[llvm-dev] Question about ObjectSizeOffsetVisitor::visitGlobalVariable

[Karl Rehm via llvm-dev]

Is the size == 0 check going to work for all cases? I don't know how
flexible array types are represented but if we have an int[1] in a global
would this fail? Maybe have a check instead for: if constructor is not
definitive and we are an array type then return false

- Karl

On Wed, Jan 29, 2020, 11:40 PM Doerfert, Johannes <jdoerfert at anl.gov> wrote:

> I tried to track down more of the reasons behind the strict definitive
> initializer requirement because of other IPO/global opts we do.
> Unfortunately, I failed.
>
> It seems clear we cannot allow any global, as we cannot do IPO depending
> on the linkage. I suspect exact definitions are the least we need to
> make sure we don't get something entirely different at link time. Then
> there is the variable-length/empty size issue that seems to me as if it
> is a "on-off" special case.
>
> Based on the above assumptions I tried out what happens if we relax the
> condition, here is what I came up with: https://reviews.llvm.org/D73679
>
>
> On 01/29, Karl Rehm via llvm-dev wrote:
> > Interesting, so I guess we can check for whether it's an array type and
> > adjust accordingly instead? Blocking *all* global variables without
> > definitive initializers feels a bit much to me, especially if they have
> > primitive types (i.e. integers) that don't have the potential to be
> screwed
> > around with like this.
> >
> > On Wed, Jan 29, 2020 at 8:14 PM George Burgess IV <
> > george.burgess.iv at gmail.com> wrote:
> >
> > > Looks like the commit that added that was
> > >
> https://github.com/llvm/llvm-project/commit/16e42128c24f5df8a61cfd2e6cdf3bd3966db0c5
> > > . A reduced example might look something like this:
> > > https://godbolt.org/z/r4G_g-
> > >
> > > At the C source level, static object size detection has to be similarly
> > > conservative in the face of 'variable-length' structs like `struct
> > > my_string { size_t size; char data[0]; };`, though I don't know how
> > > relevant that is here.
> > >
> > > On Tue, Jan 28, 2020 at 11:41 PM Karl Rehm via llvm-dev <
> > > llvm-dev at lists.llvm.org> wrote:
> > >
> > >> In this function (used to check the size of a global) there is an
> initial
> > >> check for whether the initializer to this function is "definitive." My
> > >> question is: why do we need this? How does the object's size change
> if a
> > >> global's initializer is defined at link time?
> > >>
> > >> Thanks,
> > >> Karl
> > >> _______________________________________________
> > >> LLVM Developers mailing list
> > >> llvm-dev at lists.llvm.org
> > >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > >>
> > >
>
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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>
>
> --
>
> Johannes Doerfert
> Researcher
>
> Argonne National Laboratory
> Lemont, IL 60439, USA
>
> jdoerfert at anl.gov
>
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