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

Thu Nov 10 08:35:04 PST 2016

Hi Stefan,

Okay, thanks to having your code as a working example, I figured out 
how to fix my problem. The reason was simply that my symbol name began 
with a ".".
>I think the ExecutionEngine just did the query and pass on 
>implicitly.
If it did, it did something to get around this… some day, I'll look 
into what that was.  For now, I'll just use different names.

>If you want your generated code to have implicit access to functions in
>your binary, then have a look at these two lines:
I precisely do not want that to happen. Anything implicit stinks of 
magic I have no control over, especially across 5 compilers on three 
platforms.

Greetings,
Julius

PS: Your example doesn't compile with gcc-5.4 (because of the 
DataLayout DataLayout-declaration), and the assertion that the mangled 
symbol name starts with an underscore does not hold, at least not on 
my system.

On  8.11 18:48, Stefan Gränitz wrote:
>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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>
>-- 
>https://about.me/stefan.graenitz
>