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pdqsort: Fixed overflow and simplified block partition logic. (2dc2bbd)

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Bartosz Taudul 2024-03-13 00:42:59 +01:00
parent 3b9e7a1838
commit 00e24e2b8c
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1 changed files with 84 additions and 96 deletions
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server/tracy_pdqsort.h
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@ -171,18 +171,18 @@ namespace pdqsort_detail {
template<class Iter>
tracy_force_inline void swap_offsets(Iter first, Iter last,
unsigned char* offsets_l, unsigned char* offsets_r,
int num, bool use_swaps) {
size_t num, bool use_swaps) {
typedef typename std::iterator_traits<Iter>::value_type T;
if (use_swaps) {
// This case is needed for the descending distribution, where we need
// to have proper swapping for pdqsort to remain O(n).
for (int i = 0; i < num; ++i) {
for (size_t i = 0; i < num; ++i) {
std::iter_swap(first + offsets_l[i], last - offsets_r[i]);
}
} else if (num > 0) {
Iter l = first + offsets_l[0]; Iter r = last - offsets_r[0];
T tmp(PDQSORT_PREFER_MOVE(*l)); *l = PDQSORT_PREFER_MOVE(*r);
for (int i = 1; i < num; ++i) {
for (size_t i = 1; i < num; ++i) {
l = first + offsets_l[i]; *r = PDQSORT_PREFER_MOVE(*l);
r = last - offsets_r[i]; *l = PDQSORT_PREFER_MOVE(*r);
}
@ -219,108 +219,94 @@ namespace pdqsort_detail {
if (!already_partitioned) {
std::iter_swap(first, last);
++first;
}
// The following branchless partitioning is derived from "BlockQuicksort: How Branch
// Mispredictions don’t affect Quicksort" by Stefan Edelkamp and Armin Weiss.
unsigned char offsets_l_storage[block_size + cacheline_size];
unsigned char offsets_r_storage[block_size + cacheline_size];
unsigned char* offsets_l = align_cacheline(offsets_l_storage);
unsigned char* offsets_r = align_cacheline(offsets_r_storage);
int num_l, num_r, start_l, start_r;
num_l = num_r = start_l = start_r = 0;
while (last - first > 2 * block_size) {
// Fill up offset blocks with elements that are on the wrong side.
if (num_l == 0) {
start_l = 0;
Iter it = first;
for (unsigned char i = 0; i < block_size;) {
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
// The following branchless partitioning is derived from "BlockQuicksort: How Branch
// Mispredictions don’t affect Quicksort" by Stefan Edelkamp and Armin Weiss, but
// heavily micro-optimized.
unsigned char offsets_l_storage[block_size + cacheline_size];
unsigned char offsets_r_storage[block_size + cacheline_size];
unsigned char* offsets_l = align_cacheline(offsets_l_storage);
unsigned char* offsets_r = align_cacheline(offsets_r_storage);
Iter offsets_l_base = first;
Iter offsets_r_base = last;
size_t num_l, num_r, start_l, start_r;
num_l = num_r = start_l = start_r = 0;
while (first < last) {
// Fill up offset blocks with elements that are on the wrong side.
// First we determine how much elements are considered for each offset block.
size_t num_unknown = last - first;
size_t left_split = num_l == 0 ? (num_r == 0 ? num_unknown / 2 : num_unknown) : 0;
size_t right_split = num_r == 0 ? (num_unknown - left_split) : 0;
// Fill the offset blocks.
if (left_split >= block_size) {
for (size_t i = 0; i < block_size;) {
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
}
} else {
for (size_t i = 0; i < left_split;) {
offsets_l[num_l] = i++; num_l += !comp(*first, pivot); ++first;
}
}
}
if (num_r == 0) {
start_r = 0;
Iter it = last;
for (unsigned char i = 0; i < block_size;) {
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
if (right_split >= block_size) {
for (size_t i = 0; i < block_size;) {
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
}
} else {
for (size_t i = 0; i < right_split;) {
offsets_r[num_r] = ++i; num_r += comp(*--last, pivot);
}
}
// Swap elements and update block sizes and first/last boundaries.
size_t num = std::min(num_l, num_r);
swap_offsets(offsets_l_base, offsets_r_base,
offsets_l + start_l, offsets_r + start_r,
num, num_l == num_r);
num_l -= num; num_r -= num;
start_l += num; start_r += num;
if (num_l == 0) {
start_l = 0;
offsets_l_base = first;
}
if (num_r == 0) {
start_r = 0;
offsets_r_base = last;
}
}
// Swap elements and update block sizes and first/last boundaries.
int num = std::min(num_l, num_r);
swap_offsets(first, last, offsets_l + start_l, offsets_r + start_r,
num, num_l == num_r);
num_l -= num; num_r -= num;
start_l += num; start_r += num;
if (num_l == 0) first += block_size;
if (num_r == 0) last -= block_size;
}
int l_size = 0, r_size = 0;
int unknown_left = (int)(last - first) - ((num_r || num_l) ? block_size : 0);
if (num_r) {
// Handle leftover block by assigning the unknown elements to the other block.
l_size = unknown_left;
r_size = block_size;
} else if (num_l) {
l_size = block_size;
r_size = unknown_left;
} else {
// No leftover block, split the unknown elements in two blocks.
l_size = unknown_left/2;
r_size = unknown_left - l_size;
}
// Fill offset buffers if needed.
if (unknown_left && !num_l) {
start_l = 0;
Iter it = first;
for (unsigned char i = 0; i < l_size;) {
offsets_l[num_l] = i++; num_l += !comp(*it, pivot); ++it;
// We have now fully identified [first, last)'s proper position. Swap the last elements.
if (num_l) {
offsets_l += start_l;
while (num_l--) std::iter_swap(offsets_l_base + offsets_l[num_l], --last);
first = last;
}
}
if (unknown_left && !num_r) {
start_r = 0;
Iter it = last;
for (unsigned char i = 0; i < r_size;) {
offsets_r[num_r] = ++i; num_r += comp(*--it, pivot);
if (num_r) {
offsets_r += start_r;
while (num_r--) std::iter_swap(offsets_r_base - offsets_r[num_r], first), ++first;
last = first;
}
}
int num = std::min(num_l, num_r);
swap_offsets(first, last, offsets_l + start_l, offsets_r + start_r, num, num_l == num_r);
num_l -= num; num_r -= num;
start_l += num; start_r += num;
if (num_l == 0) first += l_size;
if (num_r == 0) last -= r_size;
// We have now fully identified [first, last)'s proper position. Swap the last elements.
if (num_l) {
offsets_l += start_l;
while (num_l--) std::iter_swap(first + offsets_l[num_l], --last);
first = last;
}
if (num_r) {
offsets_r += start_r;
while (num_r--) std::iter_swap(last - offsets_r[num_r], first), ++first;
last = first;
}
// Put the pivot in the right place.
Iter pivot_pos = first - 1;
*begin = PDQSORT_PREFER_MOVE(*pivot_pos);
@ -329,6 +315,8 @@ namespace pdqsort_detail {
return std::make_pair(pivot_pos, already_partitioned);
}
// Partitions [begin, end) around pivot *begin using comparison function comp. Elements equal
// to the pivot are put in the right-hand partition. Returns the position of the pivot after
// partitioning and whether the passed sequence already was correctly partitioned. Assumes the