On: Does a compiler use all x86 instructions? (2010)

[rwmj]

And the counterpoint is that some instructions that are commonly believed to be slow (eg. the string instructions like rep movs, lods, stos), are in fact fast on modern processors.

[nneonneo → rwmj]

Now I'm curious - do you have performance numbers somewhere for this? The rep instructions can actually be shorter than a call to str*, so if rep is actually fast enough then it might make a nice optimization.

[rwmj → nneonneo]

It depends if the strings are aligned or not, the size of the copy, and also on the generation of processor. There are some good answers here:

http://stackoverflow.com/a/9177369

https://stackoverflow.com/questions/12359228/reliable-inform...

https://stackoverflow.com/questions/8425022/performance-of-x...

[veddan → nneonneo]

Reasonably modern versions of GCC will emit various rep instructions in some cases.

Some code I just compiled with GCC 6.1.1 had several snippets like this emitted for zeroing with memset:

    xor eax,eax
    ...
    rep stos QWORD PTR es:[rdi],rax

and some rep movs for memcpy/memmove.

[chkras → veddan]

really? I wonder if the timing of rep and string instructions have improved at all?

[userbinator → chkras]

They operate on cacheline-sized pieces ever since the P6.