Quo vadis, thunk? (Was: Re: [PATCH/RFA] PR target/16665: SH thunk with -fPIC (fwd))

[Mark Mitchell]

Joern Rennecke wrote:

>I see that you wrote the comment in cp/method.c .  Is target port supposed
>to be able to rely on the thunk being emitted together with the function
>that is being thunked to?
>  
>
Yes, where "together with" means "in the same object file".  That much 
is required by the C++ ABI.  The ABI does not seem to say whether or not 
the thunks must be COMDAT, but it would make sense to make them COMDAT 
only when their associated function is also COMDAT, and in that case 
they should all be in the same group.

Thus, I would consider the fact that G++ presently makes the thunks weak 
even when the underlying function is not weak to be an odd quirk, if not 
a bug.  I think that could safely be changed without endangering 
anything.  It may be that it is the way it is for historical reasons; it 
could also be that until my recent linkage cleanup, one could not be 
sure whether the associated function would be COMDAT or not.  In any 
case, I think you could change it now.

In the case that the associated function is weak, you cannot (since GCC 
does not really do COMDAT groups) assume that the thunks will be "near" 
the associated function; they will still be in separate .gnu.linkonce 
sections.

-- 
Mark Mitchell
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mark@codesourcery.com