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

[Andrew Morton]

On Wed, 16 Sep 2015 04:32:19 +0200 Andi Kleen <andi at firstfloor.org> wrote:

> > __iowrite32_copy() is marked __visible.  I don't actually know what
> > that does and Andi's d47d5c8194579bc changelog (which sucks the big
> > one) didn't explain it.  Apparently it has something to do with being
> > implemented in assembly, but zillions of functions are implemented in
> > assembly, so why are only two functions marked this way?  Anyway,
> > __ioread32_copy() is implemented in C so I guess __visible isn't needed
> > there.
> 
> __visible is needed for C functions that are called from assembler.
> Otherwise the compiler may optimize them away.

Under what circumstances will the compiler (or linker?) do this? 
LTO enabled?