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

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Date:	Thu, 17 Oct 2013 11:27:13 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Will Deacon <will.deacon@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Jiang Liu <jiang.liu@...wei.com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 6/7] arm64, jump label: optimize jump label
 implementation

On Thu, 17 Oct 2013 22:40:32 +0800
Jiang Liu <liuj97@...il.com> 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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