[PATCH v9 0/3] read-cache: speed up add_index_entry

[PATCH v9 0/3] read-cache: speed up add_index_entry

[git]

From: Jeff Hostetler <jeffhost@microsoft.com>

Version 9 addresses the following:
  () p0006 perf test can now run using either synthetic repos
     from t/perf/repos/many-repos.sh -OR- an actual real-world
     repo.
  () The commit message has been updated to include results of
     p0006 on linux.git.
  () Line 0006.3 shows a positive value, but results for that
     line had a lot of variance between runs.  I measured the
     blocks of code I added and they only added 0.007 to 0.015
     seconds.  I suspect the overall time difference and variance
     is due to file I/O to update the worktree when switching
     branches.

I think this version has addressed everything raise so far,
so I think I'm ready to let this one rest.  Thanks for all
the help and feedback.


Jeff Hostetler (3):
  read-cache: add strcmp_offset function
  p0006-read-tree-checkout: perf test to time read-tree
  read-cache: speed up add_index_entry during checkout

 Makefile                           |   1 +
 cache.h                            |   1 +
 read-cache.c                       |  66 +++++++++++++++++++++-
 t/helper/.gitignore                |   1 +
 t/helper/test-strcmp-offset.c      |  22 ++++++++
 t/perf/p0006-read-tree-checkout.sh |  69 +++++++++++++++++++++++
 t/perf/repos/.gitignore            |   1 +
 t/perf/repos/many-files.sh         | 110 +++++++++++++++++++++++++++++++++++++
 t/t0065-strcmp-offset.sh           |  21 +++++++
 9 files changed, 290 insertions(+), 2 deletions(-)
 create mode 100644 t/helper/test-strcmp-offset.c
 create mode 100755 t/perf/p0006-read-tree-checkout.sh
 create mode 100644 t/perf/repos/.gitignore
 create mode 100755 t/perf/repos/many-files.sh
 create mode 100755 t/t0065-strcmp-offset.sh

-- 
2.9.3

[PATCH v9 1/3] read-cache: add strcmp_offset function

[git]

From: Jeff Hostetler <jeffhost@microsoft.com>

Add strcmp_offset() function to also return the offset of the
first change.

Add unit test and helper to verify.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 Makefile                      |  1 +
 cache.h                       |  1 +
 read-cache.c                  | 20 ++++++++++++++++++++
 t/helper/.gitignore           |  1 +
 t/helper/test-strcmp-offset.c | 22 ++++++++++++++++++++++
 t/t0065-strcmp-offset.sh      | 21 +++++++++++++++++++++
 6 files changed, 66 insertions(+)
 create mode 100644 t/helper/test-strcmp-offset.c
 create mode 100755 t/t0065-strcmp-offset.sh

diff --git a/Makefile b/Makefile
index 9ec6065..4c4c246 100644
--- a/Makefile
+++ b/Makefile
@@ -631,6 +631,7 @@ TEST_PROGRAMS_NEED_X += test-scrap-cache-tree
 TEST_PROGRAMS_NEED_X += test-sha1
 TEST_PROGRAMS_NEED_X += test-sha1-array
 TEST_PROGRAMS_NEED_X += test-sigchain
+TEST_PROGRAMS_NEED_X += test-strcmp-offset
 TEST_PROGRAMS_NEED_X += test-string-list
 TEST_PROGRAMS_NEED_X += test-submodule-config
 TEST_PROGRAMS_NEED_X += test-subprocess
diff --git a/cache.h b/cache.h
index 80b6372..3c55047 100644
--- a/cache.h
+++ b/cache.h
@@ -574,6 +574,7 @@ extern int write_locked_index(struct index_state *, struct lock_file *lock, unsi
 extern int discard_index(struct index_state *);
 extern int unmerged_index(const struct index_state *);
 extern int verify_path(const char *path);
+extern int strcmp_offset(const char *s1, const char *s2, size_t *first_change);
 extern int index_dir_exists(struct index_state *istate, const char *name, int namelen);
 extern void adjust_dirname_case(struct index_state *istate, char *name);
 extern struct cache_entry *index_file_exists(struct index_state *istate, const char *name, int namelen, int igncase);
diff --git a/read-cache.c b/read-cache.c
index 9054369..97f13a1 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -887,6 +887,26 @@ static int has_file_name(struct index_state *istate,
 	return retval;
 }
 
