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

Alexey Bataev via cfe-dev cfe-dev at lists.llvm.org
Tue Jul 31 09:55:14 PDT 2018


Hi Serguei,

Actually your problem can be fixed easily with a simple patch that
changes the linkage of the

`.omp_offloading.img_[start|end]` symbols from external to external weak. After this change your example compiles and works perfectly without any additional changes. I'm going to commit this patch in few minutes.

-------------
Best regards,
Alexey Bataev

30.07.2018 19:50, Dmitriev, Serguei N via cfe-dev пишет:
> 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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