Curious about loop optimization C++ - assembly

Hi,

Out of curiousity, I sometimes look at the produced assembly after compilation in release mode.

What you often see, is that CPP, always fully addresses registers to copy values from a to b...

While stosb,stosw, stosd etc and the same for movs[x] are one statement, and internally use registers ESI and EDI (source, destination) to copy data.

This seems (imho) more efficient, however, CPP never uses this construct...
it always uses a lot more instructions.

imagine this loop (I simplified the idea, of course, memcpy would be normally used)


DWORD anArray [10000];

// copy array while skipping uneven element positions

for (int mycounter=5000; mycounter != 0; mycounter--, element+=2)
anArray[element] = somesource[element];


could be optimized to

setup source and destination

MOV EDI, [anArray]
MOV ESI, [somesource]
MOV ECX, myCounter
DEC ECX
CLD // forward copy

mylabel:
MOVSD <--- actual loop and copy instruction
LOOP mylabel <-- decrement ECX until ECX == 0


Q: is the mentioned construct, simply not so efficient or is there a reason the C++ compiler team decided not to try to optimize to this level?



.

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