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std: hash_map: optimize isFree/isTombstone (#10562)

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- Add an `Metadata.isFree` helper method.
- Implement `Metadata.isTombstone` and `Metadata.isFree` with `@bitCast` then comparing to a constant. I assume `@bitCast`-then-compare is faster than the old method because it only involves one comparison, and doesn't require bitmasking.
- Summary of benchmarked changes (`gotta-go-fast`, run locally, compared to master):
  - 3/4 of the hash map benchmarks used ~10% fewer cycles
  - The last one (project Euler) shows 4% fewer cycles.
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djg 2022-01-10 20:54:45 -08:00 • committed by GitHub
parent ada8e17137
commit 4731a6e5d5
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1 changed files with 10 additions and 3 deletions
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13
lib/std/hash_map.zig
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@ -750,12 +750,19 @@ pub fn HashMapUnmanaged(
fingerprint: FingerPrint = free,
used: u1 = 0,
const slot_free = @bitCast(u8, Metadata{ .fingerprint = free });
const slot_tombstone = @bitCast(u8, Metadata{ .fingerprint = tombstone });
pub fn isUsed(self: Metadata) bool {
return self.used == 1;
}
pub fn isTombstone(self: Metadata) bool {
return !self.isUsed() and self.fingerprint == tombstone;
return @bitCast(u8, self) == slot_tombstone;
}
pub fn isFree(self: Metadata) bool {
return @bitCast(u8, self) == slot_free;
}
pub fn takeFingerprint(hash: Hash) FingerPrint {
@ -1115,7 +1122,7 @@ pub fn HashMapUnmanaged(
var idx = @truncate(usize, hash & mask);
var metadata = self.metadata.? + idx;
while ((metadata[0].isUsed() or metadata[0].isTombstone()) and limit != 0) {
while (!metadata[0].isFree() 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
@ -1294,7 +1301,7 @@ pub fn HashMapUnmanaged(
var first_tombstone_idx: usize = self.capacity(); // invalid index
var metadata = self.metadata.? + idx;
while ((metadata[0].isUsed() or metadata[0].isTombstone()) and limit != 0) {
while (!metadata[0].isFree() 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