【发布时间】：2014-11-09 19:28:27
【问题描述】：
我的程序如下所示：
void Process1(unsigned char* data)
{
}
void Process2(unsigned char* data)
{
}
void Process3(unsigned char* data)
{
}
#define FLAG1 (1 << 1)
#define FLAG2 (1 << 2)
#define FLAG3 (1 << 3)
void ProcessData(unsigned char* data, unsigned int bytes, unsigned int flags)
{
bool b1 = !!(flags & FLAG1);
bool b2 = !!(flags & FLAG2);
bool b3 = !!(flags & FLAG3);
for (unsigned int i = 0; i < bytes; i ++)
{
if (b1) Process1(data + i);
if (b2) Process2(data + i);
if (b3) Process3(data + i);
}
}
看起来，flags & FLAG1 A.K.A b1 在所有循环中都不会改变。但是我们仍然必须在每个循环中进行分支。我只是想知道是否有办法动态避免这种不必要的分支。
这是 Lundin 解决方案的演示。
#include <windows.h>
#include <stdio.h>
#include <time.h>
LARGE_INTEGER ls, le, ll;
#define START_CLOCK() QueryPerformanceCounter(&ls)
#define END_CLOCK() printf ("%.0lf ns\n", (QueryPerformanceCounter(&le), ((double)le.QuadPart - ls.QuadPart) / ll.QuadPart * 1000000));
void Process1(unsigned char* data)
{
(*data)++;
}
void Process2(unsigned char* data)
{
(*data)--;
}
void Process3(unsigned char* data)
{
(*data) *= (*data);
}
#define FLAG1 (1 << 1)
#define FLAG2 (1 << 2)
#define FLAG3 (1 << 3)
void ProcessData(unsigned char* data, unsigned int bytes, unsigned int flags)
{
bool b1 = !!(flags & FLAG1);
bool b2 = !!(flags & FLAG2);
bool b3 = !!(flags & FLAG3);
for (unsigned int i = 0; i < bytes; i ++)
{
if (b1) Process1(data + i);
if (b2) Process2(data + i);
if (b3) Process3(data + i);
}
}
typedef void (*proc_t)(unsigned char*);
inline static void do_nothing (unsigned char* ptr)
{
(void)ptr;
}
void ProcessData_x(unsigned char* data, unsigned int bytes, unsigned int flags)
{
bool b1 = (flags & FLAG1) != 0; // de-obfuscate the boolean logic
bool b2 = (flags & FLAG2) != 0;
bool b3 = (flags & FLAG3) != 0;
proc_t p1 = b1 ? Process1 : do_nothing;
proc_t p2 = b2 ? Process2 : do_nothing;
proc_t p3 = b3 ? Process3 : do_nothing;
for (unsigned int i = 0; i<bytes; i++)
{
p1(data + i);
p2(data + i);
p3(data + i);
}
}
int main()
{
if (!QueryPerformanceFrequency(&ll)) return 1;
const unsigned int bytes = 0xffff;
srand((unsigned int)time(NULL));
unsigned int flags = rand() & 0x7;
unsigned char* data = new unsigned char[bytes];
for (unsigned int i = 0; i < bytes; i++)
{
data[i] = (unsigned char)(rand() & 0xff);
}
START_CLOCK();
ProcessData(data, bytes, flags);
END_CLOCK();
START_CLOCK();
ProcessData_x(data, bytes, flags);
END_CLOCK();
}
这是输出：
134 ns
272 ns
我已经运行了好几次，但出乎意料的是，它花费了更多的时间:(..它也编译为'vs2010 Release x86'
【问题讨论】：
-
过程像加法一样简单吗？
标签： c++ c loops optimization branch