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2.0_bsr.cpp
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2.0_bsr.cpp
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#include <cassert>
#include <cstdint>
#include <iostream>
#include <stdio.h>
#include <string.h>
#include <random>
#include <cstdlib> // for rand()
#include <time.h> // for clock()
#include <chrono>
#include <climits> // for DBL_MAX
#include <x86intrin.h> // for intrinsics (function that use powerful underlying assembly instruction)
#pragma intrinsic(bit_scan_reverse)
// g++ -std=c++17 -masm=intel highestSetBit.cpp && ./a.out
#include <immintrin.h>
//#define F(x) &uint8_t ()(uint32_t) ("highestSetBitIndex".append('x'))
#include <fcntl.h>
#include <stdio.h>
#include <elf.h>
#include <libelf.h>
#include <stdlib.h>
//#include <string.h>
using namespace std;
using namespace std::chrono;
/******************************************************************************/
/* Helpers */
/******************************************************************************/
//uint32_t highestSetBitOnly( uint32_t n ) __attribute__((optimize("-O3")));
uint8_t onPurposeSlowFunction(uint32_t n)
{
return sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) *sqrt(n * n) *sqrt(n * n) *sqrt(n * n) * sqrt(n * n) \
* sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) *sqrt(n * n) *sqrt(n * n) *sqrt(n * n) * sqrt(n * n)
* sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) * sqrt(n * n) *sqrt(n * n) *sqrt(n * n) *sqrt(n * n) * sqrt(n * n);
}
void int_to_bin(int num) {
char str[9] = {0};
int i;
for (i=7; i>=0; i--) {
str[i] = (num&1)?'1':'0';
num >>= 1;
}
printf("%s\n", str);
}
uint8_t exampleFunctionForInlineAssembly(uint32_t n) // undefined behavior if n == 0
{
uint8_t a = 0;
__asm__ __volatile__ (R"(
.intel_syntax noprefix
bsr eax, edi
.att_syntax noprefix
)"
// "mov %%edx, %1\n\t"
"mov %0, %%al\n\t"
:[a]"=r" (a)
// :[n]"r" (n)
);
return a;
}
uint8_t highestSetBitIndex100(uint32_t n) // undefined behavior if n == 0
{
uint8_t a = 0;
__asm__ __volatile__ (R"(
.intel_syntax noprefix
bsr eax, edi
.att_syntax noprefix
)"
// "mov %%edx, %1\n\t"
"mov %0, %%al\n\t"
:[a]"=r" (a)
// :[n]"r" (n)
);
return a;
}
/******************************************************************************/
/* Functions Test ******************************/
/******************************************************************************/
uint32_t lowestSetBitOnly(uint32_t n)
{
return n &= -n ;
}
void propagateBits(uint32_t *n) {
*n |= *n >> 1;
*n |= *n >> 2;
*n |= *n >> 4;
*n |= *n >> 8;
*n |= *n >> 16;
}
uint32_t highestSetBitOnly(uint32_t n)
{
propagateBits(&n);
return n - (n >> 1);
}
uint32_t vsNextPowerOfTwo(uint32_t value)
{
uint32_t n = highestSetBitOnly(value);
return n << (!!(value ^ n)); // it's over 9000!
}
uint8_t highestSetBitIndex1(uint32_t n)
{
uint8_t r = 0;
while (n >>= 1)
r++;
return r;
}
uint8_t highestSetBitIndex2(uint32_t n) {
return (uint8_t)(log(n) / log(2));
}
uint8_t highestSetBitIndex3(uint32_t b)
{
static const uint32_t deBruijnMagic = 0x06EB14F9;
static const uint8_t deBruijnTable[64] = {
0, 0, 0, 1, 0, 16, 2, 0, 29, 0, 17, 0, 0, 3, 0, 22,
30, 0, 0, 20, 18, 0, 11, 0, 13, 0, 0, 4, 0, 7, 0, 23,
31, 0, 15, 0, 28, 0, 0, 21, 0, 19, 0, 10, 12, 0, 6, 0,
0, 14, 27, 0, 0, 9, 0, 5, 0, 26, 8, 0, 25, 0, 24, 0,
};
return deBruijnTable[(highestSetBitOnly(b) * deBruijnMagic) >> 26];
}
uint8_t highestSetBitIndex4(uint32_t n) // undefined behavior if n == 0
{
__asm__ __volatile__ (R"(
.intel_syntax noprefix
bsr eax, edi
.att_syntax noprefix
)"
);
}
uint8_t highestSetBitIndex5(uint32_t n)
{
return _bit_scan_reverse(n); // undefined behavior if n == 0
}
uint8_t highestSetBitIndex6(uint32_t n)
{
return 31 - __builtin_clz(n); // undefined behavior if n == 0
}
uint8_t highestSetBitIndex7(uint32_t n) // undefined behavior if n == 0
{
__asm__ __volatile__ (R"(.intel_syntax noprefix
lzcnt ecx, edi
mov eax, 31
sub eax, ecx
.att_syntax noprefix
)");
}
uint8_t highestSetBitIndex8(uint32_t b)
