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

Mehdi AMINI via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 12 09:51:00 PDT 2017

2017-07-11 14:54 GMT-07:00 Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org>

> 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).

I think ThinLTO can fix most of it. My idea was to propagate function
attribute even for simple function declarations!

The reason I believe LTO can circumvent the "derefinement" issue is because
the attributes computed after optimization during the compile phase are
propagated only after the linker decided which "version" of an ODR function
we will pick in the final binary.

I believe this is valid, as long as later optimizations in the ThinLTO
backend can't "undo" an attribute (or can't change the attribute list to be
less restrictive). For example if a function is `read_none` after the
compile phase. During the ThinLTO thin-link we can propagate this attribute
to every uses (after deciding that this is the copy we want to keep for
this function). However if a later optimization invalidate the read_none
attributes, it's over. I doubt this can happen though (as in general it
would invalidate previous optimizations).


> 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
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