From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH 0/3] Add __ioread32_copy() and use it
Date: Sat, 19 Sep 2015 22:16:29 +0200
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>, Stephen Boyd <sboyd@codeaurora.org>, Andy Gross <agross@codeaurora.org>, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-soc@vger.kernel.org, linux-mips@linux-mips.org, Hauke Mehrtens <hauke@hauke-m.de>, Paul Walmsley <paul@pwsan.com>, =?utf-8?B?UmFmYcWCIE1pxYJlY2tp?= <zajec5@gmail.com>, Bjorn Andersson <bjorn.andersson@sonymobile.com>
List-Id: linux-arm-msm@vger.kernel.org

On Fri, Sep 18, 2015 at 12:19:19PM -0700, Andrew Morton wrote:
> On Wed, 16 Sep 2015 04:55:46 +0200 Andi Kleen <andi@firstfloor.org> wrote:
> 
> > > Under what circumstances will the compiler (or linker?) do this? 
> > 
> > Compiler.
> > 
> > > LTO enabled?
> > 
> > Yes it's for LTO.  The optimization allows the compiler to drop unused
> > functions, which is very popular with users (a lot use it to get smaller
> > kernel images)
> > 
> 
> Does this look truthful and complete?
> 
> 
> --- a/include/linux/compiler-gcc.h~a
> +++ a/include/linux/compiler-gcc.h
> @@ -205,7 +205,10 @@
>  
>  #if GCC_VERSION >= 40600
>  /*
> - * Tell the optimizer that something else uses this function or variable.
> + * When used with Link Time Optimization, gcc can optimize away C functions or
> + * variables which are referenced only from assembly code.  __visible tells the
> + * optimizer that something else uses this function or variable, thus preventing
> + * this.

Yes,

In a few cases I also used it to work around LTO bugs in older gcc
releases. I don't think any of those fixes made it into mainline though,
and they are not needed anymore with 5.x

-Andi