+
+/*
+ * Like strcmp(), but also return the offset of the first change.
+ * If strings are equal, return the length.
+ */
+int strcmp_offset(const char *s1, const char *s2, size_t *first_change)
+{
+	size_t k;
+
+	if (!first_change)
+		return strcmp(s1, s2);
+
+	for (k = 0; s1[k] == s2[k]; k++)
+		if (s1[k] == '\0')
+			break;
+
+	*first_change = k;
+	return (unsigned char)s1[k] - (unsigned char)s2[k];
+}
+
 /*
  * Do we have another file with a pathname that is a proper
  * subset of the name we're trying to add?
diff --git a/t/helper/.gitignore b/t/helper/.gitignore
index d6e8b36..0a89531 100644
--- a/t/helper/.gitignore
+++ b/t/helper/.gitignore
@@ -25,6 +25,7 @@
 /test-sha1
 /test-sha1-array
 /test-sigchain
+/test-strcmp-offset
 /test-string-list
 /test-submodule-config
 /test-subprocess
diff --git a/t/helper/test-strcmp-offset.c b/t/helper/test-strcmp-offset.c
new file mode 100644
index 0000000..4a45a54
--- /dev/null
+++ b/t/helper/test-strcmp-offset.c
@@ -0,0 +1,22 @@
+#include "cache.h"
+
+int cmd_main(int argc, const char **argv)
+{
+	int result;
+	size_t offset;
+
+	if (!argv[1] || !argv[2])
+		die("usage: %s <string1> <string2>", argv[0]);
+
+	result = strcmp_offset(argv[1], argv[2], &offset);
+
+	/*
+	 * Because differnt CRTs behave differently, only rely on signs
+	 * of the result values.
+	 */
+	result = (result < 0 ? -1 :
+			  result > 0 ? 1 :
+			  0);
+	printf("%d %"PRIuMAX"\n", result, (uintmax_t)offset);
+	return 0;
+}
diff --git a/t/t0065-strcmp-offset.sh b/t/t0065-strcmp-offset.sh
new file mode 100755
index 0000000..7d6d214
--- /dev/null
+++ b/t/t0065-strcmp-offset.sh
@@ -0,0 +1,21 @@
+#!/bin/sh
+
+test_description='Test strcmp_offset functionality'
+
+. ./test-lib.sh
+
+while read s1 s2 expect
+do
+	test_expect_success "strcmp_offset($s1, $s2)" '
+		echo "$expect" >expect &&
+		test-strcmp-offset "$s1" "$s2" >actual &&
+		test_cmp expect actual
+	'
+done <<-EOF
+abc abc 0 3
+abc def -1 0
+abc abz -1 2
+abc abcdef -1 3
+EOF
+
+test_done
-- 
2.9.3

[PATCH v9 2/3] p0006-read-tree-checkout: perf test to time read-tree

[git]

From: Jeff Hostetler <jeffhost@microsoft.com>

Created t/perf/repos/many-files.sh to generate large, but
artificial repositories.

