bool judgement is so slow? [closed] - c++

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I'm optimizing the function, I try every way and even sse, and modified code to return from different position to see the calculate timespan but finally I found most of the time spends on the bool judgement. Even I replace all code in the if statement with a simple add operation in it, it still cost 6000ms.
My platform is gcc 4.7.1 e5506 cpu. Its input 'a' and 'b' is a 1000size int array, and 'asize', 'bsize' are corresponding array size. MATCH_MASK = 16383, I run the function 100000 times to statistics a timespan. Is there any good idea to the problem. Thank you!
if (aoffsets[i] && boffsets[i]) // this line costs most time
Code:
uint16_t aoffsets[DOUBLE_MATCH_MASK] = {0}; // important! or it will only be right on the first time
uint16_t* boffsets = aoffsets + MATCH_MASK;
uint8_t* seen = (uint8_t *)aoffsets;
auto fn_init_offsets = [](const int32_t* x, int n_size, uint16_t offsets[])->void
{
for (int i = 0; i < n_size; ++i)
offsets[MATCH_STRIP(x[i])] = i;
};
fn_init_offsets(a, asize, aoffsets);
fn_init_offsets(b, bsize, boffsets);
uint8_t topcount = 0;
int topoffset = 0;
{
std::vector<uint8_t> count_vec(asize + bsize + 1, 0); // it's the fastest way already, very near to tls
uint8_t* counts = &(count_vec[0]);
//return aoffsets[0]; // cost 1375 ms
for (int i = 0; i < MATCH_MASK; ++i)
{
if (aoffsets[i] && boffsets[i]) // this line costs most time
{
//++affsets[i]; // for test
int offset = (aoffsets[i] -= boffsets[i]);
if ((-n_maxoffset <= offset && offset <= n_maxoffset))
{
offset += bsize;
uint8_t n_cur_count = ++counts[offset];
if (n_cur_count > topcount)
{
topcount = n_cur_count;
topoffset = offset;
}
}
}
}
}
return aoffsets[0]; // cost 6000ms

Answer 1

First, memset(count_vec,0, N); of a memaligned buffer wins over std::vector by 30%.
You can try to use the branchless expression (aoffsets[i] * boffsets[i]) and calculate some of not-to-be-used expressions simultaneously: offset = aoffset[i]-boffset[i]; offset+bsize; offset+n_maxoffset;.
Depending on the typical range of offset, one could be tempted to calculate the min/max of (offset+bsize) to restrict the needed memset(count_vec) at the next iteration: no need to clear already zero values.
As pointed by philipp, it's good to interleave the operations -- then again, one can read both aoffset[i] and boffset[i] simultaneously from uint32_t aboffset[N]; with some clever bit masking (that generates change mask for: aoffset[i], aoffset[i+1]) one could possibly handle 2 sets in parallel using 64-bit simulated SIMD in pure c (up to the histogram accumulation part).

