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Preallocate hash tables when the number of inserts are known in advance

This avoids unnecessary re-allocations and reinsertions. On webkit.git
(i.e. about 182k inserts to the name hash table), this reduces about
100ms out of 3s user time.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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1 parent 03a0fb0 commit c73592812d36ccc7eccfd0489177970d246fa522 @pclouds pclouds committed with gitster Mar 17, 2013
Showing with 10 additions and 0 deletions.
  1. +1 −0 diffcore-rename.c
  2. +7 −0 hash.h
  3. +2 −0 name-hash.c
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1 diffcore-rename.c
@@ -389,6 +389,7 @@ static int find_exact_renames(struct diff_options *options)
struct hash_table file_table;
init_hash(&file_table);
+ preallocate_hash(&file_table, rename_src_nr + rename_dst_nr);
for (i = 0; i < rename_src_nr; i++)
insert_file_table(&file_table, -1, i, rename_src[i].p->one);
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7 hash.h
@@ -40,4 +40,11 @@ static inline void init_hash(struct hash_table *table)
table->array = NULL;
}
+static inline void preallocate_hash(struct hash_table *table, unsigned int elts)
+{
+ assert(table->size == 0 && table->nr == 0 && table->array == NULL);
+ table->size = elts * 2;
+ table->array = xcalloc(sizeof(struct hash_table_entry), table->size);
+}
+
#endif
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2 name-hash.c
@@ -92,6 +92,8 @@ static void lazy_init_name_hash(struct index_state *istate)
if (istate->name_hash_initialized)
return;
+ if (istate->cache_nr)
+ preallocate_hash(&istate->name_hash, istate->cache_nr);
for (nr = 0; nr < istate->cache_nr; nr++)
hash_index_entry(istate, istate->cache[nr]);
istate->name_hash_initialized = 1;

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