[llvm-dev] Orc JIT vs. implicit template instanciation in LLVM 8

[David Blaikie via llvm-dev]

+Lang for ORC questions

This sounds like it's beyond the domain of ORC though - if the
function definition is missing, ORC (at least the bits in LLVM) know
nothing about your frontend or how to request it to manifest that
function. In your old version, when/where/how did your frontend
generate LLVM IR for this function? I'd check that and then see why a
similar thing isn't happening in the new version.

- Dave

On Fri, Sep 13, 2019 at 8:03 AM Geoff Levner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> The short version:
>
> When I port our application from LLVM 7 to 8, some JIT-compiled code
> seems to break the implicit instanciation of C++ templates. That is,
> when I try to resolve a symbol defined by the code, I get undefined
> symbols corresponding to methods that are actually function templates.
> If the code adds explicit template instanciations, the problem goes
> away.
>
> Does anybody have an intuition as to what might cause this?
>
> Now the longer version:
>
> Specifically, we use the Orc JIT (v1) to compile and execute C++ code
> on the fly. To move to LLVM 8, I am using the corresponding "legacy"
> classes before attempting the move to Orc v2. But when I do that, some
> code which compiles fine with LLVM 7 breaks. I might get an undefined
> symbol like the following when I try to resolve a symbol defined by
> the code, after running constructors (successfully):
>
>     _ZNSt6vectorIfSaIfEE17_M_default_appendEm
>
> Unmangled, that's:
>
>     std::vector<float, std::allocator<float> >::_M_default_append(unsigned long)
>
> which is a function template defined by the C++ vector header file. If
> I add an explicit instanciation like so:
>
>     template class std::vector<float>;
>
> the problem goes away. I have tried to reproduce the problem with the
> Clang front end to no avail, so I assume it is specific to symbol
> resolution in Orc JIT. Has something changed there between LLVM 7 and
> 8?
>
> Any help would be much appreciated!
>
> Thanks,
> Geoff
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