 1;
 	}
 	return 0;
@@ -2452,11 +2452,11 @@ int for_each_loose_object(each_loose_object_fn cb, void *data,
 static int append_loose_object(const struct object_id *oid, const char *path,
 			       void *data)
 {
-	oid_array_append(data, oid);
+	oidtree_insert(data, oid);
 	return 0;
 }
 
-struct oid_array *odb_loose_cache(struct object_directory *odb,
+struct oidtree *odb_loose_cache(struct object_directory *odb,
 				  const struct object_id *oid)
 {
 	int subdir_nr = oid->hash[0];
@@ -2472,24 +2472,25 @@ struct oid_array *odb_loose_cache(struct object_directory *odb,
 
 	bitmap = &odb->loose_objects_subdir_seen[word_index];
 	if (*bitmap & mask)
-		return &odb->loose_objects_cache[subdir_nr];
-
+		return odb->loose_objects_cache;
+	if (!odb->loose_objects_cache) {
+		ALLOC_ARRAY(odb->loose_objects_cache, 1);
+		oidtree_init(odb->loose_objects_cache);
+	}
 	strbuf_addstr(&buf, odb->path);
 	for_each_file_in_obj_subdir(subdir_nr, &buf,
 				    append_loose_object,
 				    NULL, NULL,
-				    &odb->loose_objects_cache[subdir_nr]);
+				    odb->loose_objects_cache);
 	*bitmap |= mask;
 	strbuf_release(&buf);
-	return &odb->loose_objects_cache[subdir_nr];
+	return odb->loose_objects_cache;
 }
 
 void odb_clear_loose_cache(struct object_directory *odb)
 {
-	int i;
-
-	for (i = 0; i < ARRAY_SIZE(odb->loose_objects_cache); i++)
-		oid_array_clear(&odb->loose_objects_cache[i]);
+	oidtree_clear(odb->loose_objects_cache);
+	FREE_AND_NULL(odb->loose_objects_cache);
 	memset(&odb->loose_objects_subdir_seen, 0,
 	       sizeof(odb->loose_objects_subdir_seen));
 }
diff --git a/object-name.c b/object-name.c
index 64202de60b..3263c19457 100644
--- a/object-name.c
+++ b/object-name.c
@@ -87,27 +87,21 @@ static void update_candidates(struct disambiguate_state *ds, const struct object
 
 static int match_hash(unsigned, const unsigned char *, const unsigned char *);
 
+static enum cb_next match_prefix(const struct object_id *oid, void *arg)
+{
+	struct disambiguate_state *ds = arg;
+	/* no need to call match_hash, oidtree_each did prefix match */
+	update_candidates(ds, oid);
+	return ds->ambiguous ? CB_BREAK : CB_CONTINUE;
+}
+
 static void find_short_object_filename(struct disambiguate_state *ds)
 {
 	struct object_directory *odb;
 
-	for (odb = ds->repo->objects->odb; odb && !ds->ambiguous; odb = odb->next) {
-		int pos;
-		struct oid_array *loose_objects;
-
-		loose_objects = odb_loose_cache(odb, &ds->bin_pfx);
-		pos = oid_array_lookup(loose_objects, &ds->bin_pfx);
-		if (pos < 0)
-			pos = -1 - pos;
-		while (!ds->ambiguous && pos < loose_objects->nr) {
-			const struct object_id *oid;
-			oid = loose_objects->oid + pos;
-			if (!match_hash(ds->len, ds->bin_pfx.hash, oid->hash))
-				break;
-			update_candidates(ds, oid);
-			pos++;
-		}
-	}
+	for (odb = ds->repo->objects->odb; odb && !ds->ambiguous; odb = odb->next)
+		oidtree_each(odb_loose_cache(odb, &ds->bin_pfx),
+				&ds->bin_pfx, ds->len, match_prefix, ds);
 }
 
 static int match_hash(unsigned len, const unsigned char *a, const unsigned char *b)
diff --git a/object-store.h b/object-store.h
index ab6d469970..e679acc4c3 100644
--- a/object-store.h
+++ b/object-store.h
@@ -9,6 +9,7 @@
 #include "thread-utils.h"
 #include "khash.h"
 #include "dir.h"
+#include "oidtree.h"
 
 struct object_directory {
 	struct object_directory *next;
@@ -23,7 +24,7 @@ struct object_directory {
 	 * Be sure to call odb_load_loose_cache() before using.
 	 */
 	uint32_t loose_objects_subdir_seen[8]; /* 256 bits */
-	struct oid_array loose_objects_cache[256];
+	struct oidtree *loose_objects_cache;
 
 	/*
 	 * Path to the alternative object store. If this is a relative path,
@@ -59,7 +60,7 @@ void add_to_alternates_memory(const char *dir);
  * Populate and return the loose object cache array corresponding to the
  * given object ID.
  */
-struct oid_array *odb_loose_cache(struct object_directory *odb,
+struct oidtree *odb_loose_cache(struct object_directory *odb,
 				  const struct object_id *oid);
 
