commit-slab: avoid large realloc

Instead of using a single "slab" and keep reallocating it as we find
that we need to deal with commits with larger values of commit->index,
make a "slab" an array of many "slab_piece"s. Each access may need
two levels of indirections, but we only need to reallocate the first
level array of pointers when we have to grow the table this way.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -501,34 +501,56 @@ struct commit *pop_commit(struct commit_list **stack)
 	return item;
 }
 
+struct commit_slab_piece {
+	int buf;
+};
+
 struct commit_slab {
-	int *buf;
+	int piece_size;
-	int alloc;
+	int piece_count;
+	struct commit_slab_piece **piece;
 };
 
 static void slab_init(struct commit_slab *s)
 {
-	memset(s, 0, sizeof(*s));
+	/* allocate ~512kB at once, allowing for malloc overhead */
+	int size = (512*1024-32) / sizeof(struct commit_slab_piece);
+
+	s->piece_size = size;
+	s->piece_count = 0;
+	s->piece = NULL;
 }
 
 static void slab_clear(struct commit_slab *s)
 {
-	free(s->buf);
+	int i;
-	slab_init(s);
+
+	for (i = 0; i < s->piece_count; i++)
+		free(s->piece[i]);
+	s->piece_count = 0;
+	free(s->piece);
+	s->piece = NULL;
 }
 
-static inline int *slab_at(struct commit_slab *s, const struct commit *c)
+static inline struct commit_slab_piece *slab_at(struct commit_slab *s,
+						const struct commit *c)
 {
-	if (s->alloc <= c->index) {
+	int nth_piece, nth_slot;
-		int new_alloc = alloc_nr(s->alloc);
+
-		if (new_alloc <= c->index)
+	nth_piece = c->index / s->piece_size;
-			new_alloc = c->index + 1;
+	nth_slot = c->index % s->piece_size;
+
+	if (s->piece_count <= nth_piece) {
+		int i;
 
-		s->buf = xrealloc(s->buf, new_alloc * sizeof(*s->buf));
+		s->piece = xrealloc(s->piece, (nth_piece + 1) * sizeof(s->piece));
-		memset(s->buf + s->alloc, 0, new_alloc - s->alloc);
+		for (i = s->piece_count; i <= nth_piece; i++)
-		s->alloc = new_alloc;
+			s->piece[i] = NULL;
+		s->piece_count = nth_piece + 1;
 	}
-	return s->buf + c->index;
+	if (!s->piece[nth_piece])
+		s->piece[nth_piece] = xcalloc(s->piece_size, sizeof(**s->piece));
+	return &s->piece[nth_piece][nth_slot];
 }
 
 /*
@@ -550,7 +572,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 	/* Mark them and clear the indegree */
 	for (next = orig; next; next = next->next) {
 		struct commit *commit = next->item;
-		*slab_at(&indegree, commit) = 1;
+		slab_at(&indegree, commit)->buf = 1;
 	}
 
 	/* update the indegree */
@@ -558,7 +580,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 		struct commit_list * parents = next->item->parents;
 		while (parents) {
 			struct commit *parent = parents->item;
-			int *pi = slab_at(&indegree, parent);
+			int *pi = &slab_at(&indegree, parent)->buf;
 
 			if (*pi)
 				(*pi)++;
@@ -578,7 +600,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 	for (next = orig; next; next = next->next) {
 		struct commit *commit = next->item;
 
-		if (*slab_at(&indegree, commit) == 1)
+		if (slab_at(&indegree, commit)->buf == 1)
 			insert = &commit_list_insert(commit, insert)->next;
 	}
 
@@ -599,7 +621,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 		commit = work_item->item;
 		for (parents = commit->parents; parents ; parents = parents->next) {
 			struct commit *parent = parents->item;
-			int *pi = slab_at(&indegree, parent);
+			int *pi = &slab_at(&indegree, parent)->buf;
 
 			if (!*pi)
 				continue;
@@ -620,7 +642,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
 		 * work_item is a commit all of whose children
 		 * have already been emitted. we can emit it now.
 		 */
-		*slab_at(&indegree, commit) = 0;
+		slab_at(&indegree, commit)->buf = 0;
 		*pptr = work_item;
 		pptr = &work_item->next;
 	}