Re: [PATCH v3 6/7] arm64, jump label: optimize jump label implementation

[Jiang Liu (Gerry)]

On 2013/10/17 23:27, Steven Rostedt wrote:
> On Thu, 17 Oct 2013 22:40:32 +0800
> Jiang Liu <liuj97@xxxxxxxxx> wrote:
>
>
> > > > > You could make the code more concise by limiting your patching ability to
> > > > > branch immediates. Then a nop is simply a branch to the next instruction (I
> > > > > doubt any modern CPUs will choke on this, whereas the architecture requires
> > > > > a NOP to take time).
> > > > I guess a NOP should be more effecient than a "B #4" on real CPUs:)
> > >
> > > Well, I was actually questioning that. A NOP *has* to take time (the
> > > architecture prevents implementations from discaring it) whereas a static,
> > > unconditional branch will likely be discarded early on by CPUs with even
> > > simple branch prediction logic.
> > I naively thought "NOP" is cheaper than a "B" :(
> > Will use a "B #1" to replace "NOP".
>
> Really?? What's the purpose of a NOP then? It seems to me that an
> architecture is broken if a NOP is slower than a static branch.
>
> -- Steve
Hi Steve and Will,
	I have discussed this with one of our chip design members.
He thinks "NOP" should be better than "B #1" because jump instruction
is one of the most complex instructions for microarchitecture, which
may stall the pipeline. And NOP should be friendly enough for all
microarchitectures. So I will keep the "NOP" version.
Thanks!
Gerry


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