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https://issues.apache.org/jira/browse/IMPALA-11412?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Daniel Becker resolved IMPALA-11412.
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Resolution: Fixed
> CodegenFnPtr<FuncType>::store() has a compile time error when instantiated
> --------------------------------------------------------------------------
>
> Key: IMPALA-11412
> URL: https://issues.apache.org/jira/browse/IMPALA-11412
> Project: IMPALA
> Issue Type: Bug
> Components: Backend
> Reporter: Daniel Becker
> Assignee: Daniel Becker
> Priority: Major
>
> The function template {{CodegenFnPtr<FuncType>::store()}} tries to implicitly
> cast a function pointer (of type {{FuncType}}) to {{void*}}, which is a
> compile time error. The reason this didn't come up in the builds is that this
> function template is currently not used anywhere, and the function pointers
> are stored through the parent class, using {{{}CodegenFnPtrBase::store(){}}},
> which takes a {{{}void*{}}}.
> We should either
> # remove the hitherto unused {{CodegenFnPtr<FuncType>::store()}} function
> template
> OR
> # add the correct explicit cast from function pointer to {{void*}} AND add a
> test which instantiates (and tests) this function template so we can be sure
> that the new implementation is correct.
> I'm inclined to choose the second option because I think the interface of
> {{CodegenFnPtr<FuncType>}} is more complete if we have this function as well,
> even if it is currently not used.
> Note:
> After digging a bit on the internet I found that the reason that implicit
> function pointer to {{void*}} cast is not allowed (as opposed to implicit
> regular pointer to {{void*}}) is because the standard doesn't guarantee that
> regular and function pointers have the same size, and there are some
> architectures where they actually don't.
> However, according to 8) on
> [https://en.cppreference.com/w/cpp/language/reinterpret_cast], POSIX
> compliant systems do have this guarantee, so it shouldn't be a problem that
> we store function pointers as {{{}void*{}}}. We don't really have a choice
> because LLVM does the same as
> {{llvm::ExecutionEngine::getPointerToFunction()}} returns a {{void*}} (see
> [https://llvm.org/doxygen/classllvm_1_1ExecutionEngine.html#acc46759a6acfc3d116c3f22110326ffa]);
> we call that function
> [here|https://github.com/apache/impala/blob/fefb9f24be1f99ac0077a8d6ef00834d8e90ef45/be/src/codegen/llvm-codegen.cc#L1315].
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