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Alex Rivera
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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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