[Openmp-dev] Using clang-offload-bundler to bundle application/executable

[Lukas Sommer via Openmp-dev]

Hi Jonas,

thanks for your help!

I tried the suggested command, unfortunately it still fails:

> clang -fopenmp -fopenmp-targets=x86_64-unknown-linux-gnu app.o
yields:

> /usr/bin/ld: /tmp/app-c48732.o: relocation R_X86_64_32 against `.data'
> can not be used when making a shared object; recompile with -fPIC
> /usr/bin/ld: final link failed: nonrepresentable section on output
> /usr/bin/ld: cannot find /tmp/app-23d932.out inside /
> /usr/bin/ld: cannot find /tmp/app-23d932.out inside /
> clang-9: error: linker command failed with exit code 1 (use -v to see
> invocation)
> clang-9: error: linker command failed with exit code 1 (use -v to see
> invocation)
I also tried to generate and bundle assembly (.asm) and run that through
clang (with openmp-flags as above), but that gave the same error.

Lukas

Lukas Sommer, M.Sc.
TU Darmstadt
Embedded Systems and Applications Group (ESA)
Hochschulstr. 10, 64289 Darmstadt, Germany
Phone: +49 6151 1622429
www.esa.informatik.tu-darmstadt.de

On 11/4/19 5:01 PM, Jonas Hahnfeld wrote:
> Hi Lukas,
>
> Am Montag, den 28.10.2019, 13:42 +0100 schrieb Lukas Sommer via
> Openmp-dev:
>> Hello,
>> I'm trying to use the clang-offload-bundler to merge the results of the
>> compilation for device and host for an application using OpenMP target
>> pragmas.
>> My goal is to run a custom pass on the device code and then bundle an
>> application from the modified device code and the host code. For testing
>> purposes, I use a very simple vector addition with target offloading
>> pragmas and the "offloading" to x86_64 (LLVM/Clang release 9.0).
>> I use clang to generate LLVM IR from the application code. Then I use
>> the clang-offload-bundler to unbundle the LLVM IR for device and host. I
>> run opt and llc separately on the LLVM IR for host and device.
>> To this end, on my application code (code.c), I run the following steps:
>>> clang -fopenmp -fopenmp-targets=x86_64-unknown-linux-gnu -S -emit-llvm
>>> -O3 -o bundled.ll code.c
>>> clang-offload-bundler --inputs=bundled.ll
>>> --outputs=host.in.ll,device.in.ll
>>> --targets=host-x86_64-unknown-linux-gnu,openmp-x86_64-unknown-linux-gnu
>>> --type=ll --unbundle
>>> opt -o host.opt.ll -O3 host.in.ll
>>> llc -o host.o -filetype=obj host.opt.ll
>>> opt -o device.opt.ll -O3 device.in.ll (custom pass should be added
>>> here later on)
>>> llc -o device.o -filetype=obj device.opt.ll
>> Afterwards, I tried to bundle the object files for host & device:
>>> clang-offload-bundler --inputs=host.o,device.o --outputs=app.o
>>> --targets=host-x86_64-unknown-linux-gnu,openmp-x86_64-unknown-linux-gnu
>>> --type=o
>> This command completes without any warning or error. 
>> I tried to invoke gcc on the output to generate the executable from the
>> object file (app.o):
>>> gcc app.o -lomp -lomptarget
>> However, I get the following error:
>>> /usr/bin/ld:
>>> app.o:(.rodata..omp_offloading.device_images[.omp_offloading.descriptor_reg.x86_64-unknown-linux-gnu]+0x10):
>>> undefined reference to `.omp_offloading.entries_begin'
>>> /usr/bin/ld:
>>> app.o:(.rodata..omp_offloading.device_images[.omp_offloading.descriptor_reg.x86_64-unknown-linux-gnu]+0x18):
>>> undefined reference to `.omp_offloading.entries_end'
>>> /usr/bin/ld:
>>> app.o:(.rodata..omp_offloading.descriptor[.omp_offloading.descriptor_reg.x86_64-unknown-linux-gnu]+0x10):
>>> undefined reference to `.omp_offloading.entries_begin'
>>> /usr/bin/ld:
>>> app.o:(.rodata..omp_offloading.descriptor[.omp_offloading.descriptor_reg.x86_64-unknown-linux-gnu]+0x18):
>>> undefined reference to `.omp_offloading.entries_end'
>> Do I need to run any additional steps before going through gcc or pass
>> additional flags? Is there another way of accomplishing the desired
>> behavior?
>
> I think you should use clang with the full -fopenmp
> -fopenmp-targets=x86_64-unknown-linux-gnu to build the final
> executable, ie
> $ clang -fopenmp -fopenmp-targets=x86_64-unknown-linux-gnu app.o
> If I remember correctly, the above symbols are provided by a linker
> script in Clang 9.0, which gcc has no clue about.
>
> Please note that there have been recent changes in that area and trunk
> now relies on a new tool clang-offload-wrapper AFAICS
> (see https://github.com/llvm/llvm-project/commit/5836c356fa6e17d0e10a2f9e0e111b7236dc15fb).
> As before this should be called transparently by clang, as long as
> it's correctly invoked with a consistent value for -fopenmp-targets.
>
> I haven't tested above command, but I hope it works. Please let me
> know if it doesn't and I'll take a closer look!
> Jonas
>
>> Thanks a lot in advance,
>> Best regards
>> Lukas
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