Created t/perf/p0006-read-tree-checkout.sh to measure
performance on various read-tree, checkout, and update-index
operations.  This test can run using either artificial repos
described above or normal repos.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 t/perf/p0006-read-tree-checkout.sh |  69 +++++++++++++++++++++++
 t/perf/repos/.gitignore            |   1 +
 t/perf/repos/many-files.sh         | 110 +++++++++++++++++++++++++++++++++++++
 3 files changed, 180 insertions(+)
 create mode 100755 t/perf/p0006-read-tree-checkout.sh
 create mode 100644 t/perf/repos/.gitignore
 create mode 100755 t/perf/repos/many-files.sh

diff --git a/t/perf/p0006-read-tree-checkout.sh b/t/perf/p0006-read-tree-checkout.sh
new file mode 100755
index 0000000..5881251
--- /dev/null
+++ b/t/perf/p0006-read-tree-checkout.sh
@@ -0,0 +1,69 @@
+#!/bin/sh
+##
+## This test measures the performance of various read-tree
+## and checkout operations.  It is primarily interested in
+## the algorithmic costs of index operations and recursive
+## tree traversal -- and NOT disk I/O on thousands of files.
+
+test_description="Tests performance of read-tree"
+
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+## If the test repo was generated by ./repos/many-files.sh
+## then we know something about the data shape and branches,
+## so we can isolate testing to the ballast-related commits
+## and setup sparse-checkout so we don't have to populate
+## the ballast files and directories.
+##
+## Otherwise, we make some general assumptions about the
+## repo and consider the entire history of the current
+## branch to be the ballast.
+
+git branch | grep p0006-ballast >/dev/null 2>&1
+synthetic=$?
+if test "$synthetic" = 0
+then
+    echo Assuming synthetic repo from many-files.sh
+    git branch br_base            master
+    git branch br_ballast         p0006-ballast^
+    git branch br_ballast_alias   p0006-ballast^
+    git branch br_ballast_plus_1  p0006-ballast
+    echo '/*'          >.git/info/sparse-checkout
+    echo '!ballast/*' >>.git/info/sparse-checkout
+    git config --local core.sparsecheckout 1
+else
+    echo Assuming non-synthetic repo...
+    git branch br_base            $(git rev-list HEAD | tail -n 1)
+    git branch br_ballast         HEAD^ || error "no ancestor commit from current head"
+    git branch br_ballast_alias   HEAD^
+    git branch br_ballast_plus_1  HEAD
+fi
+
+test_expect_success 'setup' '
+	git checkout -q br_ballast
+'
+
+nr_files=$(git ls-files | wc -l)
+
+test_perf "read-tree br_base br_ballast ($nr_files)" '
+	git read-tree -m br_base br_ballast -n
+'
+
+test_perf "switch between br_base br_ballast ($nr_files)" '
+	git checkout -q br_base &&
+	git checkout -q br_ballast
+'
+
+test_perf "switch between br_ballast br_ballast_plus_1 ($nr_files)" '
+	git checkout -q br_ballast_plus_1 &&
+	git checkout -q br_ballast
+'
+
+test_perf "switch between aliases ($nr_files)" '
+	git checkout -q br_ballast_alias &&
+	git checkout -q br_ballast
+'
+
+test_done
diff --git a/t/perf/repos/.gitignore b/t/perf/repos/.gitignore
new file mode 100644
index 0000000..72e3dc3
--- /dev/null
+++ b/t/perf/repos/.gitignore
@@ -0,0 +1 @@
+gen-*/
diff --git a/t/perf/repos/many-files.sh b/t/perf/repos/many-files.sh
