[PATCH 0/3] Add __ioread32_copy() and use it

[Andi Kleen]

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 at 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