--- /dev/null
+#include "cache.h"
+#include "sha1-lookup.h"
+
+/*
+ * Conventional binary search loop looks like this:
+ *
+ * unsigned lo, hi;
+ * do {
+ * unsigned mi = (lo + hi) / 2;
+ * int cmp = "entry pointed at by mi" minus "target";
+ * if (!cmp)
+ * return (mi is the wanted one)
+ * if (cmp > 0)
+ * hi = mi; "mi is larger than target"
+ * else
+ * lo = mi+1; "mi is smaller than target"
+ * } while (lo < hi);
+ *
+ * The invariants are:
+ *
+ * - When entering the loop, lo points at a slot that is never
+ * above the target (it could be at the target), hi points at a
+ * slot that is guaranteed to be above the target (it can never
+ * be at the target).
+ *
+ * - We find a point 'mi' between lo and hi (mi could be the same
+ * as lo, but never can be as same as hi), and check if it hits
+ * the target. There are three cases:
+ *
+ * - if it is a hit, we are happy.
+ *
+ * - if it is strictly higher than the target, we set it to hi,
+ * and repeat the search.
+ *
+ * - if it is strictly lower than the target, we update lo to
+ * one slot after it, because we allow lo to be at the target.
+ *
+ * If the loop exits, there is no matching entry.
+ *
+ * When choosing 'mi', we do not have to take the "middle" but
+ * anywhere in between lo and hi, as long as lo <= mi < hi is
+ * satisfied. When we somehow know that the distance between the
+ * target and lo is much shorter than the target and hi, we could
+ * pick mi that is much closer to lo than the midway.
+ *
+ * Now, we can take advantage of the fact that SHA-1 is a good hash
+ * function, and as long as there are enough entries in the table, we
+ * can expect uniform distribution. An entry that begins with for
+ * example "deadbeef..." is much likely to appear much later than in
+ * the midway of the table. It can reasonably be expected to be near
+ * 87% (222/256) from the top of the table.
+ *
+ * However, we do not want to pick "mi" too precisely. If the entry at
+ * the 87% in the above example turns out to be higher than the target
+ * we are looking for, we would end up narrowing the search space down
+ * only by 13%, instead of 50% we would get if we did a simple binary
+ * search. So we would want to hedge our bets by being less aggressive.
+ *
+ * The table at "table" holds at least "nr" entries of "elem_size"
+ * bytes each. Each entry has the SHA-1 key at "key_offset". The
+ * table is sorted by the SHA-1 key of the entries. The caller wants
+ * to find the entry with "key", and knows that the entry at "lo" is
+ * not higher than the entry it is looking for, and that the entry at
+ * "hi" is higher than the entry it is looking for.
+ */
+int sha1_entry_pos(const void *table,
+ size_t elem_size,
+ size_t key_offset,
+ unsigned lo, unsigned hi, unsigned nr,
+ const unsigned char *key)
+{
+ const unsigned char *base = table;
+ const unsigned char *hi_key, *lo_key;
+ unsigned ofs_0;
+ static int debug_lookup = -1;
+
+ if (debug_lookup < 0)
+ debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
+
+ if (!nr || lo >= hi)
+ return -1;
+
+ if (nr == hi)
+ hi_key = NULL;
+ else
+ hi_key = base + elem_size * hi + key_offset;
+ lo_key = base + elem_size * lo + key_offset;
+
+ ofs_0 = 0;
+ do {
+ int cmp;
+ unsigned ofs, mi, range;
+ unsigned lov, hiv, kyv;
+ const unsigned char *mi_key;
+
+ range = hi - lo;
+ if (hi_key) {
+ for (ofs = ofs_0; ofs < 20; ofs++)
+ if (lo_key[ofs] != hi_key[ofs])
+ break;
+ ofs_0 = ofs;
+ /*
+ * byte 0 thru (ofs-1) are the same between
+ * lo and hi; ofs is the first byte that is
+ * different.
