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Date:   Tue, 15 Sep 2015 19:50:31 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Andi Kleen <andi@firstfloor.org>
Cc:     Stephen Boyd <sboyd@codeaurora.org>,
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        Bjorn Andersson <bjorn.andersson@sonymobile.com>
Subject: Re: [PATCH 0/3] Add __ioread32_copy() and use it
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On Wed, 16 Sep 2015 04:32:19 +0200 Andi Kleen <andi@firstfloor.org> wrote:

> > __iowrite32_copy() is marked __visible.  I don't actually know what
> > that does and Andi's d47d5c8194579bc changelog (which sucks the big
> > one) didn't explain it.  Apparently it has something to do with being
> > implemented in assembly, but zillions of functions are implemented in
> > assembly, so why are only two functions marked this way?  Anyway,
> > __ioread32_copy() is implemented in C so I guess __visible isn't needed
> > there.
> 
> __visible is needed for C functions that are called from assembler.
> Otherwise the compiler may optimize them away.

Under what circumstances will the compiler (or linker?) do this? 
LTO enabled?