[llvm-dev] [llvm] To link or not to link

Tue Nov 8 09:48:20 PST 2016

Hi Julius, I had similar questions a while ago. First of all: the
GlobalMappingLayer is used for explicit mappings from strings to
function pointers, so that your generated code can access these
functions. It doesn't do much more.

I didn't inspect your attached example, but I think you can look at one
of my examples to find out how I wire it up. Maybe that helps:
https://github.com/weliveindetail/JitFromScratch/commit/70277db033d75599eff24ccd802cdf7fdfbcde99

> The problem I have with that conceptually is: The name seems to
> matter. The abstraction I'd like is that I get an llvm::Function*, and
> that is mapped to &somefunc, the name "bla" I gave it should be
> irrelevant.
Just query the name of your llvm::Function and pass it also to the
GlobalMappingLayer (if you want to use it).

> (And that's what I'd like that to be like for all functions and
> function calls and whatnot: The name I give them is irrelevant, only
> the llvm::Function*/llvm::Value* matters.) Am I using the wrong API
> and there is a way to do this without the names, without any symbol
> resolution at all? (I don't want to use symbols from the current
> executable or anywhere else, either.) 
I think the ExecutionEngine just did the query and pass on implicitly.

> The problem I have with that implementationally: It just doesn't work.
> I get a LLVM ERROR: Undefined temporary symbol from the
> ELFObjectWriter (and the error is actually about that symbol I'm
> trying to map)
Query your mapping during symbol resolution like this:
https://github.com/weliveindetail/JitFromScratch/commit/70277db033d75599eff24ccd802cdf7fdfbcde99#diff-db46172f086b2c8918df9eea96b799c2R165

If you want your generated code to have implicit access to functions in
your binary, then have a look at these two lines:
https://github.com/weliveindetail/JitFromScratch/commit/860bec7e7fae1a1f31ef794bf78d191c44355c4c#diff-db46172f086b2c8918df9eea96b799c2R22
https://github.com/weliveindetail/JitFromScratch/commit/860bec7e7fae1a1f31ef794bf78d191c44355c4c#diff-db46172f086b2c8918df9eea96b799c2R64

It registers your binary as dynamic library and resolves symbols from
within your process. In order to avoid C++ mangling trouble, the
functions that get used may be extern "C" qualified. If you use a
frontend like Clang to generate your code, take care about compatible
settings, e.g. debug vs. release and ABI-related compile flags like
exceptions and RTTI.

Cheers,
Stefan

Am 07.11.16 um 11:47 schrieb Julius Michaelis via llvm-dev:
> Hi,
>
> I have migrated an LLVM front-end from LLVM 3.5 to 3.8 and now to 3.9
> and ORC, and there is a concept which I could not transfer. Consider:
>
> extern "C" { void somefunc() {} }
> …
> auto llvmfunc = llvm::Function::Create(type,
> llvmFunction::PrivateLinkage, "bla", module));
> executionengine.addGlobalMapping(llvmfunc, &somefunc);
> // now I have llvmfunc to work with and don't need to consider
> anything else
>
> The GlobalMappingLayer class seems to provide the functionality I
> want. I oriented myself on the example from
> https://github.com/AndySomogyi/cayman/blob/aaa809c/src/llvm_orc_initial.cpp#L1172
> (just oriented, because at least on 3.9, I can not stack the
> GlobalMappingLayer on the ObjectLinkingLayer. I have to put it onto
> the IRCompileLayer. One question I'd have: How does that order matter?)
>
> The problem I have with that conceptually is: The name seems to
> matter. The abstraction I'd like is that I get an llvm::Function*, and
> that is mapped to &somefunc, the name "bla" I gave it should be
> irrelevant. (And that's what I'd like that to be like for all
> functions and function calls and whatnot: The name I give them is
> irrelevant, only the llvm::Function*/llvm::Value* matters.) Am I using
> the wrong API and there is a way to do this without the names, without
> any symbol resolution at all? (I don't want to use symbols from the
> current executable or anywhere else, either.)
>
> The problem I have with that implementationally: It just doesn't work.
> I get a LLVM ERROR: Undefined temporary symbol from the
> ELFObjectWriter (and the error is actually about that symbol I'm
> trying to map), none of the llvm assertions fail. I compiled a minimal
> example of what I'm doing and attached it. The curious part is that
> the lambda resolver does not even get invoked… Can anybody tell me
> what I am doing wrong?
>
> Thank you
> Julius
>
>
> PS: Related thread:
> http://lists.llvm.org/pipermail/llvm-dev/2015-August/thread.html#89230
>
> PPS:
> The whole thing I'm building is supposed to be a script engine for a
> game. As the scripts aren't supposed to have any access outside the
> game, I'd like as little linking magic as possible.
> It's still in early stages.
> https://git.openclonk.org/jcaesar/experimental.git/blob/7be141ce667f4689428943592bae0a7568134213:/src/script/C4AulCompiler.cpp
>
>
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> llvm-dev at lists.llvm.org
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-- 
https://about.me/stefan.graenitz

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