[llvm-dev] Questions after playing around with KaleidoscopeJIT -Update (With source files)

[David Blaikie via llvm-dev]

Replies to the original would generally be easier to keep track of the
conversation - but I've cc'd the folks from that thread here in case they
can pick it up here.

On Tue, Oct 1, 2019 at 11:03 PM Gaier, Bjoern via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Heyho,
>
>
>
> I resend this mail, including the attachments to mark the questions that
> were answered, but also to append a new question. I hope to keep track
> about the questions – and not to spam…
>
>
>
> *From:* Gaier, Bjoern
> *Sent:* 25 September 2019 11:37
> *To:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Questions after playing around with KaleidoscopeJIT (With
> source files)
>
>
>
> Hello LLVM people,
>
>
>
> after finishing Chapter 1 and 2 of the KaleidoscopeJIT tutorial, I started
> to play around with the code and now, I have even more questions than
> before. I hope that the people reading this could help me with it, to
> improve my understanding about the LLVM and the JIT.
>
> I have also the source code and binaries (Windows) available but because I
> don't know how the mailing list treats attachments (like .zip) I decided to
> only attach the source files - hoping it will help >o<
>
>
>
> ---
>
>
>
> 1.) Resolving undefined references
>
> I wonder what the best practice is to resolve symbols that are not defined
> in my llvm::Module.
>
> For that purpose I created "CM_ExternalConstant", that is using "extern
> const int planschiValue;" and compiled it with Clang. (See
> CM_ExternalConstant.cpp)
>
>
>
> "planschiValue" is not defined in the module, so it will be unreferenced
> when I jit it. What is the best way to do this? I used 3 different ways:
>
>
>
> 1.) I went over the llvm::Module before adding it to the layers. I
> searched for the symbol "?planschiValue@@3HB" and called
> "replaceAllUsesWith" to replace the symbol with the correct address. (See
> PlanschbeckenJIT.cpp:31)
>
> 2.) I set my own GeneratorFunction and provided addresses for the symbols
> (See PlanschbeckenJIT.cpp:95)
>
> 3.) I can use this->es.getMainJITDylib().define
>
>
>
> Each way worked fine. But I felt that way 2.) was the most annoying way,
> because I had to provide many other addresses. So what way is the best? Or
> can I 'freely' choose?
>
> Is it possible to have multiple GeneratorFunctions? I would like to use
> the 'default' DynamicLibrarySearchGenerator plus my own lookup function.
>
> Would it also be possible to replace the uses of "?planschiValue@@3HB"
> directly with the actuall value of "plaschiValue"? Because it is a constant?
>
>
>
> 2.) Loading multiple modules
>
> I created "CM_Orc", which has a function "int helloOrc()" -
> "CM_ExternalConstant" has the same function, but a different
> implementation. (See CM_Orc.cpp and CM_ExternalConstant.cpp) Adding both
> modules to the JIT will lead to an error message:
>
> "Failure value returned from cantFail wrapped call
>
> UNREACHABLE executed at c:\program
> files\llvm\include\llvm\support\error.h:708!"
>
> I always thought that in this situation, one of the functions would be
> renamed...
>
>
>
> 3.) Loading multiple modules - controlled resolving
>
> Finally I created "CM_PlanschiValue" this file defines a value for
> "planschiValue" that was referenced in "CM_ExternalConstant". (See CM_
> PlanschiValue.cpp and CM_ExternalConstant.cpp) Surprisingly, if I wrote:
>
> "const int planschiValue = 543210;"
>
> Clang would not generate code for it.
>
> But when writing:
>
> "extern const int planschiValue = 543210;" it worked. Why? Why can Clang
> optimize away the code, when it could be referenced by a different module?
> Like it was!
>
>
>
> I loaded "CM_ExternalConstant" and "CM_PlanschiValue" together, while
> using my own GeneratorFunction. But there was no need any more to provide
> an address for "?planschiValue@@3HB". Obviously, the two modules where
> linked - which is correct. But what, if I don't want that for whatever
> reason? Would I have to rename the referenced value in the llvm::Module
> before adding the second module? Or should I run instead two different JITs?
>
> The idea behind this is... what should I do if I have multiple modules,
> that are not meant to be used together but should run in the same process.
>
>
>
> [Answered] 4.) Clang and JIT optimization
>
> If I compile my modules with Clang and all optimizations turned on - do I
> still need the TransformLayer in my JIT?
>
>
>
> [Answered] 5.) Mangling names
>
> Does Clang or the LLVM provide a function to mangle from the string "const
> int planschiValue;" to "?planschiValue@@3HB"? Or vice versa?
>
>
>
> 6.) Delayed resolving
>
> This question goes along with question 1.) – I wondered if there is a way,
> when emitting code for my module, if I can resolve the undefined values
> with a “don’t know yet value”. The idea is to still create code for the
> module, so that I can already use some functions of it – at a later point I
> would like to revisit the “don’t know values” to resolve them.
>
> Only because function “foo” is missing a certain reference, why shouldn’t
> I be allowed to already use “bar” which is fully resolved? That is the
> basic idea behind my question.
>
>
>
> ---
>
>
>
> I know that this are a lot questions and that I'm not good in explaining
> stuff - but I really hope you guys can help me to understand everything
> better.
>
>
>
> Kind greetings and many thanks
>
> Björn
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert
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> Junichi Tajika
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