+ */
+ hiv = hi_key[ofs_0];
+ if (ofs_0 < 19)
+ hiv = (hiv << 8) | hi_key[ofs_0+1];
+ } else {
+ hiv = 256;
+ if (ofs_0 < 19)
+ hiv <<= 8;
+ }
+ lov = lo_key[ofs_0];
+ kyv = key[ofs_0];
+ if (ofs_0 < 19) {
+ lov = (lov << 8) | lo_key[ofs_0+1];
+ kyv = (kyv << 8) | key[ofs_0+1];
+ }
+ assert(lov < hiv);
+
+ if (kyv < lov)
+ return -1 - lo;
+ if (hiv < kyv)
+ return -1 - hi;
+
+ /*
+ * Even if we know the target is much closer to 'hi'
+ * than 'lo', if we pick too precisely and overshoot
+ * (e.g. when we know 'mi' is closer to 'hi' than to
+ * 'lo', pick 'mi' that is higher than the target), we
+ * end up narrowing the search space by a smaller
+ * amount (i.e. the distance between 'mi' and 'hi')
+ * than what we would have (i.e. about half of 'lo'
+ * and 'hi'). Hedge our bets to pick 'mi' less
+ * aggressively, i.e. make 'mi' a bit closer to the
+ * middle than we would otherwise pick.
+ */
+ kyv = (kyv * 6 + lov + hiv) / 8;
+ if (lov < hiv - 1) {
+ if (kyv == lov)
+ kyv++;
+ else if (kyv == hiv)
+ kyv--;
+ }
+ mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
+
+ if (debug_lookup) {
+ printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
+ printf("ofs %u lov %x, hiv %x, kyv %x\n",
+ ofs_0, lov, hiv, kyv);
+ }
+ if (!(lo <= mi && mi < hi))
+ die("assertion failure lo %u mi %u hi %u %s",
+ lo, mi, hi, sha1_to_hex(key));
+
+ mi_key = base + elem_size * mi + key_offset;
+ cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
+ if (!cmp)
+ return mi;
+ if (cmp > 0) {
+ hi = mi;
+ hi_key = mi_key;
+ } else {
+ lo = mi + 1;
+ lo_key = mi_key + elem_size;
+ }
+ } while (lo < hi);
+ return -lo-1;
+}
#include "tree.h"
#include "refs.h"
#include "pack-revindex.h"
+#include "sha1-lookup.h"
#ifndef O_NOATIME
#if defined(__linux__) && (defined(__i386__) || defined(__PPC__))
{
const uint32_t *level1_ofs = p->index_data;
const unsigned char *index = p->index_data;
- unsigned hi, lo;
+ unsigned hi, lo, stride;
+ static int use_lookup = -1;
+ static int debug_lookup = -1;
+
+ if (debug_lookup < 0)
+ debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
if (!index) {
if (open_pack_index(p))
index += 4 * 256;
hi = ntohl(level1_ofs[*sha1]);
lo = ((*sha1 == 0x0) ? 0 : ntohl(level1_ofs[*sha1 - 1]));
+ if (p->index_version > 1) {
+ stride = 20;
+ } else {
+ stride = 24;
+ index += 4;
+ }
+
+ if (debug_lookup)
+ printf("%02x%02x%02x... lo %u hi %u nr %u\n",
+ sha1[0], sha1[1], sha1[2], lo, hi, p->num_objects);
+
+ if (use_lookup < 0)
+ use_lookup = !!getenv("GIT_USE_LOOKUP");
+ if (use_lookup) {
+ int pos = sha1_entry_pos(index, stride, 0,
+ lo, hi, p->num_objects, sha1);
+ if (pos < 0)
+ return 0;
+ return nth_packed_object_offset(p, pos);
+ }
do {
unsigned mi = (lo + hi) / 2;
- unsigned x = (p->index_version > 1) ? (mi * 20) : (mi * 24 + 4);
- int cmp = hashcmp(index + x, sha1);
+ int cmp = hashcmp(index + mi * stride, sha1);
+
+ if (debug_lookup)
+ printf("lo %u hi %u rg %u mi %u\n",
+ lo, hi, hi - lo, mi);
if (!cmp)
return nth_packed_object_offset(p, mi);
if (cmp > 0)