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mormord sort more branchless plus extra edge-case handling for empty sized calls

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This commit is contained in:
Richard Thier 2024-04-11 19:45:30 +02:00
parent d16505a297
commit 2d2cad2c5a
1 changed files with 28 additions and 18 deletions
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46
ypsu.cpp
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@ -150,6 +150,9 @@ static inline uint32_t morgrab(uint32_t elem) noexcept {
}
template<int j>
static inline void mormord_sort_impl(uint32_t *a, int n) noexcept {
/* Edge-case */
if(n == 0) return;
/* Preparation */
uint32_t radics[256] = {0};
uint32_t radics2[256] = {0};
@ -184,34 +187,41 @@ static inline void mormord_sort_impl(uint32_t *a, int n) noexcept {
uint32_t reali = 0;
#pragma GCC unroll 16
for(int i = 0; i < 256; ++i) {
radics[i] += prev;
if(radics[i] != 0) {
radics[i] += prev;
real_radics[reali] = prev;
real_radics[reali + 1] = radics[i];
prev = radics[i];
reali += 2;
} else {
radics[i] += prev;
prev = radics[i];
}
prev = radics[i];
}
// Inplace swap
// Inplace swap - own ideas + some ideas based-on "famous" ct-swap (for branchless / more ILP):
// void ct-swap(bool secret, uint64_t a[], uint64_t b[], size_t len) {
// uint64_t mask = ~((uint64_t)secret - 1); // 1->111....111; 0->000....000
// for (size_t i = 0; i < len; i++) {
// uint64_t delta = (a[i] ^ b[i]) & mask;
// a[i] = a[i] ^ delta; // b[i], ha secret - amúgy a[i]
// b[i] = b[i] ^ delta; // fordítva
// }
// }
uint32_t pivoti = 0;
while(pivoti < n) {
uint32_t radixval = morgrab<j>(a[pivoti]);
uint32_t targeti = radics[radixval] - 1;
if(targeti > pivoti) {
// swap
uint32_t tmp = a[pivoti];
a[pivoti] = a[targeti];
a[targeti] = tmp;
// dec index
--radics[radixval];
} else {
// progress pivot
++pivoti;
}
uint32_t targeti = --radics[radixval]; // dec index (!)
// Bitmask: true -> 11.....1; false -> 00.....0
uint32_t mask = ~((targeti > pivoti) - 1);
// Branchless swap (using bitmask)
uint32_t delta = (a[pivoti] ^ a[targeti]) & mask;
a[pivoti] = a[pivoti] ^ delta;
a[targeti] = a[targeti] ^ delta;
// "else" branch
pivoti += !mask;
radics[radixval] += !mask; // undec index (!)
}
// Ends recursion