 /* Empty the loose object cache for the specified object directory. */
diff --git a/oidtree.c b/oidtree.c
new file mode 100644
index 0000000000..7eb0e9ba05
--- /dev/null
+++ b/oidtree.c
@@ -0,0 +1,104 @@
+/*
+ * A wrapper around cbtree which stores oids
+ * May be used to replace oid-array for prefix (abbreviation) matches
+ */
+#include "oidtree.h"
+#include "alloc.h"
+#include "hash.h"
+
+struct oidtree_node {
+	/* n.k[] is used to store "struct object_id" */
+	struct cb_node n;
+};
+
+struct oidtree_iter_data {
+	oidtree_iter fn;
+	void *arg;
+	size_t *last_nibble_at;
+	int algo;
+	uint8_t last_byte;
+};
+
+void oidtree_init(struct oidtree *ot)
+{
+	cb_init(&ot->tree);
+	mem_pool_init(&ot->mem_pool, 0);
+}
+
+void oidtree_clear(struct oidtree *ot)
+{
+	if (ot) {
+		mem_pool_discard(&ot->mem_pool, 0);
+		oidtree_init(ot);
+	}
+}
+
+void oidtree_insert(struct oidtree *ot, const struct object_id *oid)
+{
+	struct oidtree_node *on;
+
+	if (!oid->algo)
+		BUG("oidtree_insert requires oid->algo");
+
+	on = mem_pool_alloc(&ot->mem_pool, sizeof(*on) + sizeof(*oid));
+	oidcpy_with_padding((struct object_id *)on->n.k, oid);
+
+	/*
+	 * n.b. Current callers won't get us duplicates, here.  If a
+	 * future caller causes duplicates, there'll be a a small leak
+	 * that won't be freed until oidtree_clear.  Currently it's not
+	 * worth maintaining a free list
+	 */
+	cb_insert(&ot->tree, &on->n, sizeof(*oid));
+}
+
+
+int oidtree_contains(struct oidtree *ot, const struct object_id *oid)
+{
+	struct object_id k;
+	size_t klen = sizeof(k);
+
+	oidcpy_with_padding(&k, oid);
+
+	if (oid->algo == GIT_HASH_UNKNOWN)
+		klen -= sizeof(oid->algo);
+
+	/* cb_lookup relies on memcmp on the struct, so order matters: */
+	klen += BUILD_ASSERT_OR_ZERO(offsetof(struct object_id, hash) <
+				offsetof(struct object_id, algo));
+
+	return cb_lookup(&ot->tree, (const uint8_t *)&k, klen) ? 1 : 0;
+}
+
+static enum cb_next iter(struct cb_node *n, void *arg)
+{
+	struct oidtree_iter_data *x = arg;
+	const struct object_id *oid = (const struct object_id *)n->k;
+
+	if (x->algo != GIT_HASH_UNKNOWN && x->algo != oid->algo)
+		return CB_CONTINUE;
+
+	if (x->last_nibble_at) {
+		if ((oid->hash[*x->last_nibble_at] ^ x->last_byte) & 0xf0)
+			return CB_CONTINUE;
+	}
+
+	return x->fn(oid, x->arg);
+}
+
+void oidtree_each(struct oidtree *ot, const struct object_id *oid,
+			size_t oidhexsz, oidtree_iter fn, void *arg)
+{
+	size_t klen = oidhexsz / 2;
+	struct oidtree_iter_data x = { 0 };
+	assert(oidhexsz <= GIT_MAX_HEXSZ);
+
+	x.fn = fn;
+	x.arg = arg;
+	x.algo = oid->algo;
+	if (oidhexsz & 1) {
+		x.last_byte = oid->hash[klen];
+		x.last_nibble_at = &klen;
+	}
+	cb_each(&ot->tree, (const uint8_t *)oid, klen, iter, &x);
+}
diff --git a/oidtree.h b/oidtree.h
new file mode 100644
index 0000000000..77898f510a
--- /dev/null
+++ b/oidtree.h
@@ -0,0 +1,22 @@
+#ifndef OIDTREE_H
+#define OIDTREE_H
+
+#include "cbtree.h"
+#include "hash.h"
+#include "mem-pool.h"
+
+struct oidtree {
+	struct cb_tree tree;
+	struct mem_pool mem_pool;
+};
+
+void oidtree_init(struct oidtree *);
+void oidtree_clear(struct oidtree *);
+void oidtree_insert(struct oidtree *, const struct object_id *);
+int oidtree_contains(struct oidtree *, const struct object_id *);
+
+typedef enum cb_next (*oidtree_iter)(const struct object_id *, void *data);
+void oidtree_each(struct oidtree *, const struct object_id *,
+			size_t oidhexsz, oidtree_iter, void *data);
+
+#endif /* OIDTREE_H */
diff --git a/t/helper/test-oidtree.c b/t/helper/test-oidtree.c
