linux-kernel - Re: [PATCH 0/3] Add __ioread32

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Message-ID: <20150919201628.GI1747@two.firstfloor.org>
Date:	Sat, 19 Sep 2015 22:16:29 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Andy Gross <agross@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-soc@...r.kernel.org,
	linux-mips@...ux-mips.org, Hauke Mehrtens <hauke@...ke-m.de>,
	Paul Walmsley <paul@...an.com>,
	Rafał Miłecki <zajec5@...il.com>,
	Bjorn Andersson <bjorn.andersson@...ymobile.com>
Subject: Re: [PATCH 0/3] Add __ioread32_copy() and use it

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@...stfloor.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
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