new file mode 100755
index 0000000..5a1d25e
--- /dev/null
+++ b/t/perf/repos/many-files.sh
@@ -0,0 +1,110 @@
+#!/bin/sh
+## Generate test data repository using the given parameters.
+## When omitted, we create "gen-many-files-d-w-f.git".
+##
+## Usage: [-r repo] [-d depth] [-w width] [-f files]
+##
+## -r repo: path to the new repo to be generated
+## -d depth: the depth of sub-directories
+## -w width: the number of sub-directories at each level
+## -f files: the number of files created in each directory
+##
+## Note that all files will have the same SHA-1 and each
+## directory at a level will have the same SHA-1, so we
+## will potentially have a large index, but not a large
+## ODB.
+##
+## Ballast will be created under "ballast/".
+
+EMPTY_BLOB=e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+
+set -e
+
+## (5, 10, 9) will create 999,999 ballast files.
+## (4, 10, 9) will create  99,999 ballast files.
+depth=5
+width=10
+files=9
+
+while test "$#" -ne 0
+do
+    case "$1" in
+	-r)
+	    shift;
+	    test "$#" -ne 0 || { echo 'error: -r requires an argument' >&2; exit 1; }
+	    repo=$1;
+	    shift ;;
+	-d)
+	    shift;
+	    test "$#" -ne 0 || { echo 'error: -d requires an argument' >&2; exit 1; }
+	    depth=$1;
+	    shift ;;
+	-w)
+	    shift;
+	    test "$#" -ne 0 || { echo 'error: -w requires an argument' >&2; exit 1; }
+	    width=$1;
+	    shift ;;
+	-f)
+	    shift;
+	    test "$#" -ne 0 || { echo 'error: -f requires an argument' >&2; exit 1; }
+	    files=$1;
+	    shift ;;
+	*)
+	    echo "error: unknown option '$1'" >&2; exit 1 ;;
+	esac
+done
+
+## Inflate the index with thousands of empty files.
+## usage: dir depth width files
+fill_index() {
+	awk -v arg_dir=$1 -v arg_depth=$2 -v arg_width=$3 -v arg_files=$4 '
+		function make_paths(dir, depth, width, files, f, w) {
+			for (f = 1; f <= files; f++) {
+				print dir "/file" f
+			}
+			if (depth > 0) {
+				for (w = 1; w <= width; w++) {
+					make_paths(dir "/dir" w, depth - 1, width, files)
+				}
+			}
+		}
+		END { make_paths(arg_dir, arg_depth, arg_width, arg_files) }
+		' </dev/null |
+	sed "s/^/100644 $EMPTY_BLOB	/" |
+	git update-index --index-info
+	return 0
+}
+
+[ -z "$repo" ] && repo=gen-many-files-$depth.$width.$files.git
+
+mkdir $repo
+cd $repo
+git init .
+
+## Create an initial commit just to define master.
+touch many-files.empty
+echo "$depth $width $files" >many-files.params
+git add many-files.*
+git commit -q -m params
+
+## Create ballast for p0006 based upon the given params and
+## inflate the index with thousands of empty files and commit.
+git checkout -b p0006-ballast
+fill_index "ballast" $depth $width $files
+git commit -q -m "ballast"
+
+nr_files=$(git ls-files | wc -l)
+
+## Modify 1 file and commit.
+echo "$depth $width $files" >>many-files.params
+git add many-files.params
+git commit -q -m "ballast plus 1"
+
+## Checkout master to put repo in canonical state (because
+## the perf test may need to clone and enable sparse-checkout
+## before attempting to checkout a commit with the ballast
+## (because it may contain 100K directories and 1M files)).
+git checkout master
+
+echo "Repository "$repo" ($depth, $width, $files) created.  Ballast $nr_files."
+exit 0
-- 
2.9.3

