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Revert "std: optimize hash_map probe loop condition"

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This reverts commit 11803a3a56.

Observations from the performance dashboard:
 * strictly worse in terms of CPU instructions
 * slightly worse wall time (but this can be noisy)
 * sometimes better, sometimes worse for branch predictions

Given that the commit was introducing complexity for optimization's
sake, these performance changes do not seem worth it.
This commit is contained in:
Andrew Kelley 2021-12-17 16:56:43 -07:00
parent 11803a3a56
commit 6d04de706a
1 changed files with 13 additions and 60 deletions
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73
lib/std/hash_map.zig
View file

@ -773,11 +773,6 @@ pub fn HashMapUnmanaged(
self.used = 0;
self.fingerprint = tombstone;
}
pub fn reset(self: *Metadata) void {
self.used = 0;
self.fingerprint = free;
}
};
comptime {
@ -1115,24 +1110,12 @@ pub fn HashMapUnmanaged(
}
const mask = self.capacity() - 1;
const fingerprint = Metadata.takeFingerprint(hash);
var start = @truncate(usize, hash & mask);
var idx = start;
// Don't loop indefinitely when there are no empty slots.
var limit = self.capacity();
var idx = @truncate(usize, hash & mask);
var metadata = self.metadata.? + idx;
if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
const test_key = &self.keys()[idx];
const eql = ctx.eql(key, test_key.*);
if (eql) return idx;
}
// Temporarily mark the start position as "free" so that the probe loop
// doesn't infinitely loop when all other slots are filled or tombstones.
const saved = metadata[0];
metadata[0].reset();
defer self.metadata.?[start] = saved;
idx = (idx + 1) & mask; // initial idx was already checked
metadata = self.metadata.? + idx;
while (metadata[0].isUsed() or metadata[0].isTombstone()) {
while ((metadata[0].isUsed() or metadata[0].isTombstone()) and limit != 0) {
if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
const test_key = &self.keys()[idx];
// If you get a compile error on this line, it means that your generic eql
@ -1148,6 +1131,7 @@ pub fn HashMapUnmanaged(
}
}
limit -= 1;
idx = (idx + 1) & mask;
metadata = self.metadata.? + idx;
}
@ -1305,39 +1289,12 @@ pub fn HashMapUnmanaged(
}
const mask = self.capacity() - 1;
const fingerprint = Metadata.takeFingerprint(hash);
var start = @truncate(usize, hash & mask);
var idx = start;
var limit = self.capacity();
var idx = @truncate(usize, hash & mask);
var first_tombstone_idx: usize = self.capacity(); // invalid index
var metadata = self.metadata.? + idx;
if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
const test_key = &self.keys()[idx];
const eql = ctx.eql(key, test_key.*);
if (eql) {
return GetOrPutResult{
.key_ptr = test_key,
.value_ptr = &self.values()[idx],
.found_existing = true,
};
}
} else if (metadata[0].isTombstone()) {
first_tombstone_idx = idx;
}
// Temporarily mark the start position as "free" so that the probe loop
// doesn't infinitely loop when all other slots are filled or tombstones.
const saved = metadata[0];
metadata[0].reset();
defer {
// Don't restore when the 'home' slot was originally a tombstone
// and the probe missed, since it is now occupied by the new key.
const home = &self.metadata.?[start];
assert(!home.isUsed() or home.fingerprint == fingerprint);
if (!home.isUsed()) home.* = saved;
}
idx = (idx + 1) & mask; // initial idx was already checked
metadata = self.metadata.? + idx;
while (metadata[0].isUsed() or metadata[0].isTombstone()) {
while ((metadata[0].isUsed() or metadata[0].isTombstone()) and limit != 0) {
if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
const test_key = &self.keys()[idx];
// If you get a compile error on this line, it means that your generic eql
@ -1359,6 +1316,7 @@ pub fn HashMapUnmanaged(
first_tombstone_idx = idx;
}
limit -= 1;
idx = (idx + 1) & mask;
metadata = self.metadata.? + idx;
}
@ -1944,7 +1902,6 @@ test "std.hash_map repeat putAssumeCapacity/remove" {
try map.ensureTotalCapacity(20);
const limit = map.unmanaged.available;
const cycles = 100;
var i: u32 = 0;
while (i < limit) : (i += 1) {
@ -1954,7 +1911,7 @@ test "std.hash_map repeat putAssumeCapacity/remove" {
// Repeatedly delete/insert an entry without resizing the map.
// Put to different keys so entries don't land in the just-freed slot.
i = 0;
while (i < cycles * limit) : (i += 1) {
while (i < 10 * limit) : (i += 1) {
try testing.expect(map.remove(i));
if (i % 2 == 0) {
map.putAssumeCapacityNoClobber(limit + i, i);
@ -1963,13 +1920,9 @@ test "std.hash_map repeat putAssumeCapacity/remove" {
}
}
i = (cycles - 1) * limit;
while (i < cycles * limit) : (i += 1) {
try expectEqual(map.get(i), null); // (removed) key miss
try expectEqual(map.get(limit + i), i); // key hit
const gop = map.getOrPutAssumeCapacity(limit + i);
try testing.expect(gop.found_existing);
try testing.expectEqual(gop.value_ptr.*, i);
i = 9 * limit;
while (i < 10 * limit) : (i += 1) {
try expectEqual(map.get(limit + i), i);
}
try expectEqual(map.unmanaged.available, 0);
try expectEqual(map.unmanaged.count(), limit);