Re: [RFC][PATCH 0/5] arch: atomic rework

[Torvald Riegel <triegel@redhat.com>]

On Tue, 2014-02-18 at 22:52 +0100, Peter Zijlstra wrote:
> > > 4.	Some drivers allow user-mode code to mmap() some of their
> > > 	state.  Any changes undertaken by the user-mode code would
> > > 	be invisible to the compiler.
> > 
> > A good point, but a compiler that doesn't try to (incorrectly) assume
> > something about the semantics of mmap will simply see that the mmap'ed
> > data will escape to stuff if can't analyze, so it will not be able to
> > make a proof.
> > 
> > This is different from, for example, malloc(), which is guaranteed to
> > return "fresh" nonaliasing memory.
> 
> The kernel side of this is different.. it looks like 'normal' memory, we
> just happen to allow it to end up in userspace too.
> 
> But on that point; how do you tell the compiler the difference between
> malloc() and mmap()? Is that some function attribute?

Yes:

malloc
        The malloc attribute is used to tell the compiler that a
        function may be treated as if any non-NULL pointer it returns
        cannot alias any other pointer valid when the function returns
        and that the memory has undefined content. This often improves
        optimization. Standard functions with this property include
        malloc and calloc. realloc-like functions do not have this
        property as the memory pointed to does not have undefined
        content.

I'm not quite sure whether GCC assumes malloc() to be indeed C's malloc
even if the function attribute isn't used, and/or whether that is
different for freestanding environments.