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

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Message-Id: <20150918121919.fa20552946703ae772d636e9@linux-foundation.org>
Date:	Fri, 18 Sep 2015 12:19:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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 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.
  */
 #define __visible	__attribute__((externally_visible))
 #endif
_

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