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Asked: 2026-06-11T03:45:19+00:00

Common widsom is that rep movsb is much slower than rep movsd (or on

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Common widsom is that rep movsb is much slower than rep movsd (or on 64-bit, rep movsq) when performing identical operations. However, I’ve been testing on a few modern machines, and the run times are coming out identical (up to measurement noise) across a huge range of buffer sizes (10 bytes to 2 megs). So far I have just tested on 2 machines (32-bit Intel Atom D510 and 64-bit AMD FX 8120).

  • Are there any modern x86 (32- or 64-bit) machines where rep movsb is slower than rep movsd (or rep movsq)?

  • If not, what was the last machine where the difference was significant, and how significant was it?

I’m asking this question from a standpoint of wanting to avoid cargo-culting a bunch of tests to break memory up into unaligned head/tail and aligned middle for the sake of using rep movsd or rep movsq if there’s no actual benefit to doing this…

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  1. Editorial Team

    Lots of benchmarks here: instlatx64.atw.hu

    For example (Intel Core 2 Duo E6700):

    REP MOVSB   BW in L1D:13.04 B/c  34829MiB/s
REP MOVSW   BW in L1D:13.29 B/c  35493MiB/s
REP MOVSD   BW in L1D:13.40 B/c  35783MiB/s

    Which shows that there is a difference, but it’s tiny.

    This one for SandyBridge is a little weird:

    REP MOVSB   BW in L1D:25.50 B/c  86986MiB/s
REP MOVSW   BW in L1D:18.09 B/c  61721MiB/s
REP MOVSD   BW in L1D:27.47 B/c  93693MiB/s

    Seems there is a big difference on some Atoms (seems to have disappeared with the D5xx, so you just missed it):

    REP MOVSB   BW in L1D: 0.53 B/c    990MiB/s
REP MOVSW   BW in L1D: 1.93 B/c   3598MiB/s
REP MOVSD   BW in L1D: 3.74 B/c   6960MiB/s

    I haven’t found such big difference on anything else that can be considered new.

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