Subject: Re: [PATCH v2] last-modified: implement faster algorithm

From: Junio C Hamano <[email-masked]>
Date: Tue, 21 Oct 2025 10:52:11 -0700

Toon Claes <[email-masked]> writes:

> +static size_t path_idx(struct last_modified *lm, char *path)
> +{
> +	struct last_modified_entry *ent;
> +	ent = hashmap_get_entry_from_hash(&lm->paths, strhash(path), path,
> +					  struct last_modified_entry, hashent);
> +
> +	return ent ? ent->diff_idx : -1;
> +}

size_t is unsigned and cannot reutrn -1 sanely, unless the caller
knows that ((size_t)-1) signals an error.  The compiler warns, and
we compile with -Werror, so we end up getting

    builtin/last-modified.c: In function 'path_idx':
    builtin/last-modified.c:235:38: error: operand of '?:' changes signedness from 'int' to 'size_t' {aka 'long unsigned int'} due to unsignedness of other operand [-Werror=sign-compare]
      235 |         return ent ? ent->diff_idx : -1;
          |                                      ^~

> +static void pass_to_parent(struct last_modified *lm,
> +			   struct bitmap *c,
> +			   struct bitmap *p,
> +			   size_t pos)
> +{
> +	bitmap_unset(c, pos);
> +	bitmap_set(p, pos);
> +}

Mark lm as UNUSED, or we'd get 

    builtin/last-modified.c: In function 'pass_to_parent':
    builtin/last-modified.c:238:50: error: unused parameter 'lm' [-Werror=unused-parameter]
      238 | static void pass_to_parent(struct last_modified *lm,
          |                            ~~~~~~~~~~~~~~~~~~~~~~^~

> @@ -220,42 +273,195 @@ static bool maybe_changed_path(struct last_modified *lm, struct commit *origin)
>  	return false;
>  }
>  
> +static void process_parent(struct last_modified *lm,
> +			   struct prio_queue *queue,
> +			   struct commit *c, struct bitmap *active_c,
> +			   struct commit *parent, int parent_i)
> +{
> +	size_t i;
> +...
> +	for (i = 0; i < diff_queued_diff.nr; i++) {

Our -Wsign-compare forces the compiler to give a stupid warning
here, that i is unsigned and diff_queued_diff.nr is signed.  

    builtin/last-modified.c: In function 'process_parent':
    builtin/last-modified.c:304:23: error: comparison of integer expressions of different signedness: 'size_t' {aka 'long unsigned int'} and 'int' [-Werror=sign-compare]
      304 |         for (i = 0; i < diff_queued_diff.nr; i++) {
          |                       ^

Yes, it is comparing unsigned with signed but so what?

As people may now know, my preference is to wean ourselves off of
this dogmatic trust in -Wsign-compare but those who disagree and want
to remove #define DISABLE_SIGN_COMPARE_WARNINGS should help our poor
compiler here by telling it that this comparison is perfectly fine.

We know diff_queued_diff.nr is an int, and i is size_t, but we also
know diff_queued_diff.nr won't be negative (or we have much bigger
problems) and cannot be larger than what size_t can represent.

> +		struct diff_filepair *fp = diff_queued_diff.queue[i];
> +		size_t k = path_idx(lm, fp->two->path);
> +		if (0 <= k && bitmap_get(active_c, k))
> +			bitmap_set(lm->scratch, k);
> +	}

Earlier path_idx() wanted to signal an error by returning negative,
but the type is size_t that is unsigned so it cannot do so.  We
instead get

    builtin/last-modified.c:307:23: error: comparison of unsigned expression in '>= 0' is always true [-Werror=type-limits]
      307 |                 if (0 <= k && bitmap_get(active_c, k))
          |                       ^~

and in this case we actually deserve it (in the sense that this is
not the fault of dogmatic trust in -Wsign-compare; this is caused by
using size_t to count things).

And the solution for this is *not* "size_t" -> "ssize_t", because
ssize_t is not "store half the range of size_t with negative values
reserved for something else like errors".  Its width can be much
narrower (this came up in a separate thread very recently [*]).
Instead we'd need something ugly like

	if (k != (size_t)-1 && bitmap_get(active_c, k))

A quick band-aid patch to make it compile is attached at the end,
but it does not try to address the root causes, which are abuse of
size_t as count_t and religious use of "-Wsign-compare" [*].

[Reference]

* https://lore.kernel.org/git/[email-masked]/
* https://staticthinking.wordpress.com/2023/07/25/wsign-compare-is-garbage/

 builtin/last-modified.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git c/builtin/last-modified.c w/builtin/last-modified.c
index e9050485a9..6135bcc584 100644
--- c/builtin/last-modified.c
+++ w/builtin/last-modified.c
@@ -232,10 +232,10 @@ static size_t path_idx(struct last_modified *lm, char *path)
 	ent = hashmap_get_entry_from_hash(&lm->paths, strhash(path), path,
 					  struct last_modified_entry, hashent);
 
-	return ent ? ent->diff_idx : -1;
+	return ent ? ent->diff_idx : (size_t)-1;
 }
 
-static void pass_to_parent(struct last_modified *lm,
+static void pass_to_parent(struct last_modified *lm UNUSED,
 			   struct bitmap *c,
 			   struct bitmap *p,
 			   size_t pos)
@@ -278,7 +278,6 @@ static void process_parent(struct last_modified *lm,
 			   struct commit *c, struct bitmap *active_c,
 			   struct commit *parent, int parent_i)
 {
-	size_t i;
 	struct bitmap *active_p;
 
 	repo_parse_commit(lm->rev.repo, parent);
@@ -301,13 +300,13 @@ static void process_parent(struct last_modified *lm,
 	 * First, collect all paths that are *not* TREESAME in 'scratch'.
 	 * Then, pass paths that *are* TREESAME and active to the parent.
 	 */
-	for (i = 0; i < diff_queued_diff.nr; i++) {
+	for (int i = 0; i < diff_queued_diff.nr; i++) {
 		struct diff_filepair *fp = diff_queued_diff.queue[i];
 		size_t k = path_idx(lm, fp->two->path);
-		if (0 <= k && bitmap_get(active_c, k))
+		if (k != (size_t)-1 && bitmap_get(active_c, k))
 			bitmap_set(lm->scratch, k);
 	}
-	for (i = 0; i < lm->all_paths_nr; i++) {
+	for (size_t i = 0; i < lm->all_paths_nr; i++) {
 		if (bitmap_get(active_c, i) && !bitmap_get(lm->scratch, i))
 			pass_to_parent(lm, active_c, active_p, i);
 	}