Re: [whopr] Design/implementation alternatives for the driver and WPA

[Ian Lance Taylor <iant at google dot com>]

Nick Kledzik <kledzik@apple.com> writes:

> On Jun 4, 2008, at 5:00 PM, Ian Lance Taylor wrote:
>> Nick Kledzik <kledzik@apple.com> writes:
>>
>>> I don't claim our current implementation is bug free, but the lto
>>> interface
>>> matches the Apple linker internal model, so we don't expect and have
>>> not encountered any problems mixing mach-o and llvm bitcode files.
>>
>> Hmmm, OK, how about this example:
>>
>> a.o: contains LTO information, refers to S
>> b.o: no LTO information, defines S
>> c.o: contains LTO information, defines S at version V, S/V is not
>> hidden
>>
>> In the absence of b.o, the reference to S in a.o will be resolved
>> against the definition of S in c.o.  In the presence of b.o, the
>> reference to S in a.o will be resolved against the definition of S in
>> b.o.
>>
>> I suppose we could refuse to inline versioned symbols, but that
>> doesn't seem desirable since it is normally fine.
>
>
> As Chris mentioned earlier today, the Apple tool chain does not
> support versioned symbols.
> But if versioned symbols are a naming convention (that is everything
> is encoded in
> the symbol name), then this would work the same as your previous
> example.  Namely,
> the linker would coalesce away S in c.o, which in turns tell the LTO
> engine that it
> can't inline/optimize away c.o's S and after LTO is done, the linker
> throws away
> the LTO generated S and uses b.o's S instead.

Versioned symbols are not a naming convention, but they aren't all
that different from one.  Basically every symbol may have an optional
version, and when a symbol has a version the version may be hidden or
not.  A symbol definition with a hidden version may only be matched by
a symbol reference with that exact version.  A symbol definition with
a non-hidden version definition may be matched by a symbol reference
with the same name without a version.  This is most interesting in the
dynamic linker, of course.

How does the linker inform the plugin that the plugin is not permitted
to use c.o's S?

Ian