new file mode 100644
index 0000000000..180ee28dd9
--- /dev/null
+++ b/t/helper/test-oidtree.c
@@ -0,0 +1,49 @@
+#include "test-tool.h"
+#include "cache.h"
+#include "oidtree.h"
+
+static enum cb_next print_oid(const struct object_id *oid, void *data)
+{
+	puts(oid_to_hex(oid));
+	return CB_CONTINUE;
+}
+
+int cmd__oidtree(int argc, const char **argv)
+{
+	struct oidtree ot;
+	struct strbuf line = STRBUF_INIT;
+	int nongit_ok;
+	int algo = GIT_HASH_UNKNOWN;
+
+	oidtree_init(&ot);
+	setup_git_directory_gently(&nongit_ok);
+
+	while (strbuf_getline(&line, stdin) != EOF) {
+		const char *arg;
+		struct object_id oid;
+
+		if (skip_prefix(line.buf, "insert ", &arg)) {
+			if (get_oid_hex_any(arg, &oid) == GIT_HASH_UNKNOWN)
+				die("insert not a hexadecimal oid: %s", arg);
+			algo = oid.algo;
+			oidtree_insert(&ot, &oid);
+		} else if (skip_prefix(line.buf, "contains ", &arg)) {
+			if (get_oid_hex(arg, &oid))
+				die("contains not a hexadecimal oid: %s", arg);
+			printf("%d\n", oidtree_contains(&ot, &oid));
+		} else if (skip_prefix(line.buf, "each ", &arg)) {
+			char buf[GIT_MAX_HEXSZ + 1] = { '0' };
+			memset(&oid, 0, sizeof(oid));
+			memcpy(buf, arg, strlen(arg));
+			buf[hash_algos[algo].hexsz] = '\0';
+			get_oid_hex_any(buf, &oid);
+			oid.algo = algo;
+			oidtree_each(&ot, &oid, strlen(arg), print_oid, NULL);
+		} else if (!strcmp(line.buf, "clear")) {
+			oidtree_clear(&ot);
+		} else {
+			die("unknown command: %s", line.buf);
+		}
+	}
+	return 0;
+}
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index c5bd0c6d4c..9d37debf28 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -43,6 +43,7 @@ static struct test_cmd cmds[] = {
 	{ "mktemp", cmd__mktemp },
 	{ "oid-array", cmd__oid_array },
 	{ "oidmap", cmd__oidmap },
+	{ "oidtree", cmd__oidtree },
 	{ "online-cpus", cmd__online_cpus },
 	{ "parse-options", cmd__parse_options },
 	{ "parse-pathspec-file", cmd__parse_pathspec_file },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index e8069a3b22..f683a2f59c 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -32,6 +32,7 @@ int cmd__match_trees(int argc, const char **argv);
 int cmd__mergesort(int argc, const char **argv);
 int cmd__mktemp(int argc, const char **argv);
 int cmd__oidmap(int argc, const char **argv);
+int cmd__oidtree(int argc, const char **argv);
 int cmd__online_cpus(int argc, const char **argv);
 int cmd__parse_options(int argc, const char **argv);
 int cmd__parse_pathspec_file(int argc, const char** argv);
diff --git a/t/t0069-oidtree.sh b/t/t0069-oidtree.sh
new file mode 100755
index 0000000000..bfb1397d7b
--- /dev/null
+++ b/t/t0069-oidtree.sh
@@ -0,0 +1,49 @@
+#!/bin/sh
+
+test_description='basic tests for the oidtree implementation'
+. ./test-lib.sh
+
+maxhexsz=$(test_oid hexsz)
+echoid () {
+	prefix="${1:+$1 }"
+	shift
+	while test $# -gt 0
+	do
+		shortoid="$1"
+		shift
+		difference=$(($maxhexsz - ${#shortoid}))
+		printf "%s%s%0${difference}d\\n" "$prefix" "$shortoid" "0"
+	done
+}
+
+test_expect_success 'oidtree insert and contains' '
+	cat >expect <<-\EOF &&
+		0
+		0
+		0
+		1
+		1
+		0
+	EOF
+	{
+		echoid insert 444 1 2 3 4 5 a b c d e &&
+		echoid contains 44 441 440 444 4440 4444
+		echo clear
+	} | test-tool oidtree >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'oidtree each' '
+	echoid "" 123 321 321 >expect &&
+	{
+		echoid insert f 9 8 123 321 a b c d e
+		echo each 12300
+		echo each 3211
+		echo each 3210
+		echo each 32100
+		echo clear
+	} | test-tool oidtree >actual &&
+	test_cmp expect actual
+'
+
+test_done