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Subject: Re: [PATCH] arm64/io: Don't use WZR in writel
To:     Marc Gonzalez <marc.w.gonzalez@free.fr>,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>
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        Bjorn Andersson <bjorn.andersson@linaro.org>,
        MSM <linux-arm-msm@vger.kernel.org>,
        AngeloGioacchino Del Regno <kholk11@gmail.com>,
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From:   Robin Murphy <robin.murphy@arm.com>
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Date:   Mon, 18 Mar 2019 17:59:17 +0000
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On 18/03/2019 17:30, Marc Gonzalez wrote:
> On 18/03/2019 18:19, Robin Murphy wrote:
> 
>> For the context bank reset, yes, I am assuming that no complier will
>> ever be perverse enough to detect that cfg is not written after the
>> NULL check and immediately reallocate it to XZR for no good reason.
>> I'd like to think that assumption is going to hold for the reasonable
>> scope of this particular workaround, though.
> 
> I'm not sure I understand the above paragraph.
> 
> In code such as:
> 
> 	if (val == 0) foo(val);
> 
> gcc's algorithm is likely to figure out that the code is equivalent to
> 
> 	if (val == 0) foo(0)
> 
> and perform constant-propagation, etc.
> 
> Is that what we're talking about?

As may already be apparent, the way I expect compilers to behave, and 
the way compilers actually behave, are two overlapping but distinct sets 
of concepts...

Robin.