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thier3: realized that I can run with 256 bucketed float prepass (many us faster!) + experimented with a 1bit split trick (too much overhead to win, despite less cache misses)

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Richard Thier 2025-10-01 00:29:32 +02:00
parent 45820cf81c
commit 1f6ef0f2ea
1 changed files with 81 additions and 6 deletions
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87
thiersort3.h
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@ -1,9 +1,19 @@
#ifndef THIER_SORT3_H
#define THIER_SORT3_H
#include <stdint.h>
#include "threepass_xbit.h"
/* A non-implace tricky float-hackz based bucket sort variant. Uses threepass_xbit and removes some compies! */
/* Disables extra 1-bit split processing before threepass_xb */
#define NO_EXTRA_BIT /* I measure extra split to be slightly slower because overhead - despite less cache misses */
#ifndef NO_EXTRA_BIT
#define CUSTOM_TPBX_BITS
#define TPBX1 9 /* top - not 10 because our extra bit split / partitioning trickery! */
#define TPBX2 9 /* mid */
#define TPBX3 9 /* bottom */
#endif /* NO_EXTRA_BIT */
#include "threepass_xbit.h"
/* Float and unsigned32 reinterpreter */
union th3_fu {
float f;
@ -26,11 +36,12 @@ static inline uint32_t witch_bucket3(uint32_t key) {
as.f = (float) key;
uint32_t witch_base = (key <= 2) ? 0 : (as.u >> 23) - 128; // 0, [127..159] -> [0..31]
// return witch_base * 8 + ((as.u >> (23 - 3)) & 7); // 0..255
return witch_base * 128 + ((as.u >> (23 - 7)) & 127); // 0..2047
/* Alternative (but I measure it being worse):
return (as.u >> 23);
*/
return witch_base * 8 + ((as.u >> (23 - 3)) & 7); // 0..255
/* All below ones were worse, because three-ways 28 bit sorting would just make unnecessary work at cost of some locality */
//return witch_base * 128 + ((as.u >> (23 - 7)) & 127); // 0..4095
//return witch_base * 64 + ((as.u >> (23 - 6)) & 63); // 0..2047
//return witch_base * 32 + ((as.u >> (23 - 5)) & 31); // 0..1023
//return witch_base; // not enough!
}
/**
@ -45,6 +56,11 @@ static inline void thiersort3(uint32_t *arr, uint32_t *temparr, int n) {
int bucket[4096]; /* Inclusive */
int bucket_end[4096]; /* Not inclusive */
#ifndef NO_EXTRA_BIT
int bucket_leftend[4096]; /* for extra 1bit split processing */
int bucket_left[4096]; /* for extra 1bit split processing */
#endif /* NO_EXTRA_BIT */
/* Check if need to sort at all - needed for invariants later */
if(n < 2) {
return;
@ -74,7 +90,19 @@ static inline void thiersort3(uint32_t *arr, uint32_t *temparr, int n) {
bucket_end[i] = bucket[i];
}
#ifndef NO_EXTRA_BIT
/* Save left-offsets */
bucket_left[0] = 0;
bucket_leftend[0] = 0;
#pragma GCC unroll 64
for(int i = 0; i < 4095; ++i) {
bucket_left[1 + i] = bucket[i];
bucket_leftend[1 + i] = bucket[i];
}
#endif /* NO_EXTRA_BIT */
/* arr -> temparr */
#ifdef NO_EXTRA_BIT
/* Move to the buckets */
/* Rem.: This also changes bucket[i] so they will point to bucket beginnings */
#pragma GCC unroll 64
@ -104,6 +132,53 @@ static inline void thiersort3(uint32_t *arr, uint32_t *temparr, int n) {
uint32_t *buf = &(arr[begin]);
threepass_xb(a, buf, n);
}
#else
/* Move to the buckets by split-partitioning on the 28th bit */
/* Rem.: This also changes bucket[i] so they will point to bucket.right beginnings */
/* Rem.: This also changes bucket_leftend[i] so they will point to bucket.left endings (needed in-process only) */
#pragma GCC unroll 64
for(int i = 0; i < n; ++i) {
uint32_t num = arr[i];
uint32_t witch = witch_bucket3(num);
int offset = (num & (1 << 27)) ?
(--bucket[witch]) :
(bucket_leftend[witch]++);
temparr[offset] = num;
}
/* temparr -> arr each bucket and sort them in-place */
#pragma GCC unroll 64
for(int b = 0; b < 4096; ++b) {
assert(bucket_leftend[b] == bucket[b]);
int lbegin = bucket_left[b];
int lend = bucket[b]; /* non-inclusive */
int rbegin = bucket[b];
int rend = bucket_end[b]; /* non-inclusive */
/* Ensure exists and process left part */
if(lbegin < lend) {
/* Use our specialized threepass */
/* - which does not use up all the bits */
/* - which does not allocate and copy back to arr here */
uint32_t n = lend - lbegin;
uint32_t *a = &(temparr[lbegin]);
uint32_t *buf = &(arr[lbegin]);
threepass_xb(a, buf, n);
}
/* Ensure exists and process right part */
if(rbegin < rend) {
/* Use our specialized threepass */
/* - which does not use up all the bits */
/* - which does not allocate and copy back to arr here */
uint32_t n = rend - rbegin;
uint32_t *a = &(temparr[rbegin]);
uint32_t *buf = &(arr[rbegin]);
threepass_xb(a, buf, n);
}
}
#endif /* NO_EXTRA_BIT */
}
#endif /* THIER_SORT3_H */