builtin/rev-parse.c |  5 +++--
 diff.c              |  2 +-
 environment.c       |  2 +-
 sha1_name.c         | 39 ++++++++++++++++++++++++++++++++++++++-
 4 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 4da1f1da2..cfb0f1510 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -671,8 +671,9 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				filter &= ~(DO_FLAGS|DO_NOREV);
 				verify = 1;
 				abbrev = DEFAULT_ABBREV;
-				if (arg[7] == '=')
-					abbrev = strtoul(arg + 8, NULL, 10);
+				if (!arg[7])
+					continue;
+				abbrev = strtoul(arg + 8, NULL, 10);
 				if (abbrev < MINIMUM_ABBREV)
 					abbrev = MINIMUM_ABBREV;
 				else if (40 <= abbrev)
diff --git a/diff.c b/diff.c
index 59920747d..c6d445915 100644
--- a/diff.c
+++ b/diff.c
@@ -3421,7 +3421,7 @@ void diff_setup_done(struct diff_options *options)
 			 */
 			read_cache();
 	}
-	if (options->abbrev <= 0 || 40 < options->abbrev)
+	if (options->abbrev > 40)
 		options->abbrev = 40; /* full */
 
 	/*
diff --git a/environment.c b/environment.c
index c1442df9a..fd6681e46 100644
--- a/environment.c
+++ b/environment.c
@@ -16,7 +16,7 @@ int trust_executable_bit = 1;
 int trust_ctime = 1;
 int check_stat = 1;
 int has_symlinks = 1;
-int minimum_abbrev = 4, default_abbrev = 7;
+int minimum_abbrev = 4, default_abbrev = -1;
 int ignore_case;
 int assume_unchanged;
 int prefer_symlink_refs;
diff --git a/sha1_name.c b/sha1_name.c
index 3b647fd7c..684b36dba 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -15,6 +15,7 @@ typedef int (*disambiguate_hint_fn)(const unsigned char *, void *);
 
 struct disambiguate_state {
 	int len; /* length of prefix in hex chars */
+	unsigned int nrobjects;
 	char hex_pfx[GIT_SHA1_HEXSZ + 1];
 	unsigned char bin_pfx[GIT_SHA1_RAWSZ];
 
@@ -118,6 +119,12 @@ static void find_short_object_filename(struct disambiguate_state *ds)
 
 			if (strlen(de->d_name) != 38)
 				continue;
+
+			// We only look at the one subdirectory, and we assume
+			// each subdirectory is roughly similar, so each object
+			// we find probably has 255 other objects in the other
+			// fan-out directories
+			ds->nrobjects += 256;
 			if (memcmp(de->d_name, ds->hex_pfx + 2, ds->len - 2))
 				continue;
 			memcpy(hex + 2, de->d_name, 38);
@@ -151,6 +158,7 @@ static void unique_in_pack(struct packed_git *p,
 
 	open_pack_index(p);
 	num = p->num_objects;
+	ds->nrobjects += num;
 	last = num;
 	while (first < last) {
 		uint32_t mid = (first + last) / 2;
@@ -455,17 +463,46 @@ int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
 	return ret;
 }
 
+static int get_automatic_abbrev(const char *hex)
+{
+	static int len;
+	struct disambiguate_state ds;
+
+	if (init_object_disambiguation(hex, 7, &ds) < 0)
+		return 7;
+
+	find_short_object_filename(&ds);
+	find_short_packed_object(&ds);
+
+	for (len = 7; len < 16; len++) {
+		unsigned int expect_collision = 1 << (len * 2);
+		if (ds.nrobjects < expect_collision)
+			break;
+	}
+	return len;
+}
+
 int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
 {
 	int status, exists;
+	int flags = GET_SHA1_QUIETLY;
 
 	sha1_to_hex_r(hex, sha1);
 	if (len == 40 || !len)
 		return 40;
+
+	if (len < 0) {
+		static int automatic_abbrev = -1;
+
+		if (automatic_abbrev < 0)
+			automatic_abbrev = get_automatic_abbrev(hex);
+		len = automatic_abbrev;
+	}
+
 	exists = has_sha1_file(sha1);
 	while (len < 40) {
 		unsigned char sha1_ret[20];
-		status = get_short_sha1(hex, len, sha1_ret, GET_SHA1_QUIETLY);
+		status = get_short_sha1(hex, len, sha1_ret, flags);
 		if (exists
 		    ? !status
 		    : status == SHORT_NAME_NOT_FOUND) {