llvm dev - Jul 2017 - [ThinLTO] Making ThinLTO functions not fail hasExactDefinition (specifically preventing it from being derefined)

Hey all,

I'm working on adding interprocedural FunctionAttrs optimization (
http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/FunctionAttrs.cpp) to ThinLTO
so it does something similar to what LTO is doing (
https://bugs.llvm.org/show_bug.cgi?id=33648). I've hit a problem with how
the FunctionAttrs optimization expects linkage types.

In ThinLTO since the linkage type is set to External or AvailableExternally
it is unable to apply any of the functionattr optimizations that LTO can
(since LTO is able to convert everything to Internal). This is a problem
because every functionattr optimization expects an exact definition by
calling hasExactDefinition (http://llvm-cs.pcc.me.uk/
include/llvm/IR/GlobalValue.h#387) which doesn't want a derefined function
(mayBeDerefined http://llvm-cs.pcc.me.uk/include/llvm/IR/GlobalValue.h#119).
According to the current definition, an AvailableExternally function could
be derefined so hasExactDefinition fails.

Does anyone have any ideas on how to make a function with
External/AvailableExternally linkage type have an exact definition?

Thanks,
Charles
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On Mon, Jul 10, 2017 at 8:55 AM, Charles Saternos via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hey all,
>
> I'm working on adding interprocedural FunctionAttrs optimization
> (http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/FunctionAttrs.cpp) to ThinLTO
> so it does something similar to what LTO is doing
> (https://bugs.llvm.org/show_bug.cgi?id=33648). I've hit a problem with
how
> the FunctionAttrs optimization expects linkage types.
>
> In ThinLTO since the linkage type is set to External or AvailableExternally
> it is unable to apply any of the functionattr optimizations that LTO can
> (since LTO is able to convert everything to Internal). This is a problem
> because every functionattr optimization expects an exact definition by
> calling hasExactDefinition
> (http://llvm-cs.pcc.me.uk/include/llvm/IR/GlobalValue.h#387) which
doesn't
> want a derefined function (mayBeDerefined
> http://llvm-cs.pcc.me.uk/include/llvm/IR/GlobalValue.h#119). According to
> the current definition, an AvailableExternally function could be derefined
> so hasExactDefinition fails.
>
> Does anyone have any ideas on how to make a function with
> External/AvailableExternally linkage type have an exact definition?
>
+ Sanjoy, who did all the derefinement work originally.

--
Davide

Hi Charles,

On Tue, Jul 11, 2017 at 12:27 PM, Davide Italiano <davide at freebsd.org>
wrote:
>> I'm working on adding interprocedural FunctionAttrs optimization
>> (http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/FunctionAttrs.cpp) to
ThinLTO
>> so it does something similar to what LTO is doing
>> (https://bugs.llvm.org/show_bug.cgi?id=33648). I've hit a problem
with how
>> the FunctionAttrs optimization expects linkage types.
>>
>> In ThinLTO since the linkage type is set to External or
AvailableExternally
FunctionAttrs should still kick in for external linkage, can you post
an IR sample of where that's not happening (since that's a bug IIUC)?

As far as available_externally is concerned, preventing FunctionAttrs
on them is by design and I'm not sure if there is anything in ThinLTO
to fix that (assuming that the functions are, in fact,
available_externally).

There is some background information on this issue at
https://www.playingwithpointers.com/ipo-and-derefinement.html

-- Sanjoy

>> it is unable to apply any of the functionattr optimizations that LTO
can
>> (since LTO is able to convert everything to Internal). This is a
problem
>> because every functionattr optimization expects an exact definition by
>> calling hasExactDefinition
>> (http://llvm-cs.pcc.me.uk/include/llvm/IR/GlobalValue.h#387) which
doesn't
>> want a derefined function (mayBeDerefined
>> http://llvm-cs.pcc.me.uk/include/llvm/IR/GlobalValue.h#119). According
to
>> the current definition, an AvailableExternally function could be
derefined
>> so hasExactDefinition fails.
>>
>> Does anyone have any ideas on how to make a function with
>> External/AvailableExternally linkage type have an exact definition?
>>
>
> + Sanjoy, who did all the derefinement work originally.
>
> --
> Davide