[llvm-dev] ThinLTO and linkonce_odr + unnamed_addr

Wed Feb 7 12:36:31 PST 2018

> But it is interesting in general because according to the definition for
local_unnamed_addr, the symbol has to be linkonce_odr to be auto hide as
well. ThinLTO promotion can break that as well.

What do you mean that the promotion can break that?

The original description I found here: https://reviews.llvm.org/D20348 says
that it is possible to exclude a global from the symbol table if three
things are true:

   - This attribute is present on every instance of the global (which means
   that the normal rule that the global must have a unique address can be
   broken without being observable by the program by performing comparisons
   against the global's address)
   - The global has linkonce_odr linkage (which means that each linkage
   unit must have its own copy of the global if it requires one, and the copy
   in each linkage unit must be the same)
   - It is a constant or a function (which means that the program cannot
   observe that the unique-address rule has been broken by writing to the
   global)

When promoting from a linkonce_odr, it seems safe to me to *keep* the
local_unnamed_addr
on the weak_odr since we know the symbol was linkonce_odr in the first
place.

-- 
Mehdi

2018-02-07 12:12 GMT-08:00 Steven Wu <stevenwu at apple.com>:

> We didn't drop unnamed_addr. I just don't think weakodr_addr +
> unnamed_addr is safe to hide in all cases.
>
> I don't know if I interpreted local_unnamed_addr correctly but I think it
> is mostly the same in thinLTO for ld64. The code generator only looks at
> the individual module and ld64 will be in charge of merging all the symbols
> with autohide. It doesn't really help in this case. But it is interesting
> in general because according to the definition for local_unnamed_addr, the
> symbol has to be linkonce_odr to be auto hide as well. ThinLTO promotion
> can break that as well.
>
> Steven
>
>
>
> On Feb 7, 2018, at 11:44 AM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>
> Something I haven't seen mentioned: why are we dropping the unnamed_addr?
> Can't we preserve it with the weak symbol? Would it be OK to add auto-hide
> in this case (maybe it would already happen)?
> Can we use the new local_unnamed_addr that was added (by pcc or Rafael I
> don't remember)? I think this attribute matches exactly the `auto-hide`
> semantic. Wasn't the idea that this could be added any time by a
> module-level `infer_attribute` pass?
>
> --
> Mehdi
>
>
> 2018-02-07 11:36 GMT-08:00 Steven Wu via llvm-dev <llvm-dev at lists.llvm.org
> >:
>
>> I didn't realize that that WeakDefCanBeHiddenDirective is only available
>> on Darwin. So if we are doing it for #2, it should be a Darwin only fix as
>> well.
>>
>> Steven
>>
>>
>> On Feb 7, 2018, at 11:29 AM, Reid Kleckner <rnk at google.com> wrote:
>>
>> I agree with Teresa, we should probably do #2 to preserve behavior for
>> now.
>>
>> On Wed, Feb 7, 2018 at 9:34 AM, Teresa Johnson via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi Steven,
>>>
>>> I'd prefer not to inhibit importing. I am also concerned about putting
>>> these symbols in the llvm.compiler_used (I don't recall earlier discussion
>>> around this, but it seems like it could have effects on optimization as you
>>> mention).
>>>
>>> What are the downsides of #2 (adding visibility hidden)? We already do
>>> this when promoting internal linkage to external due to importing. I'm not
>>> an expert on how this would affect link semantics.
>>>
>>> Thanks,
>>> Teresa
>>>
>>> On Tue, Feb 6, 2018 at 5:35 PM, Steven Wu <stevenwu at apple.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> I recently found that thinLTO doesn't deal with globals that has
>>>> linkonce_odr and unnamed_addr (for macho at least) because it prohibits the
>>>> autohide optimization during link time.
>>>>
>>>> In LLVM, we tagged a global linkonce_odr and unnamed_addr to indicate
>>>> to the linker can hide them from symbol table if they were picked (aka,
>>>> linkonce_odr_auto_hide linkage). It is very commonly used for some type of
>>>> Tables for c++ code in clang for example.
>>>> However, thinLTO is promoting these symbols to weak_odr + unnamed_addr,
>>>> which lose the property. As a result, it introduces unnecessary weak
>>>> external symbols and weak external are not good for performance on darwin
>>>> platforms.
>>>>
>>>> I have few proposed solutions for this issue but I don't know which one
>>>> works the best for none macho platforms and other LTO clients like lld.
>>>>
>>>> 1. Use llvm.compiler_used.
>>>> As far as I know, the linkage promote are just there to keep the symbol
>>>> through internalize and codegen so adding them to compiler used should
>>>> solve this issue. I was told that there was some objections to do that in
>>>> the first place. Is it because the globals added to compiler used is
>>>> ignored by the optimizer so they cannot be internalized and they cannot be
>>>> optimized away? This works well for the case I am looking at because c++
>>>> VTable can't really be optimized and for darwin platforms because we can
>>>> rely on ld64 to do dead_stripping if needed.
>>>>
>>>> 2. Add visibility hidden when promote linkonce_odr + unnamed_addr.
>>>> Well,this doesn't really preserve the link semantics, but neither does
>>>> promoting linkonce_odr to weak_odr. The global will still end up in the
>>>> symbol table but at least it isn't external so it doesn't come with a
>>>> performance cost.
>>>>
>>>> 3. We can teach function importer that it cannot just reference to
>>>> linkonce_odr + unnamed_addr symbols without importing them. I have some
>>>> thoughts about how to do this so I can propose something if people are
>>>> interested going down this route. I am expecting at least add an entry in
>>>> the global summery and change the cost of importing symbols that references
>>>> to linkonce_odr + unnamed_addr symbols.
>>>>
>>>> 4. As a temporary fix, just targeting at the VTables for c++. We can
>>>> put a special case for global constants that uses this linkage so they are
>>>> never promoted and their parents are never imported into other modules. The
>>>> benefit for inlining global constants is very minimal and I don't think we
>>>> are doing it currently.
>>>>
>>>> Let me know if any of those solutions work for other LTO client.
>>>>
>>>> Thanks
>>>>
>>>> Steven
>>>>
>>>
>>>
>>>
>>> --
>>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>> 408-460-2413 <(408)%20460-2413>
>>>
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>>>
>>
>>
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