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AK: Speed up BitmapView::find_next_range_of_unset_bits

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While watching Andreas' most recent video, I noticed that this function
only worked with 32 bit values, but was a serious performance
bottleneck for the kernel. As such, I reworked it to use `size_t`, so
it now can switch to 64-bit sweeps on 64-bit platforms. This caused
test-js to go from 12.5 seconds hot to 11.5 seconds hot on my machine
when running on KVM x86_64.
This commit is contained in:
Nick Johnson 2021-12-11 21:54:45 -06:00 • committed by Andreas Kling
parent 548529ace4
commit e122502f70
1 changed files with 23 additions and 21 deletions
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44
AK/BitmapView.h
View file

@ -7,8 +7,8 @@
#pragma once
#include <AK/Array.h>
#include <AK/BuiltinWrappers.h>
#include <AK/Optional.h>
#include <AK/Platform.h>
#include <AK/StdLibExtras.h>
#include <AK/Types.h>
@ -205,18 +205,20 @@ public:
return {};
}
u32* bitmap32 = (u32*)m_data;
size_t bit_size = 8 * sizeof(size_t);
size_t* bitmap = (size_t*)m_data;
// Calculating the start offset.
size_t start_bucket_index = from / 32;
size_t start_bucket_bit = from % 32;
size_t start_bucket_index = from / bit_size;
size_t start_bucket_bit = from % bit_size;
size_t* start_of_free_chunks = &from;
size_t free_chunks = 0;
for (size_t bucket_index = start_bucket_index; bucket_index < m_size / 32; ++bucket_index) {
if (bitmap32[bucket_index] == 0xffffffff) {
// Skip over completely full bucket of size 32.
for (size_t bucket_index = start_bucket_index; bucket_index < m_size / bit_size; ++bucket_index) {
if (bitmap[bucket_index] == NumericLimits<size_t>::max()) {
// Skip over completely full bucket of size bit_size.
if (free_chunks >= min_length) {
return min(free_chunks, max_length);
}
@ -224,12 +226,12 @@ public:
start_bucket_bit = 0;
continue;
}
if (bitmap32[bucket_index] == 0x0) {
// Skip over completely empty bucket of size 32.
if (bitmap[bucket_index] == 0x0) {
// Skip over completely empty bucket of size bit_size.
if (free_chunks == 0) {
*start_of_free_chunks = bucket_index * 32;
*start_of_free_chunks = bucket_index * bit_size;
}
free_chunks += 32;
free_chunks += bit_size;
if (free_chunks >= max_length) {
return max_length;
}
@ -237,26 +239,26 @@ public:
continue;
}
u32 bucket = bitmap32[bucket_index];
size_t bucket = bitmap[bucket_index];
u8 viewed_bits = start_bucket_bit;
u32 trailing_zeroes = 0;
bucket >>= viewed_bits;
start_bucket_bit = 0;
while (viewed_bits < 32) {
while (viewed_bits < bit_size) {
if (bucket == 0) {
if (free_chunks == 0) {
*start_of_free_chunks = bucket_index * 32 + viewed_bits;
*start_of_free_chunks = bucket_index * bit_size + viewed_bits;
}
free_chunks += 32 - viewed_bits;
viewed_bits = 32;
free_chunks += bit_size - viewed_bits;
viewed_bits = bit_size;
} else {
trailing_zeroes = count_trailing_zeroes_32(bucket);
trailing_zeroes = count_trailing_zeroes(bucket);
bucket >>= trailing_zeroes;
if (free_chunks == 0) {
*start_of_free_chunks = bucket_index * 32 + viewed_bits;
*start_of_free_chunks = bucket_index * bit_size + viewed_bits;
}
free_chunks += trailing_zeroes;
viewed_bits += trailing_zeroes;
@ -266,7 +268,7 @@ public:
}
// Deleting trailing ones.
u32 trailing_ones = count_trailing_zeroes_32(~bucket);
u32 trailing_ones = count_trailing_zeroes(~bucket);
bucket >>= trailing_ones;
viewed_bits += trailing_ones;
free_chunks = 0;
@ -275,8 +277,8 @@ public:
}
if (free_chunks < min_length) {
size_t first_trailing_bit = (m_size / 32) * 32;
size_t trailing_bits = size() % 32;
size_t first_trailing_bit = (m_size / bit_size) * bit_size;
size_t trailing_bits = size() % bit_size;
for (size_t i = 0; i < trailing_bits; ++i) {
if (!get(first_trailing_bit + i)) {
if (free_chunks == 0)