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Remove division from matrix multiplication

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This change reduces llama.com CPU cycles systemically by 2.5% according
to the Linux Kernel `perf stat -Bddd` utility.
This commit is contained in:
Justine Tunney 2023-05-10 21:19:54 -07:00
parent a88290e595
commit 1f6f9e6701
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GPG key ID: BE714B4575D6E328
7 changed files with 191 additions and 70 deletions
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62
examples/fastdiv.c
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@ -1,62 +0,0 @@
#if 0
/*─────────────────────────────────────────────────────────────────╗
│ To the extent possible under law, Justine Tunney has waived │
│ all copyright and related or neighboring rights to this file, │
│ as it is written in the following disclaimers: │
│ • http://unlicense.org/ │
│ • http://creativecommons.org/publicdomain/zero/1.0/ │
╚─────────────────────────────────────────────────────────────────*/
#endif
#include "libc/calls/calls.h"
#include "libc/macros.internal.h"
#include "libc/stdio/stdio.h"
#include "libc/sysv/consts/sig.h"
#include "libc/testlib/ezbench.h"
/**
* @fileoverview Fast Division Using Multiplication Tutorial
*
* Expected program output:
*
* 23 / 3 = 7
* 0x5555555555555556 1 1
* division l: 16𝑐 5𝑛𝑠
* fast div l: 5𝑐 2𝑛𝑠
* precomps l: 70𝑐 23𝑛𝑠
*/
struct Divisor {
uint64_t m;
uint8_t s;
uint8_t t;
};
struct Divisor GetDivisor(uint64_t d) {
int b;
uint128_t x;
if (!d) raise(SIGFPE);
b = __builtin_clzll(d) ^ 63;
x = -d & (((1ull << b) - 1) | (1ull << b));
return (struct Divisor){(x << 64) / d + 1, MIN(1, b + 1), MAX(0, b)};
}
uint64_t Divide(uint64_t x, struct Divisor d) {
uint128_t t;
uint64_t l, h;
t = d.m;
t *= x;
l = t;
h = t >> 64;
l = (x - h) >> d.s;
return (h + l) >> d.t;
}
int main(int argc, char *argv[]) {
printf("23 / 3 = %ld\n", Divide(23, GetDivisor(3)));
volatile struct Divisor v = GetDivisor(3);
volatile uint64_t x = 23, y = 3, z;
EZBENCH2("division", donothing, z = x / y);
EZBENCH2("fast div", donothing, z = Divide(x, v));
EZBENCH2("precomp ", donothing, v = GetDivisor(y));
return 0;
}