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Date: Fri, 18 Sep 2015 12:19:19 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>, Andy Gross <agross@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 04:55:46 +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.
  */
 #define __visible	__attribute__((externally_visible))
 #endif
_