Date: Thu, 17 Oct 2013 11:27:13 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Jiang Liu <liuj97@gmail.com>
Cc: Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <Catalin.Marinas@arm.com>,
        Sandeepa Prabhu <sandeepa.prabhu@linaro.org>,
        Jiang Liu <jiang.liu@huawei.com>, Marc Zyngier <Marc.Zyngier@arm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 6/7] arm64, jump label: optimize jump label
 implementation
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On Thu, 17 Oct 2013 22:40:32 +0800
Jiang Liu <liuj97@gmail.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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