Re: PATCH: Add pause intrinsic

[Michael Matz <matz@suse.de>]

Hi,

On Thu, 26 May 2011, Andrew Haley wrote:

> >>> +Generates the @code{pause} machine instruction.
> >>
> >> But that's missing the fact that it generates a compiler memory 
> >> barrier, which is important.  And if you think it's not a compiler 
> >> memory barrier, please explain
> >>
> >> a.  Why it's not a compiler memory barrier,
> > 
> > It is not a compiler memory barrier because it is a builtin function call
> > which is never assumed to be a barrier for local automatic storage
> > that does not have its address taken.
> 
> OK.  How would you tell the difference between the kind of barrier
> that it is and a real compiler memory barrier?

First we have to determine if this builtin really does what its users 
intend to use it for.  I believe they _do_ want to use it also with 
regards to auto variables (it includes also address-takens whose address 
doesn't escape).  A normal builtin call is not a barrier for operations on 
such entities, hence it might very well be that the implementation of HJ 
actually doesn't what he wanted.

I don't have a good word for what functions calls are in their barrierness 
part of pre/post conditions.  "global memory movement barrier" perhaps, 
with an appropriate definition of global memory (which funnily include 
address-taken escaped local storage, ugh).

> > To make it a compiler memory barrier you have to "expand" the
> > builtin already in the frontend and present the middle-end with
> > __asm__ ("...." : : : "memory").  That will serve as a compiler
> > memory barrier also covering local non-address taken storage
> > (global and practically most of address-taken local storage
> > is covered by a builtin function call already).
> 
> Well, the fact that it's also a memory clobber has to be documented
> somehow.  If the present documentation is to be changed, it should
> not be changed by deleting a vital piece of information.

It's not only about the docu.  As implemented right now it's neither an 
optimization barrier nor a memory clobber.


Ciao,
Michael.