[PATCH v9 3/3] read-cache: speed up add_index_entry during checkout

[git]

From: Jeff Hostetler <jeffhost@microsoft.com>

Teach add_index_entry_with_check() and has_dir_name()
to see if the path of the new item is greater than the
last path in the index array before attempting to search
for it.

During checkout, merge_working_tree() populates the new
index in sorted order, so this change will save at least 2
binary lookups per file.  This preserves the original
behavior but simply checks the last element before starting
the search.

This helps performance on very large repositories.

This can be seen using p0006-read-tree-checkout.sh and the
artificial repository created by t/perf/repos/many-files.sh
with parameters (5, 10, 9).   (1M files in index.)

Test                                                            HEAD^              HEAD
----------------------------------------------------------------------------------------------------------
0006.2: read-tree br_base br_ballast (1000001)                  4.15(2.72+1.41)    3.21(1.97+1.22) -22.7%
0006.3: switch between br_base br_ballast (1000001)             8.11(5.57+2.28)    6.77(4.36+2.14) -16.5%
0006.4: switch between br_ballast br_ballast_plus_1 (1000001)   13.52(8.68+4.35)   11.80(7.38+3.96) -12.7%
0006.5: switch between aliases (1000001)                        13.59(8.75+4.43)   11.85(7.49+3.87) -12.8%

On linux.git, results are:
Test                                                          HEAD^             HEAD
------------------------------------------------------------------------------------------------------
0006.2: read-tree br_base br_ballast (57994)                  0.24(0.22+0.01)   0.20(0.17+0.02) -16.7%
0006.3: switch between br_base br_ballast (57994)             9.91(5.82+2.79)   10.26(5.92+2.77) +3.5%
0006.4: switch between br_ballast br_ballast_plus_1 (57994)   0.59(0.44+0.16)   0.50(0.34+0.18) -15.3%
0006.5: switch between aliases (57994)                        0.62(0.48+0.16)   0.50(0.35+0.16) -19.4%

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 read-cache.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index 97f13a1..a8ef823 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -918,9 +918,24 @@ static int has_dir_name(struct index_state *istate,
 	int stage = ce_stage(ce);
 	const char *name = ce->name;
 	const char *slash = name + ce_namelen(ce);
+	size_t len_eq_last;
+	int cmp_last = 0;
+
+	if (istate->cache_nr > 0) {
+		/*
+		 * Compare the entry's full path with the last path in the index.
+		 * If it sorts AFTER the last entry in the index and they have no
+		 * common prefix, then there cannot be any F/D name conflicts.
+		 */
+		cmp_last = strcmp_offset(name,
+			istate->cache[istate->cache_nr-1]->name,
+			&len_eq_last);
+		if (cmp_last > 0 && len_eq_last == 0)
+			return retval;
+	}
 
 	for (;;) {
-		int len;
+		size_t len;
 
 		for (;;) {
 			if (*--slash == '/')
@@ -930,6 +945,24 @@ static int has_dir_name(struct index_state *istate,
 		}
 		len = slash - name;
 
+		if (cmp_last > 0) {
+			/*
+			 * If this part of the directory prefix (including the trailing
+			 * slash) already appears in the path of the last entry in the
+			 * index, then we cannot also have a file with this prefix (or
+			 * any parent directory prefix).
+			 */
+			if (len+1 <= len_eq_last)
+				return retval;
+			/*
+			 * If this part of the directory prefix (excluding the trailing
+			 * slash) is longer than the known equal portions, then this part
+			 * of the prefix cannot collide with a file.  Go on to the parent.
+			 */
+			if (len > len_eq_last)
+				continue;
+		}
+
 		pos = index_name_stage_pos(istate, name, len, stage);
 		if (pos >= 0) {
 			/*
@@ -1021,7 +1054,16 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 
 	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
 		cache_tree_invalidate_path(istate, ce->name);
-	pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
+
+	/*
+	 * If this entry's path sorts after the last entry in the index,
+	 * we can avoid searching for it.
+	 */
+	if (istate->cache_nr > 0 &&
+		strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
+		pos = -istate->cache_nr - 1;
+	else
+		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
 
 	/* existing match? Just replace it. */
 	if (pos >= 0) {
-- 
2.9.3

Re: [PATCH v9 0/3] read-cache: speed up add_index_entry

[Jeff King]

On Tue, Apr 11, 2017 at 07:16:59PM +0000, git@jeffhostetler.com wrote:

> I think this version has addressed everything raise so far,
> so I think I'm ready to let this one rest.  Thanks for all
> the help and feedback.

Yeah, this addresses my nitpicks with the perf stuff. Thanks for
sticking with it.

