Author: Koundinya Veluri
Date: 03:54:38 01/15/02
Go up one level in this thread
On my computer, bsf is faster if the 1-bit is less significant and bsr is faster
if the 1-bit is more significant, so the FirstBit() and LastBit() versions with
the jumps are running faster on average. How does this test on other processors?
I'm curious...
#include <iostream>
using namespace std;
#include <windows.h>
#define ITERATIONS (1000000000)
#define FB (0x8000000000000000)
#define LB (1)
inline const int FirstBit1(const unsigned __int64 bb);
inline const int FirstBit2(const unsigned __int64 bb);
inline const int LastBit1(const unsigned __int64 bb);
inline const int LastBit2(const unsigned __int64 bb);
inline const int LastBit3(const unsigned __int64 bb);
int main()
{
unsigned int a, b, t;
cout << "Press enter to start...";
cin.ignore();
cout << endl;
t = GetTickCount();
for(a = 0; a < ITERATIONS; ++a)
b = FirstBit1(FB);
t = GetTickCount() - t;
cout << t << endl;
cout.flush();
t = GetTickCount();
for(a = 0; a < ITERATIONS; ++a)
b = FirstBit2(FB);
t = GetTickCount() - t;
cout << t << endl;
cout.flush();
cout << endl;
cout.flush();
t = GetTickCount();
for(a = 0; a < ITERATIONS; ++a)
b = LastBit1(LB);
t = GetTickCount() - t;
cout << t << endl;
cout.flush();
t = GetTickCount();
for(a = 0; a < ITERATIONS; ++a)
b = LastBit2(LB);
t = GetTickCount() - t;
cout << t << endl;
cout.flush();
t = GetTickCount();
for(a = 0; a < ITERATIONS; ++a)
b = LastBit3(LB);
t = GetTickCount() - t;
cout << t << endl;
cout.flush();
return 0;
}
const int FirstBit1(const unsigned __int64 bb)
{
__asm
{
bsf eax, [bb]
jnz L10
bsf eax, [bb + 4]
add eax, 32
L10:
}
}
const int FirstBit2(const unsigned __int64 bb)
{
__asm
{
bsf eax, [bb + 4]
add eax, 32
bsf eax, [bb]
}
}
const int LastBit1(const unsigned __int64 bb)
{
__asm
{
bsr eax, [bb + 4]
jz L10
add eax, 32
jmp L11
L10:
bsr eax, [bb]
L11:
}
}
const int LastBit2(const unsigned __int64 bb)
{
__asm
{
bsr eax, [bb + 4]
jnz L10
bsr eax, [bb]
sub eax, 32
L10:
add eax, 32
}
}
const int LastBit3(const unsigned __int64 bb)
{
__asm
{
bsr eax, [bb]
sub eax, 32
bsr eax, [bb + 4]
add eax, 32
}
}
Output on my computer:
Press enter to start...
5898
5899
4747
6509
7080
--------
Regards,
Koundinya
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Last modified: Thu, 15 Apr 21 08:11:13 -0700
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