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copied!<p>I tested this some time in the past on Win32 machines and did not notice much of a penalty on 32-bit machines. On 64-bit, though, it was significantly slower. For example, I ran the following bit of code. On a 32-bit machine, the times printed were hardly changed. But on a 64-bit machine, the times for the misaligned accesses were nearly twice as long. The times follow the code.</p> <pre><code>#define UINT unsigned __int64 #define ENDPART QuadPart #else #define UINT unsigned int #define ENDPART LowPart #endif int main(int argc, char *argv[]) { LARGE_INTEGER startCount, endCount, freq; int i; int offset; int iters = atoi(argv[1]); char *p = (char*)malloc(16); double *d; for ( offset = 0; offset < 9; offset++ ) { d = (double*)( p + offset ); printf( "Address alignment = %u\n", (unsigned int)d % 8 ); *d = 0; QueryPerformanceFrequency(&freq); QueryPerformanceCounter(&startCount); for(i = 0; i < iters; ++i) *d = *d + 1.234; QueryPerformanceCounter(&endCount); printf( "Time: %lf\n", (double)(endCount.ENDPART-startCount.ENDPART)/freq.ENDPART ); } } </code></pre> <p>Here are the results on a 64-bit machine. I compiled the code as a 32-bit application.</p> <pre><code>[P:\t]pointeralignment.exe 100000000 Address alignment = 0 Time: 0.484156 Address alignment = 1 Time: 0.861444 Address alignment = 2 Time: 0.859656 Address alignment = 3 Time: 0.861639 Address alignment = 4 Time: 0.860234 Address alignment = 5 Time: 0.861539 Address alignment = 6 Time: 0.860555 Address alignment = 7 Time: 0.859800 Address alignment = 0 Time: 0.484898 </code></pre>

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