Re: Question re: Thunks (was Re: [PATCH] - Fix PR 21956 and PR 22003)

[Mark Mitchell <mark at codesourcery dot com>]

Richard Henderson wrote:
> I'll agree that it's possible to put them in to different sections
> iff it's possible to properly predict this from the backend.  We
> can then use indirect branches when necessary to perform the jump.
>
> That said, I see no reason not to always put the thunks in the
> same section.  We really should always choose a matched set of
> functions when resolving symbols.

Thunks should always be in the same section as their targets.  In fact, 
if we were smarter, some of our thunks would not end with jumps at all; 
they would fall through into the main function, if there was only one 
thunk, or be alternate entry points.

It is an invariant of the ABI design that all thunks are emitted in the 
same object file as the target function.

If we're depending on gnu.linkonce sections, life gets a little 
unpleasant, in the case, of, say, a virtual function that is a template. 
 We need some way to say that both the thunks and the targets are 
linkonce, but we have no way, so far as I know, of putting them in the 
same linkonce section, with our current machinery.  (I think the linker 
would handle it, but I don't think we know how to do it in varasm.c.)

With COMDAT, this should definitely work, but again, we probably need to 
make varasm.c know that the thunk goes in the same COMDAT group as the 
target.  (And, we need to figure out why Julian Brown's patch to not 
make COMDAT things weak doesn't work for people.)

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
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