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

[Steven Rostedt]

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
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