-Peff

Re: [PATCH v9 3/3] read-cache: speed up add_index_entry during checkout

[Junio C Hamano]

git@jeffhostetler.com writes:

> From: Jeff Hostetler <jeffhost@microsoft.com>
>
> Teach add_index_entry_with_check() and has_dir_name()
> to see if the path of the new item is greater than the
> last path in the index array before attempting to search
> for it.
>
> During checkout, merge_working_tree() populates the new
> index in sorted order, so this change will save at least 2
> binary lookups per file.

Smart and simple.

> diff --git a/read-cache.c b/read-cache.c
> index 97f13a1..a8ef823 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -918,9 +918,24 @@ static int has_dir_name(struct index_state *istate,
>  	int stage = ce_stage(ce);
>  	const char *name = ce->name;
>  	const char *slash = name + ce_namelen(ce);
> +	size_t len_eq_last;
> +	int cmp_last = 0;
> +
> +	if (istate->cache_nr > 0) {
> +		/*
> +		 * Compare the entry's full path with the last path in the index.
> +		 * If it sorts AFTER the last entry in the index and they have no
> +		 * common prefix, then there cannot be any F/D name conflicts.
> +		 */
> +		cmp_last = strcmp_offset(name,
> +			istate->cache[istate->cache_nr-1]->name,

Style?  "istate->cache[istate->cache_nr - 1]->name"

> +			&len_eq_last);
> +		if (cmp_last > 0 && len_eq_last == 0)
> +			return retval;
> +	}

Let me follow the logic aloud.  Say the last entry in the cache is
"x/y".  If we came here with ce->name == "x", we need to worry about
nuking the existing entry "x/y".  But if we have "zoo", that cannot
possibly overlap and we can safely return 0.

That sounds correct, except that it might be playing overly safe.
If we came here with "xx", we still are safe, but len_eq_last would
be non-zero.  Probably it is not worth the extra complexity to catch
it here (rather than letting the loop below to handle it).

>  	for (;;) {
> -		int len;
> +		size_t len;
>  
>  		for (;;) {
>  			if (*--slash == '/')
> @@ -930,6 +945,24 @@ static int has_dir_name(struct index_state *istate,
>  		}
>  		len = slash - name;

Mental note: cmp_last may be 0, >0 or <0 at this point in the very
first iteration of the loop.  It is not updated in the loop.  The
variable len_eq_last are used to carry the information about the
last entry we learned at the beginning of this function---the new
special case happens only when the path we are adding sorts later
than the last existing entry (i.e. cmp_last > 0).

> +		if (cmp_last > 0) {
> +			/*
> +			 * If this part of the directory prefix (including the trailing
> +			 * slash) already appears in the path of the last entry in the
> +			 * index, then we cannot also have a file with this prefix (or
> +			 * any parent directory prefix).
> +			 */
> +			if (len+1 <= len_eq_last)

Style?  "len + 1".

> +				return retval;
> +			/*
> +			 * If this part of the directory prefix (excluding the trailing
> +			 * slash) is longer than the known equal portions, then this part
> +			 * of the prefix cannot collide with a file.  Go on to the parent.
> +			 */
> +			if (len > len_eq_last)
> +				continue;

Hmph, is the reasoning used in the two conditionals above sound?
Does this work correctly even when the last existing entry in the
cache is marked with CE_REMOVE?

> +		}
> +
>  		pos = index_name_stage_pos(istate, name, len, stage);
>  		if (pos >= 0) {
>  			/*
> @@ -1021,7 +1054,16 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
>  
>  	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
>  		cache_tree_invalidate_path(istate, ce->name);
> -	pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
> +
> +	/*
> +	 * If this entry's path sorts after the last entry in the index,
> +	 * we can avoid searching for it.
> +	 */
> +	if (istate->cache_nr > 0 &&
> +		strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
> +		pos = -istate->cache_nr - 1;
> +	else
> +		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));

This one (unlike the change to has_dir_name()) is trivially and
obviously correct.

