小编use*_*735的帖子

I just noticed a pieces of my code exhibit different performance when copying memory. A test showed that a memory copying performance degraded if the address of destination buffer is greater than address of source. Sounds ridiculous, but the following code shows the difference (Delphi):

  const MEM_CHUNK = 50 * 1024 * 1024;
        ROUNDS_COUNT = 100;


  LpSrc := VirtualAlloc(0,MEM_CHUNK,MEM_COMMIT,PAGE_READWRITE);
  LpDest := VirtualAlloc(0,MEM_CHUNK,MEM_COMMIT,PAGE_READWRITE);

  QueryPerformanceCounter(LTick1);
  for i := 0 to ROUNDS_COUNT - 1 do
    CopyMemory(LpDest,LpSrc,MEM_CHUNK);
  QueryPerformanceCounter(LTick2);
    // show timings

  QueryPerformanceCounter(LTick1);
  for i …