Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

Ingo Molnar
2015-02-26T11:47:10Z

* Steven Rostedt <rostedt@goodmis.org> wrote:

> > That would require a branch insn. The whole idea of
> > masking was merely to avoid that branch. If you need a
> > branch, then you can as well just retain current code.
>
> I'm just curious, do all these micro optimizations have
> any real impact on real use cases?

The bona fide removal of a real instruction from a true hot
path is generally always worth doing, you don't even have
to 'prove' that it improves things: unless the claim is
that for some really robust reason the instruction was zero
cost to begin with.

So the main question here is not whether it's worth doing
it, the question is the cost of the removal:

- the change in syscall number overflow handling
behavior. (We might not want the new behavior)

- the increase in the syscall table size.

Thanks,

Ingo