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

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Message-ID: <fbacd3df-41c4-f98a-8875-db654aff3a5c@free.fr>
Date:   Mon, 18 Mar 2019 18:30:26 +0100
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     Robin Murphy <robin.murphy@....com>,
        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 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?

Regards.