[llvm-dev] AA and external globals

[Christian Convey via llvm-dev]

On Thu, Nov 5, 2015 at 4:30 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

> I'm curious why you don't use the existing CFL-AA implementation?
> Or is this just an experiment/learning experience/whatever?
>

The issue came up as part of me trying out some AA ideas that have no
prayer of getting into the mainline LLVM.  But in addition to that, now I'm
troubled by the mere fact that I don't know the answer :)


>
> On Thu, Nov 5, 2015 at 1:00 PM, Christian Convey via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I'm working on an AA implementation, and I'm using abstract blocks to
>> model the memory used to store the values of program variables.
>>
>> To be sound my analysis must ensure that all program objects which *might
>> possibly* occupy the same chunk of runtime memory are modeled with the
>> same abstract memory block.  I'm trying to understand if "external" linkage
>> is a problem for me.
>>
>> More concretely, suppose I have a Module which includes these
>> declarations:
>> @Bar = external global i32
>> @Foo = external global i32
>>
>> Is there any reasonable possibility that the linker will cause @Bar and
>> @Foo to name the same piece of runtime memory?
>>
>
> Do you want "what's legal in some particular language" or "what may ever
> happen"?
>

I guess both answers are interesting.  Pragmatically speaking, I need an
answer that's pretty trustworthy for any not-intentionally-evil C or C++
code compiled by clang 3.7+, and linked with the GNU linker on a modern
Linux distro, on x86-64.

But I'm also curious if there's something in the LLVM IR spec or ELF spec
that should have given me the answer.  I couldn't tell if such docs don't
exist, or if my Google-fu was too weak to find the answer.
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