[PATCH 5/8] [GSOC] ref-filter: introduction ref_filter_slopbuf[1]

["ZheNing Hu via GitGitGadget" <gitgitgadget@gmail.com>]

From: ZheNing Hu <adlternative@gmail.com>

The ref-filter code uses xstrdup("") extensively to indicate null
value.

xstrdup("") calls strdup(""), which in glibc will call malloc(1),
this means using xstrdup("") will cause unnecessary memory allocation.

Introduce the global one-byte array variable ref_filter_slopbuf[1],
which learn from strbuf_slopbuf[1] in strbuf.c. Using ref_filter_slopbuf
instead of xstrdup(""), we can reduce some malloc/free overhead.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: ZheNing Hu <adlternative@gmail.com>
---
 ref-filter.c | 53 ++++++++++++++++++++++++++--------------------------
 1 file changed, 27 insertions(+), 26 deletions(-)

diff --git a/ref-filter.c b/ref-filter.c
index 3784c0af57c..b74fcbb4806 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -24,6 +24,8 @@
 #include "hashmap.h"
 #include "strvec.h"
 
+static char ref_filter_slopbuf[1];
+
 static struct ref_msg {
 	const char *gone;
 	const char *ahead;
@@ -1209,7 +1211,7 @@ static const char *copy_name(const char *buf)
 		if (!strncmp(cp, " <", 2))
 			return xmemdupz(buf, cp - buf);
 	}
-	return xstrdup("");
+	return ref_filter_slopbuf;
 }
 
 static const char *copy_email(const char *buf, struct used_atom *atom)
@@ -1217,7 +1219,7 @@ static const char *copy_email(const char *buf, struct used_atom *atom)
 	const char *email = strchr(buf, '<');
 	const char *eoemail;
 	if (!email)
-		return xstrdup("");
+		return ref_filter_slopbuf;
 	switch (atom->u.email_option.option) {
 	case EO_RAW:
 		eoemail = strchr(email, '>');
@@ -1239,7 +1241,7 @@ static const char *copy_email(const char *buf, struct used_atom *atom)
 	}
 
 	if (!eoemail)
-		return xstrdup("");
+		return ref_filter_slopbuf;
 	return xmemdupz(email, eoemail - email);
 }
 
@@ -1293,7 +1295,7 @@ static void grab_date(const char *buf, struct atom_value *v, const char *atomnam
 	v->value = timestamp;
 	return;
  bad:
-	v->s = xstrdup("");
+	v->s = ref_filter_slopbuf;
 	v->value = 0;
 }
 
@@ -1503,7 +1505,7 @@ static void fill_missing_values(struct atom_value *val)
 	for (i = 0; i < used_atom_cnt; i++) {
 		struct atom_value *v = &val[i];
 		if (v->s == NULL)
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 	}
 }
 
@@ -1576,7 +1578,7 @@ static const char *lstrip_ref_components(const char *refname, int len)
 		switch (*start++) {
 		case '\0':
 			free((char *)to_free);
-			return xstrdup("");
+			return ref_filter_slopbuf;
 		case '/':
 			remaining--;
 			break;
@@ -1614,7 +1616,7 @@ static const char *rstrip_ref_components(const char *refname, int len)
 		char *p = strrchr(start, '/');
 		if (p == NULL) {
 			free((char *)to_free);
-			return xstrdup("");
+			return ref_filter_slopbuf;
 		} else
 			p[0] = '\0';
 	}
@@ -1645,7 +1647,7 @@ static void fill_remote_ref_details(struct used_atom *atom, const char *refname,
 				       AHEAD_BEHIND_FULL) < 0) {
 			v->s = xstrdup(msgs.gone);
 		} else if (!num_ours && !num_theirs)
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 		else if (!num_ours)
 			v->s = xstrfmt_len(&v->s_size, msgs.behind, num_theirs);
 		else if (!num_theirs)
@@ -1662,7 +1664,7 @@ static void fill_remote_ref_details(struct used_atom *atom, const char *refname,
 		if (stat_tracking_info(branch, &num_ours, &num_theirs,
 				       NULL, atom->u.remote_ref.push,
 				       AHEAD_BEHIND_FULL) < 0) {
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 			return;
 		}
 		if (!num_ours && !num_theirs)
@@ -1721,7 +1723,7 @@ char *get_head_description(void)
 static const char *get_symref(struct used_atom *atom, struct ref_array_item *ref)
 {
 	if (!ref->symref)
-		return xstrdup("");
+		return ref_filter_slopbuf;
 	else
 		return show_ref(&atom->u.refname, ref->symref);
 }
@@ -1805,7 +1807,7 @@ static char *get_worktree_path(const struct used_atom *atom, const struct ref_ar
 	e = hashmap_get(&(ref_to_worktree_map.map), &entry, ref->refname);
 
