[PATCH] Prologue support

[Peter Collingbourne]

On Fri, Nov 28, 2014 at 09:36:27PM -0500, Stephen Checkoway wrote:
> 
> On Nov 28, 2014, at 10:20 AM, Ben Gamari <bgamari at gmail.com> wrote:
> 
> > Here we redefine the notion of prefix data to instead be data which
> > occurs immediately before the function entrypoint (i.e. the symbol
> > address). Since prefix data now occurs before the function entrypoint,
> > there is no need for the data to be valid code.
> 
> I seem to recall that when prefix data was being discussed previously, it came up that putting data before the symbol wouldn't work for OS X because of .subsections_via_symbols (or maybe some other reason). Peter may remember the details better.

Oh yes, I remember this; I think .subsections_via_symbols was the problem.
I think if we wanted this to work on OS X we would need to store the metadata
as prologue data, and teach the frontend to take an offset from the function
pointer. This might need a change in IR semantics to say that calling a
(constant) offseted function pointer does the same thing as calling the
function directly.

Thanks,
-- 
Peter