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std.HashMap: optimize indexing by avoiding modulo operator 

x % y can be optimized if y is a power of two by doing x & (y-1) instead. HashMap already enforces power of two capacity, so we can take advantage of this optimization.
master
Ryan Liptak 2019-06-05 23:26:48 -07:00
parent a0d66fa1e6
commit 656ac43735
1 changed files with 13 additions and 5 deletions
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18
std/hash_map.zig
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@ -155,6 +155,8 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
/// capacity is greater than the current capacity.
/// New capacity must be a power of two.
fn ensureCapacityExact(self: *Self, new_capacity: usize) !void {
// capacity must always be a power of two to allow for modulo
// optimization in the constrainIndex fn
const is_power_of_two = new_capacity & (new_capacity - 1) == 0;
assert(is_power_of_two);
@ -209,7 +211,7 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
{
var roll_over: usize = 0;
while (roll_over <= hm.max_distance_from_start_index) : (roll_over += 1) {
const index = (start_index + roll_over) % hm.entries.len;
const index = hm.constrainIndex(start_index + roll_over);
var entry = &hm.entries[index];
if (!entry.used) return null;
@ -218,7 +220,7 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
const removed_kv = entry.kv;
while (roll_over < hm.entries.len) : (roll_over += 1) {
const next_index = (start_index + roll_over + 1) % hm.entries.len;
const next_index = hm.constrainIndex(start_index + roll_over + 1);
const next_entry = &hm.entries[next_index];
if (!next_entry.used or next_entry.distance_from_start_index == 0) {
entry.used = false;
@ -301,7 +303,7 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
roll_over += 1;
distance_from_start_index += 1;
}) {
const index = (start_index + roll_over) % self.entries.len;
const index = self.constrainIndex(start_index + roll_over);
const entry = &self.entries[index];
if (entry.used and !eql(entry.kv.key, key)) {
@ -358,7 +360,7 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
{
var roll_over: usize = 0;
while (roll_over <= hm.max_distance_from_start_index) : (roll_over += 1) {
const index = (start_index + roll_over) % hm.entries.len;
const index = hm.constrainIndex(start_index + roll_over);
const entry = &hm.entries[index];
if (!entry.used) return null;
@ -369,7 +371,13 @@ pub fn HashMap(comptime K: type, comptime V: type, comptime hash: fn (key: K) u3
}
fn keyToIndex(hm: Self, key: K) usize {
return usize(hash(key)) % hm.entries.len;
return hm.constrainIndex(usize(hash(key)));
}
fn constrainIndex(hm: Self, i: usize) usize {
// this is an optimization for modulo of power of two integers;
// it requires hm.entries.len to always be a power of two
return i & (hm.entries.len - 1);
}
};
}