{
static const uint32_t Magic = (uint32_t) 0x07C4ACDD;
static const int BitTable[32] = {
0, 9, 1, 10, 13, 21, 2, 29,
11, 14, 16, 18, 22, 25, 3, 30,
8, 12, 20, 28, 15, 17, 24, 7,
19, 27, 23, 6, 26, 5, 4, 31,
};
propagateBits(&b);
return BitTable[(b * Magic) >> 27];
}
/*
** credit: https://sites.google.com/site/sydfhd/articles-tutorials/de-bruijn-sequence-generator
*/
uint8_t highestSetBitIndex9(uint32_t b)
{
static const uint8_t u = 0;
static const uint8_t BitTable[64] = {
2, u, u, u, 9, u, 29, 5,
u, u, 3, u, u, u, 21, u,
u, 18, 10, u, 23, 13, 30, u,
6, u, 27, 1, u, u, u, 4,
u, u, 20, u, 17, 22, 12, u,
26, u, u, u, 19, 16, 11, 25,
u, u, 15, 24, 14, u, 31, u,
u, u, 7, u, u, 8, 28, u,
};
propagateBits(&b);
b += (b << 3); // Multiply by 9
b += (b << 4); // Multiply by 17
/* b = (b << 5) - b; // Multiply by 31
b = (b << 17) - b;// Multiply by 131071*/
b -= (b << 5); // Multiply by 31
b -= (b << 17);// Multiply by 131071
return BitTable[b >> 26];
}
/******************************************************************************/
/* Benchmark Main */
/******************************************************************************/
#define N 10000
#define BATTLE_NB 10000
double benchmark(uint8_t (*f)(uint32_t)) {
auto start = high_resolution_clock::now();
uint8_t a = 0;
for (int i{}; i != N; ++i)
a |= f(rand());
auto stop = high_resolution_clock::now();
double elapsedTime = duration_cast<nanoseconds>(stop - start).count();
// std::cerr << "Duration: " << duration << " ms\n";
return elapsedTime;
}
double overHead(uint8_t (*f)(uint32_t)) {
auto start = high_resolution_clock::now();
uint8_t a = 0;
for (int i{}; i != N; ++i)
a |= rand();
auto stop = high_resolution_clock::now();
double elapsedTime = duration_cast<nanoseconds>(stop - start).count();
// std::cerr << "Duration: " << duration << " ms\n";
return elapsedTime;
}
int main(void) {
srand (time (NULL));
uint64_t simulationsCount = 0;
uint8_t (*func_ptr[])(uint32_t) = {&onPurposeSlowFunction, \
&highestSetBitIndex1, &highestSetBitIndex2, &highestSetBitIndex3, \
&highestSetBitIndex4, &highestSetBitIndex5, &highestSetBitIndex6, \
&highestSetBitIndex7, &highestSetBitIndex8, &highestSetBitIndex9};
int len = sizeof(func_ptr) / sizeof(void*);
#ifdef __DEBUG__
for (uint32_t a = UINT_MAX; a > UINT_MAX - 100; a--) {
//cout << endl << "Testing with: " << a << endl;
uint32_t res = (uint32_t)func_ptr[1](a);
for (int i = 2; i < len; i++) {
uint32_t cur =(uint32_t)func_ptr[i](a);
if (res != cur)
printf("error with function %u returning %u instead of %u\n", i, cur, res);
//cout << (uint32_t)(func_ptr[i](a)) << endl;
//cout << (uint32_t)(highestSetBitIndex1(a)) << endl;
}
}
return 0;
#endif
int battle = BATTLE_NB;
simulationsCount = (N * battle) / 1000000.0f; // million of simulations
double elapsedTime[len] = {};
int score[len] = {};
while (battle--) {
int currentScore = 0;
int lowestLatencyIndex = 0;
double lowestLatency = numeric_limits<double>::max();
double currentElapsedTime = 0;
for (int i = 1; i < len; i++) {
currentElapsedTime = benchmark(func_ptr[i]);
currentElapsedTime -= overHead(func_ptr[i]);
if (currentElapsedTime < lowestLatency) {
lowestLatency = currentElapsedTime;
lowestLatencyIndex = i;
}
elapsedTime[i] += currentElapsedTime;
}
//printf("winner %d \n", lowestLatencyIndex);
score[lowestLatencyIndex]++;
}
int winner = 0;
int bestScore = 0;
for (int i = 0; i < len; i++) {
elapsedTime[i] /= N * 1000;
if (score[i] > bestScore)
{
bestScore = score[i];
winner = i;
}
}
char color[13]; // https://misc.flogisoft.com/bash/tip_colors_and_formatting
strncpy(color, "\e[1m\e[96m", 13); //bold and cyan
printf("\n%s%lu million simulations result:\n\e[0m", color, simulationsCount);
for (int i = 1; i < len; ++i) {
color[0] = '\0';
if (i == winner) // \033[5m
strncpy(color, "\e[1m\e[5m\e[92m", 13); // bold, blink and green
printf("\tFunction %2d scored %6d with an average time of %s%03.1lf\e[0m ms\n", i, score[i], color, elapsedTime[i]);
}
putchar('\n');
return 0;
}