linux-kernel - Re: [PATCH] arm64/io: Don't use WZR in writel

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Message-ID: <0c06cb51-fc33-f2ba-e2b8-ad280b89f35c@arm.com>
Date:   Mon, 18 Mar 2019 17:59:17 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....com>,
        Jens Axboe <axboe@...nel.dk>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Catalin Marinas <catalin.marinas@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        AngeloGioacchino Del Regno <kholk11@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64/io: Don't use WZR in writel

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.