>  	/* existing match? Just replace it. */
>  	if (pos >= 0) {

Re: [PATCH v9 3/3] read-cache: speed up add_index_entry during checkout

[Jeff Hostetler]

On 4/11/2017 11:12 PM, Junio C Hamano wrote:
> git@jeffhostetler.com writes:
>
>> From: Jeff Hostetler <jeffhost@microsoft.com>
>>
>> Teach add_index_entry_with_check() and has_dir_name()
>> to see if the path of the new item is greater than the
>> last path in the index array before attempting to search
>> for it.
>>
>> During checkout, merge_working_tree() populates the new
>> index in sorted order, so this change will save at least 2
>> binary lookups per file.
>
> Smart and simple.
>
>> diff --git a/read-cache.c b/read-cache.c
>> index 97f13a1..a8ef823 100644
>> --- a/read-cache.c
>> +++ b/read-cache.c
>> @@ -918,9 +918,24 @@ static int has_dir_name(struct index_state *istate,
>>  	int stage = ce_stage(ce);
>>  	const char *name = ce->name;
>>  	const char *slash = name + ce_namelen(ce);
>> +	size_t len_eq_last;
>> +	int cmp_last = 0;
>> +
>> +	if (istate->cache_nr > 0) {
>> +		/*
>> +		 * Compare the entry's full path with the last path in the index.
>> +		 * If it sorts AFTER the last entry in the index and they have no
>> +		 * common prefix, then there cannot be any F/D name conflicts.
>> +		 */
>> +		cmp_last = strcmp_offset(name,
>> +			istate->cache[istate->cache_nr-1]->name,
>
> Style?  "istate->cache[istate->cache_nr - 1]->name"
>
>> +			&len_eq_last);
>> +		if (cmp_last > 0 && len_eq_last == 0)
>> +			return retval;
>> +	}
>
> Let me follow the logic aloud.  Say the last entry in the cache is
> "x/y".  If we came here with ce->name == "x", we need to worry about
> nuking the existing entry "x/y".  But if we have "zoo", that cannot
> possibly overlap and we can safely return 0.
>
> That sounds correct, except that it might be playing overly safe.
> If we came here with "xx", we still are safe, but len_eq_last would
> be non-zero.  Probably it is not worth the extra complexity to catch
> it here (rather than letting the loop below to handle it).
>
>>  	for (;;) {
>> -		int len;
>> +		size_t len;
>>
>>  		for (;;) {
>>  			if (*--slash == '/')
>> @@ -930,6 +945,24 @@ static int has_dir_name(struct index_state *istate,
>>  		}
>>  		len = slash - name;
>
> Mental note: cmp_last may be 0, >0 or <0 at this point in the very
> first iteration of the loop.  It is not updated in the loop.  The
> variable len_eq_last are used to carry the information about the
> last entry we learned at the beginning of this function---the new
> special case happens only when the path we are adding sorts later
> than the last existing entry (i.e. cmp_last > 0).
>
>> +		if (cmp_last > 0) {
>> +			/*
>> +			 * If this part of the directory prefix (including the trailing
>> +			 * slash) already appears in the path of the last entry in the
>> +			 * index, then we cannot also have a file with this prefix (or
>> +			 * any parent directory prefix).
>> +			 */
>> +			if (len+1 <= len_eq_last)
>
> Style?  "len + 1".
>
>> +				return retval;
>> +			/*
>> +			 * If this part of the directory prefix (excluding the trailing
>> +			 * slash) is longer than the known equal portions, then this part
>> +			 * of the prefix cannot collide with a file.  Go on to the parent.
>> +			 */
>> +			if (len > len_eq_last)
>> +				continue;
>
> Hmph, is the reasoning used in the two conditionals above sound?
> Does this work correctly even when the last existing entry in the
> cache is marked with CE_REMOVE?

I'll double check my math here.  I also want to think about
the "too conservative" comment above.

WRT if the last entry is CE_REMOVE, I think we are still OK
because we are asking if the given ce is strictly greater than
the last entry so that we can append it rather than searching
for a collision or insertion point.

Thanks,
Jeff