llvm dev - Mar 2016 - [cfe-dev] RFC: A new ABI for virtual calls, and a change to the virtual call representation in the IR

> On Mar 11, 2016, at 1:40 PM, Rafael Espíndola <rafael.espindola at
gmail.com> wrote:
>>> And you can't also just produce STV_PROTECTED for every symbol.
I
>>> would love for that to be the case, but while most ELF systems
support
>>> copy relocations and related PLT hacks for functions it is not
>>> practical to do it.
>> 
>> I’m sorry, I'm not familiar with the technical problems here.
>> 
>> For example, I don’t understand why protected symbols require new
relocations.
>> I would expect that, as far as relocations go, they would be treated
the same as
>> hidden symbols within the linkage unit, and the same as default symbols
outside of it.
> 
> It is not that they require. It is the other way, they don't really
> work with copy relocations.
> The issue is that a non -fPIC program can link with a shared library.
> When it does that the only way to keep pointer equality is for the
> dynamic linker to
> * Copy data from .so to the main program
> * Use a PLT entry in the main program as the address of functions
> 
> with that a protected visibility symbols ends up living in the main
> executable. That is not well supported.
I’m sorry, but I didn’t quite get that.  The symbol is defined as protected
by the dynamic library, but the main executable can’t just use an ordinary
external symbol reference to it for some reason?
>>> Except that we still produce STV_DEFAULT. Basically we have a mode
>>> where we handle GVs as if they were protected but produce a
>>> STV_DEFAULT.
>> 
>> Yes, I don’t think this is a justifiable position.  We either do or do
not support
>> interposition of global definitions.
> 
> I don't see why it has to be black and white like that. We have been
> producing STV_DEFAULT for some time. I don't know how much we would
> drop in performance if we just started not inlining some functions
> just to keep them STV_DEFAULT.
I’m worried about the semantic model.  Our current model is that we allow
interposition, but calls within the linkage unit might ignore it, depending on
how much our optimizer sees.  That’s not a coherent position.
>>> In summary, I think it is important that
>>> 
>>> * a __attribute__((visibility("protected"))) maps to
STV_PROTECED
>>> * by default a decl with no attributes maps to STV_DEFAULT
>> 
>> It’s not really my place to decide whether or not
-fsemantic-interposition should be
>> the default, so I’ll leave this up to others to debate.
>> 
>> My instinct is that interposition is not actually important to very
many people,
>> and that the people who rely on it won’t really mind having to opt in. 
We have
>> also been optimizing as if semantic interposition were disabled for
many years,
>> and I don’t remember hearing widespread complaints about it.  So I
would
>> encourage us to turn it off by default, despite not matching GCC.  But
like I
>> said, that is not ultimately my call, and I’ll leave the decision to
others.
> 
> The problem is not interposition being on or off by default. The
> problem is that you are proposing locking that with producing
> STV_DEFAULT or STV_PROTECTED, and I don't think we can just start
> producing STV_PROTECTED everywhere.
I agree that we have to resolve that question before we can make a policy
statement here.

John.
> 
> I am personally fine having clang behave like gcc (STV_DEFAULT, assume
> regular decls with -fPIC can be interposed), but would like to know
> what other ELF users think of that before forcing them to select
> between not inlining with -fPIC or getting STV_PROTECTED.
> 
> BTW, note that if are going to produce STV_PROTECTED anyway, we don't
> need -fsemantic-interposition, we can just use -fvisibility=protected.
> 
> Cheers,
> Rafael

On Fri, Mar 11, 2016 at 01:54:36PM -0800, John McCall via cfe-dev
wrote:
> I’m sorry, but I didn’t quite get that.  The symbol is defined as protected
> by the dynamic library, but the main executable can’t just use an ordinary
> external symbol reference to it for some reason?
The main program is not position independent. A function call is fine,
the linker will provide a PLT slot for that and the issue is done. The
problems are accessing a variable OR taking the pointer of a function.
With non-PIC code on x86, you get an immediate load. But the linker
can't adjust this into a load from the GOT.

Joerg

On 11 March 2016 at 18:34, Joerg Sonnenberger via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On Fri, Mar 11, 2016 at 01:54:36PM -0800, John McCall via cfe-dev wrote:
>> I’m sorry, but I didn’t quite get that.  The symbol is defined as
protected
>> by the dynamic library, but the main executable can’t just use an
ordinary
>> external symbol reference to it for some reason?
>
> The main program is not position independent. A function call is fine,
> the linker will provide a PLT slot for that and the issue is done. The
> problems are accessing a variable OR taking the pointer of a function.
> With non-PIC code on x86, you get an immediate load. But the linker
> can't adjust this into a load from the GOT.
Yes. There is quite a bit of confusion about protected being "you get
something semantically equivalent" or "you get exactly this
entry".
With the first interpretation you cannot assume the address, with the
second one you can. See
https://sourceware.org/bugzilla/show_bug.cgi?id=19823 for example.

But this is getting into a bit more detail than what we need. The
sufficient summary is that STV_PROTECTED is sufficiently broken that
we probably cannot make it the default.

IMHO we can do the same as gcc: default visibility can be preempted
and is not inlined with -fPIC. Use -fvisibility=protected if you want
inlining and handle whatever brokenness you find in the system tools.

But that is a pretty big change. I would ask that if you want to make
that change, please start a thread in llvmdev to see what others
think. I wouldn't even mention IR changes. For now, lets just focus at
the end to end (C to .o) difference.

Cheers,
Rafael