CODEWITHSUNDEEP
c++cperformance
ZijingWu
ZijingWu

Reputation: 3490

In what platform memmove and memcpy can have significant performance difference?

I understand that memmove and memcpy difference is that memmove handles the memory overlap case. I have checked the implementation in libgcc and got this article [memcpy performance] from the intel website.

In libgcc, the memmove is similar to memcpy, both just go though one byte and byte, so the performance should be almost same even after optimization.

Someone has measured this and got this article memcopy, memmove, and Speed over Safety. Even I don't think the memmove can be faster than memcpy, but there should be no big difference at least on Intel platform.

So in what platform and how, memcpy can be significantly faster than memmove, if there is none, why providing two similiar functions instead of just memmove, and lead to a lots of bug.

Edit: I'm not asking the difference of memmove and memcpy, I know memmove can handle overlap issue. The question is about is there really any platform where memcpy is faster than memmove?

Upvotes: 6

Views: 2014

Answers (2)

Ehsan
Ehsan

Reputation: 1418

Good practice: In general, USE memmove only if you have to. USE it when there is a very reasonable chance that the source and destination regions are over-lapping.

Otherwise USE memcpy. memcpy is more efficient.

Reference: https://www.youtube.com/watch?v=Yr1YnOVG-4g Dr. Jerry Cain, (Stanford Intro Systems Lecture - 7) Time: 36:00

Upvotes: -3

Suma
Suma

Reputation: 34403

There is at least one recent case where the constraint of non-overlapping memory is used to generate faster code:

In Visual Studio memcpy can be compiled using intrinsics, while memmove cannot. This leads in memcpy being much faster for small regions of a known size because of removing the function call and setup overhead. The implementation using movsd/movsw/movsb is not suitable for overlapping blocks, as it starts copying at the lowest address, incrementing the edi/esi during the copy.

See also Make compiler copy characters using movsd.

The GCC also lists memcpy as implemented as built-ins, the implementation and motivation is likely to be similar to that of Visual Studio.

Upvotes: 3

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