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

[Dmitriev, Serguei N via cfe-dev]

Hi Alexey,

Empty object file produced by the bundler is one of the problems with that example, but I was talking about the different issue which is related to the offload initialization code.

Front end, as part of the generating offload initialization code, creates the target binary descriptor object which is passed to the libomptarget registration API. The binary descriptor object besides the start/end addresses of all target images contains the number of target images which need to be registered and compiler initializes this field to the number of offload targets that were specified in command line. So, in that example below, both a.o and b.o will have initialization code for registering only one target image because only one offload target was specified in command line. The initialization code is generated as a comdat group, so linker will choose either a.o’s or b.o’s instance of this code at link stage, but in any case it will register only one target image instead of two. So that example will not work as expected even if offload bundler problem is fixed. Delaying generation of the offload initialization code till link time would resolve this issue.

I suspect there would also be problems with the offload entry table besides these two issues. The number of offload entries on the host side and target images won’t match, and I guess libomptarget cannot handle this correctly, so I assume runtime would require some changes as well.

Thanks,
Serguei

From: Alexey Bataev [mailto:a.bataev at outlook.com]
Sent: Wednesday, August 1, 2018 11:45 AM
To: Dmitriev, Serguei N <serguei.n.dmitriev at intel.com>
Cc: cfe-dev at lists.llvm.org; Hal Finkel <hfinkel at anl.gov>
Subject: Re: [cfe-dev] [RFC][OpenMP] Usability improvement, allow dropping offload targets


Hi Serguei,

I don't see a lot of problems with this example. As I can see there is only one problem: clang-offload-bundler generates an empty object file that cannot be recognized by the linker. If we teach clang-offload-bundler to generate correct empty object files (what we need to do anyway, because currently it may produce wrong utout), your example will work without any changes.

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Best regards,

Alexey Bataev
31.07.2018 16:50, Dmitriev, Serguei N пишет:
Hi Alexey,

Such change would fix the link issue, but I believe it would be a short term solution that will still be revised in future.

Let me explain my concerns. Current implementation has one more limitation which I assume would also be addressed in future – target binaries are expected to have entries for all OpenMP target regions in the program, though it seems to be too restrictive. I assume there would be use cases when you would want to tune target regions in your program for particular targets and offloading to other targets would not make much sense for those regions (or probably won’t even be possible due to limitations of a particular target). It seems reasonable to compile those region only for the targets they were tuned for, thus I assume compiler will support the following usage model in future

clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda -c a.c
clang -fopenmp -fopenmp-targets=x86_64-pc-linux-gnu -c b.c
clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda,x86_64-pc-linux-gnu a.o b.o

And such usage model would anyway require redesigning the way how offload initialization code is generated. It has to be delayed till link time because the final set of offload targets that need to be registered in runtime would be known only at link step and thus compiler cannot create correct target binary descriptor object (which is a part of the offload initialization code) at compile time as it is done now.

Does that sound reasonable?

Thanks,
Serguei

From: Alexey Bataev [mailto:a.bataev at outlook.com]
Sent: Tuesday, July 31, 2018 9:55 AM
To: Dmitriev, Serguei N <serguei.n.dmitriev at intel.com><mailto:serguei.n.dmitriev at intel.com>; cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [RFC][OpenMP] Usability improvement, allow dropping offload targets


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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