[llvm-dev] Since MCJIT I can't get libm functions to work

[Lang Hames via llvm-dev]

Hi Frank,

This is on Linux, right? If so, you will need to pass --export-dynamic to
ld to export symbols from the main binary.

For example, given source file foo.c:

#include <stdio.h>
#include <dlfcn.h>

void bar() {}

int main(int argc, char *argv[]) {
  printf("bar is %p\n", dlsym(RTLD_DEFAULT, "bar"));
  return 0;
}

You will see:

$ clang -O3 -D_GNU_SOURCE=1 -o foo foo.c -ldl
$ ./foo
bar is (nil)
$ clang -O3 -D_GNU_SOURCE=1 -Wl,--export-dynamic -o foo foo.c -ldl
$ ./foo
bar is 0x400820

Hope this helps!

Cheers,
Lang.


On Thu, Jun 28, 2018 at 12:40 PM Frank Winter via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I am upgrading our JIT application to use LLVM 6.0.0, and with this
> transition I am making the move to use the new MCJIT facility.
>
> A problem I am encountering is that the math functions from libm are not
> resolved/found. I am using the lambda resolver from the KaleidoscopeJIT
> class which first searches the present modules and, if that is
> unsuccessful, continues the search in the whole process space.
>
> The generated modules would declare external functions like
>
> declare float @cosf(float)
>
> and would call it like:
>
>    %17 = call float @cosf(float %16)
>
> The datalayout is set to
>
> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
>
> The code snippet that adds the module is pretty much from the JIT tutorial:
>
>        auto Resolver = llvm::orc::createLambdaResolver(
>                                [&](const std::string &Name) {
>                              if (auto Sym =
> CompileLayer.findSymbol(Name, false))
>                                return Sym;
>                              return llvm::JITSymbol(nullptr);
>                                },
>                                [](const std::string &Name) {
>                              if (auto SymAddr =
> llvm::RTDyldMemoryManager::getSymbolAddressInProcess(Name))
>                                return llvm::JITSymbol(SymAddr,
> llvm::JITSymbolFlags::Exported);
>                              return llvm::JITSymbol(nullptr);
>                                });
>
>        cantFail(CompileLayer.addModule(std::move(M),
>                        std::move(Resolver)));
>
> When running the program I receive the following error:
>
> LLVM ERROR: Program used external function 'cosf' which could not be
> resolved!
>
> This is on an Intel i7 CPU with LLVM targets configured as "X86".
>
> Adding the '-lm' and '-ldl' option to the linker command that links the
> final program doesn't help. (I even called the 'cosf' function by hand
> in the host code to make sure it is mapped - didn't change the behavior
> of the MCJIT resolver.)
>
> Any ideas?
>
>
> Best wishes,
>
> Frank
>
>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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