 	if (!e)
-		return xstrdup("");
+		return ref_filter_slopbuf;
 
 	lookup_result = container_of(e, struct ref_to_worktree_entry, ent);
 
@@ -1827,7 +1829,7 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 		ref->symref = resolve_refdup(ref->refname, RESOLVE_REF_READING,
 					     NULL, NULL);
 		if (!ref->symref)
-			ref->symref = xstrdup("");
+			ref->symref = ref_filter_slopbuf;
 	}
 
 	/* Fill in specials first */
@@ -1855,7 +1857,7 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 			if (ref->kind == FILTER_REFS_BRANCHES)
 				v->s = get_worktree_path(atom, ref);
 			else
-				v->s = xstrdup("");
+				v->s = ref_filter_slopbuf;
 			continue;
 		}
 		else if (atom_type == ATOM_SYMREF)
@@ -1865,7 +1867,7 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 			/* only local branches may have an upstream */
 			if (!skip_prefix(ref->refname, "refs/heads/",
 					 &branch_name)) {
-				v->s = xstrdup("");
+				v->s = ref_filter_slopbuf;
 				continue;
 			}
 			branch = branch_get(branch_name);
@@ -1874,11 +1876,11 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 			if (refname)
 				fill_remote_ref_details(atom, refname, branch, v);
 			else
-				v->s = xstrdup("");
+				v->s = ref_filter_slopbuf;
 			continue;
 		} else if (atom_type == ATOM_PUSH && atom->u.remote_ref.push) {
 			const char *branch_name;
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 			if (!skip_prefix(ref->refname, "refs/heads/",
 					 &branch_name))
 				continue;
@@ -1891,8 +1893,6 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 				if (!refname)
 					continue;
 			}
-			/* We will definitely re-init v->s on the next line. */
-			free((char *)v->s);
 			fill_remote_ref_details(atom, refname, branch, v);
 			continue;
 		} else if (atom_type == ATOM_COLOR) {
@@ -1905,7 +1905,7 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 			if (ref->flag & REF_ISPACKED)
 				cp = copy_advance(cp, ",packed");
 			if (cp == buf)
-				v->s = xstrdup("");
+				v->s = ref_filter_slopbuf;
 			else {
 				*cp = '\0';
 				v->s = xstrdup(buf + 1);
@@ -1922,33 +1922,33 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 			continue;
 		} else if (atom_type == ATOM_ALIGN) {
 			v->handler = align_atom_handler;
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 			continue;
 		} else if (atom_type == ATOM_END) {
 			v->handler = end_atom_handler;
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 			continue;
 		} else if (atom_type == ATOM_IF) {
 			const char *s;
 			if (skip_prefix(name, "if:", &s))
 				v->s = xstrdup(s);
 			else
-				v->s = xstrdup("");
+				v->s = ref_filter_slopbuf;
 			v->handler = if_atom_handler;
 			continue;
 		} else if (atom_type == ATOM_THEN) {
 			v->handler = then_atom_handler;
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 			continue;
 		} else if (atom_type == ATOM_ELSE) {
 			v->handler = else_atom_handler;
-			v->s = xstrdup("");
+			v->s = ref_filter_slopbuf;
 			continue;
 		} else if (atom_type == ATOM_REST) {
 			if (ref->rest)
 				v->s = xstrdup(ref->rest);
 			else
-				v->s = xstrdup("");
+				v->s = ref_filter_slopbuf;
 			continue;
 		} else
 			continue;
@@ -2297,7 +2297,8 @@ static void free_array_item(struct ref_array_item *item)
 	if (item->value) {
 		int i;
 		for (i = 0; i < used_atom_cnt; i++)
-			free((char *)item->value[i].s);
+			if (item->value[i].s != ref_filter_slopbuf)
+				free((char *)item->value[i].s);
 		free(item->value);
 	}
 	free(item);
-- 
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