Answer 2

You can increase the speed of your program by reducing the cache misses: aoffsets[i] and boffsets[i] are relatively far away from each other in memory. By placing them next to each other, you speed up the program significantly. On my machine (e5400 cpu, VS2012) the execution time is reduced from 3.0 seconds to 2.3 seconds:
#include <vector>
#include <windows.h>
#include <iostream>
typedef unsigned short uint16_t;
typedef int int32_t;
typedef unsigned int uint32_t;
typedef unsigned char uint8_t;
#define MATCH_MASK 16383
#define DOUBLE_MATCH_MASK (MATCH_MASK*2)
static const int MATCH_BITS = 14;
static const int MATCH_LEFT = (32 - MATCH_BITS);
#define MATCH_STRIP(x) ((uint32_t)(x) >> MATCH_LEFT)
static const int n_maxoffset = 1000;
uint16_t test(int32_t* a, int asize, int32_t* b, int bsize)
{
uint16_t offsets[DOUBLE_MATCH_MASK] = {0};
auto fn_init_offsets = [](const int32_t* x, int n_size, uint16_t offsets[])->void
{
for (int i = 0; i < n_size; ++i)
offsets[MATCH_STRIP(x[i])*2 /*important. leave space for other offsets*/] = i;
};
fn_init_offsets(a, asize, offsets);
fn_init_offsets(b, bsize, offsets+1);
uint8_t topcount = 0;
int topoffset = 0;
{
std::vector<uint8_t> count_vec(asize + bsize + 1, 0);
uint8_t* counts = &(count_vec[0]);
for (int i = 0; i < MATCH_MASK; i+=2)
{
if (offsets[i] && offsets[i+1])
{
int offset = (offsets[i] - offsets[i+1]); //NOTE: I removed
if ((-n_maxoffset <= offset && offset <= n_maxoffset))
{
offset += bsize;
uint8_t n_cur_count = ++counts[offset];
if (n_cur_count > topcount)
{
topcount = n_cur_count;
topoffset = offset;
}
}
}
}
}
return offsets[0];
}
int main(int argc, char* argv[])
{
const int sizes = 1000;
int32_t* a = new int32_t[sizes];
int32_t* b = new int32_t[sizes];
for (int i=0;i<sizes;i++)
{
a[i] = rand()*rand();
b[i] = rand()*rand();
}
//Variablen
LONGLONG g_Frequency, g_CurentCount, g_LastCount;
QueryPerformanceFrequency((LARGE_INTEGER*)&g_Frequency);
QueryPerformanceCounter((LARGE_INTEGER*)&g_CurentCount);
int sum = 0;
for (int i=0;i<100000;i++)
{
sum += test(a,sizes,b,sizes);
}
QueryPerformanceCounter((LARGE_INTEGER*)&g_LastCount);
double dTimeDiff = (((double)(g_LastCount-g_CurentCount))/((double)g_Frequency));
std::cout << "Result: " << sum << std::endl <<"time: " << dTimeDiff << std::endl;
delete[] a;
delete[] b;
return 0;
}
compared to your version of test().
#include <vector>
#include <windows.h>
#include <iostream>
typedef unsigned short uint16_t;
typedef int int32_t;
typedef unsigned int uint32_t;
typedef unsigned char uint8_t;
#define MATCH_MASK 16383
#define DOUBLE_MATCH_MASK (MATCH_MASK*2)
static const int MATCH_BITS = 14;
static const int MATCH_LEFT = (32 - MATCH_BITS);
#define MATCH_STRIP(x) ((uint32_t)(x) >> MATCH_LEFT)
static const int n_maxoffset = 1000;
uint16_t test(int32_t* a, int asize, int32_t* b, int bsize)
{
uint16_t aoffsets[DOUBLE_MATCH_MASK] = {0}; // important! or it will only be right on the first time
uint16_t* boffsets = aoffsets + MATCH_MASK;
auto fn_init_offsets = [](const int32_t* x, int n_size, uint16_t offsets[])->void
{
for (int i = 0; i < n_size; ++i)
offsets[MATCH_STRIP(x[i])] = i;
};
fn_init_offsets(a, asize, aoffsets);
fn_init_offsets(b, bsize, boffsets);
uint8_t topcount = 0;
int topoffset = 0;
{
std::vector<uint8_t> count_vec(asize + bsize + 1, 0);
uint8_t* counts = &(count_vec[0]);
for (int i = 0; i < MATCH_MASK; ++i)
{
if (aoffsets[i] && boffsets[i])
{
int offset = (aoffsets[i] - boffsets[i]); //NOTE: I removed the -= because otherwise offset would always be positive!
if ((-n_maxoffset <= offset && offset <= n_maxoffset))
{
offset += bsize;
uint8_t n_cur_count = ++counts[offset];
if (n_cur_count > topcount)
{
topcount = n_cur_count;
topoffset = offset;
}
}
}
}
}
return aoffsets[0];
}
int main(int argc, char* argv[])
{
const int sizes = 1000;
int32_t* a = new int32_t[sizes];
int32_t* b = new int32_t[sizes];
for (int i=0;i<sizes;i++)
{
a[i] = rand()*rand();
b[i] = rand()*rand();
}
LONGLONG g_Frequency, g_CurentCount, g_LastCount;
QueryPerformanceFrequency((LARGE_INTEGER*)&g_Frequency);
QueryPerformanceCounter((LARGE_INTEGER*)&g_CurentCount);
int sum = 0;
for (int i=0;i<100000;i++)
{
sum += test(a,sizes,b,sizes);
}
QueryPerformanceCounter((LARGE_INTEGER*)&g_LastCount);
double dTimeDiff = (((double)(g_LastCount-g_CurentCount))/((double)g_Frequency));
std::cout << "Result: " << sum << std::endl <<"time: " << dTimeDiff << std::endl;
delete[] a;
delete[] b;
return 0;
}