[cfe-dev] [RFC][OpenMP] Usability improvement, allow dropping offload targets

[Hal Finkel via cfe-dev]

Hi, Serguei,

I agree that this is a problem worth solving. Two questions:

 1. Is there any interaction between your proposal and
https://reviews.llvm.org/D47394, and if so, how might that work?

 2. Might it be possible to solve this problem by providing weak
versions of the required symbols, and then having the linker find the
strong versions when linking for each target requested when linking the
executable?

Thanks,

Hal


On 07/30/2018 06:50 PM, Dmitriev, Serguei N via cfe-dev wrote:
> Motivation
>
> The existing OpenMP offloading implementation in clang does not allow dropping
> offload targets at link time. That is, if an object file is created with one set
> of offload targets you must use exactly the same set of offload targets at the
> link stage. Otherwise, linking will fail
>
> $ clang -fopenmp -fopenmp-targets=x86_64-pc-linux-gnu,nvptx64-nvidia-cuda foo.c -c
> $ clang -fopenmp -fopenmp-targets=x86_64-pc-linux-gnu foo.o
> /tmp/foo-dd79f7.o:(.rodata..omp_offloading.device_images[.omp_offloading.descriptor_reg]+0x20): undefined reference to `.omp_offloading.img_start.nvptx64-nvidia-cuda'
> /tmp/foo-dd79f7.o:(.rodata..omp_offloading.device_images[.omp_offloading.descriptor_reg]+0x28): undefined reference to `.omp_offloading.img_end.nvptx64-nvidia-cuda'
> clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
> $ 
>
> This limits OpenMP offload usability. So far, this has not been a high priority
> issue but the importance of this problem will grow once clang offload starts
> supporting static libraries with offload functionality. For instance, this
> limitation won't allow creating general purpose static libraries targeting
> multiple types of offload devices and later linking them into a program that
> uses only one offload target.
>
> Problem description
>
> Offload targets cannot be dropped at the link phase because object files
> produced by the compiler for the host have dependencies on the offload targets
> specified during compilation. These dependencies arise from the offload
> initialization code.
>
> The clang front-end adds offload initialization code to each host object in
> addition to all necessary processing of OpenMP constructs. This initialization
> code is intended to register target binaries for all offload targets in the
> runtime library at program startup. This code consists of two compiler-generated
> routines. One of these routines is added to the list of global constructors and
> the other to the global destructors. The constructor routine calls a
> libomptarget API which registers the target binaries and the destructor
> correspondingly calls a similar API for unregistering target binaries.
>
> Both these APIs accept a pointer to the target binary descriptor object which
> specifies the number of offload target binaries to register and the start/end
> addresses of target binary images. Since the start/end addresses of target
> binaries are not available at compile time, the target binary descriptors are
> initialized using link-time constants which reference (undefined) symbols
> containing the start/end addresses of all target images. These symbols are
> created by the dynamically-generated linker script which the clang driver
> creates for the host link action.
>
> References to the target specific symbols from host objects make them dependent
> on particular offload targets and prevents dropping offload targets at the link
> step. Therefore, the OpenMP offload initialization needs to be redesigned to
> make offload targets discardable.
>
> Proposed change
>
> Host objects should be independent of offload targets in order to allow dropping
> code for offload targets. That can be achieved by removing offload
> initialization code from host objects. The compiler should not inject this code
> into host objects.
>
> However, offload initialization should still be done, so it is proposed to move
> the initialization code into a special dynamically generated object file
> (referred to as 'wrapper object' here onwards), which, besides the
> initialization code, will also contain embedded images for offload targets.
>
> The wrapper object file will be generated by the clang driver with the help of
> a new tool: clang-offload-wrapper. This tool will take offload target binaries
> as input and produce bitcode files containing offload initialization code and
> embedded target images. The output bitcode is then passed to the backend and
> assembler tools from the host toolchain to produce the wrapper object which is
> then added as an input to the linker for host linking.
>
> The offload action builder in the clang driver needs to be changed to use this
> tool while building the actions graph for OpenMP offload compilations.
>
> Current status
>
> A patch with initial implementation of the proposed changes has been uploaded to
> phabricator for review - https://reviews.llvm.org/D49510.
>
> Looking for a feedback for this proposal.
>
> Thanks,
> Sergey
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory