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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Hi Alexey,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Thanks,<o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Serguei<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><a name="_____replyseparator"></a><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext">
Alexey Bataev [mailto:a.bataev@outlook.com] <br>
<b>Sent:</b> Wednesday, August 1, 2018 11:45 AM<br>
<b>To:</b> Dmitriev, Serguei N <serguei.n.dmitriev@intel.com><br>
<b>Cc:</b> cfe-dev@lists.llvm.org; Hal Finkel <hfinkel@anl.gov><br>
<b>Subject:</b> Re: [cfe-dev] [RFC][OpenMP] Usability improvement, allow dropping offload targets<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p>Hi Serguei,<o:p></o:p></p>
<p>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.
<o:p></o:p></p>
<pre>-------------<o:p></o:p></pre>
<pre>Best regards,<o:p></o:p></pre>
<pre>Alexey Bataev<o:p></o:p></pre>
<div>
<p class="MsoNormal">31.07.2018 16:50, Dmitriev, Serguei N пишет:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Hi Alexey,</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Such change would fix the link issue, but I believe it would be a short term solution that will still be revised in future.</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda -c a.c</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">clang -fopenmp -fopenmp-targets=x86_64-pc-linux-gnu -c b.c</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda,x86_64-pc-linux-gnu a.o b.o</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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.</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Does that sound reasonable?</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
<div>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks,</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Serguei</span><o:p></o:p></p>
</div>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"> </span><o:p></o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"> Alexey Bataev [<a href="mailto:a.bataev@outlook.com">mailto:a.bataev@outlook.com</a>]
<br>
<b>Sent:</b> Tuesday, July 31, 2018 9:55 AM<br>
<b>To:</b> Dmitriev, Serguei N <a href="mailto:serguei.n.dmitriev@intel.com"><serguei.n.dmitriev@intel.com></a>;
<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [cfe-dev] [RFC][OpenMP] Usability improvement, allow dropping offload targets</span><o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"><span lang="EN-US"> </span><o:p></o:p></p>
<p><span lang="EN-US">Hi Serguei,</span><o:p></o:p></p>
<p><span lang="EN-US">Actually your problem can be fixed easily with a simple patch that changes the linkage of the
</span><o:p></o:p></p>
<pre><span lang="EN-US">`.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.</span><o:p></o:p></pre>
<pre><span lang="EN-US">-------------</span><o:p></o:p></pre>
<pre><span lang="EN-US">Best regards,</span><o:p></o:p></pre>
<pre><span lang="EN-US">Alexey Bataev</span><o:p></o:p></pre>
<div>
<p class="MsoNormal"><span lang="EN-US">30.07.2018 19:50, Dmitriev, Serguei N via cfe-dev
</span>пишет<span lang="EN-US">:</span><o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<pre><span lang="EN-US">Motivation</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">The existing OpenMP offloading implementation in clang does not allow dropping</span><o:p></o:p></pre>
<pre><span lang="EN-US">offload targets at link time. That is, if an object file is created with one set</span><o:p></o:p></pre>
<pre><span lang="EN-US">of offload targets you must use exactly the same set of offload targets at the</span><o:p></o:p></pre>
<pre><span lang="EN-US">link stage. Otherwise, linking will fail</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">$ clang -fopenmp -fopenmp-targets=x86_64-pc-linux-gnu,nvptx64-nvidia-cuda foo.c -c</span><o:p></o:p></pre>
<pre><span lang="EN-US">$ clang -fopenmp -fopenmp-targets=x86_64-pc-linux-gnu foo.o</span><o:p></o:p></pre>
<pre><span lang="EN-US">/tmp/foo-dd79f7.o:(.rodata..omp_offloading.device_images[.omp_offloading.descriptor_reg]+0x20): undefined reference to `.omp_offloading.img_start.nvptx64-nvidia-cuda'</span><o:p></o:p></pre>
<pre><span lang="EN-US">/tmp/foo-dd79f7.o:(.rodata..omp_offloading.device_images[.omp_offloading.descriptor_reg]+0x28): undefined reference to `.omp_offloading.img_end.nvptx64-nvidia-cuda'</span><o:p></o:p></pre>
<pre><span lang="EN-US">clang-7: error: linker command failed with exit code 1 (use -v to see invocation)</span><o:p></o:p></pre>
<pre><span lang="EN-US">$ </span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">This limits OpenMP offload usability. So far, this has not been a high priority</span><o:p></o:p></pre>
<pre><span lang="EN-US">issue but the importance of this problem will grow once clang offload starts</span><o:p></o:p></pre>
<pre><span lang="EN-US">supporting static libraries with offload functionality. For instance, this</span><o:p></o:p></pre>
<pre><span lang="EN-US">limitation won't allow creating general purpose static libraries targeting</span><o:p></o:p></pre>
<pre><span lang="EN-US">multiple types of offload devices and later linking them into a program that</span><o:p></o:p></pre>
<pre><span lang="EN-US">uses only one offload target.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Problem description</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Offload targets cannot be dropped at the link phase because object files</span><o:p></o:p></pre>
<pre><span lang="EN-US">produced by the compiler for the host have dependencies on the offload targets</span><o:p></o:p></pre>
<pre><span lang="EN-US">specified during compilation. These dependencies arise from the offload</span><o:p></o:p></pre>
<pre><span lang="EN-US">initialization code.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">The clang front-end adds offload initialization code to each host object in</span><o:p></o:p></pre>
<pre><span lang="EN-US">addition to all necessary processing of OpenMP constructs. This initialization</span><o:p></o:p></pre>
<pre><span lang="EN-US">code is intended to register target binaries for all offload targets in the</span><o:p></o:p></pre>
<pre><span lang="EN-US">runtime library at program startup. This code consists of two compiler-generated</span><o:p></o:p></pre>
<pre><span lang="EN-US">routines. One of these routines is added to the list of global constructors and</span><o:p></o:p></pre>
<pre><span lang="EN-US">the other to the global destructors. The constructor routine calls a</span><o:p></o:p></pre>
<pre><span lang="EN-US">libomptarget API which registers the target binaries and the destructor</span><o:p></o:p></pre>
<pre><span lang="EN-US">correspondingly calls a similar API for unregistering target binaries.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Both these APIs accept a pointer to the target binary descriptor object which</span><o:p></o:p></pre>
<pre><span lang="EN-US">specifies the number of offload target binaries to register and the start/end</span><o:p></o:p></pre>
<pre><span lang="EN-US">addresses of target binary images. Since the start/end addresses of target</span><o:p></o:p></pre>
<pre><span lang="EN-US">binaries are not available at compile time, the target binary descriptors are</span><o:p></o:p></pre>
<pre><span lang="EN-US">initialized using link-time constants which reference (undefined) symbols</span><o:p></o:p></pre>
<pre><span lang="EN-US">containing the start/end addresses of all target images. These symbols are</span><o:p></o:p></pre>
<pre><span lang="EN-US">created by the dynamically-generated linker script which the clang driver</span><o:p></o:p></pre>
<pre><span lang="EN-US">creates for the host link action.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">References to the target specific symbols from host objects make them dependent</span><o:p></o:p></pre>
<pre><span lang="EN-US">on particular offload targets and prevents dropping offload targets at the link</span><o:p></o:p></pre>
<pre><span lang="EN-US">step. Therefore, the OpenMP offload initialization needs to be redesigned to</span><o:p></o:p></pre>
<pre><span lang="EN-US">make offload targets discardable.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Proposed change</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Host objects should be independent of offload targets in order to allow dropping</span><o:p></o:p></pre>
<pre><span lang="EN-US">code for offload targets. That can be achieved by removing offload</span><o:p></o:p></pre>
<pre><span lang="EN-US">initialization code from host objects. The compiler should not inject this code</span><o:p></o:p></pre>
<pre><span lang="EN-US">into host objects.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">However, offload initialization should still be done, so it is proposed to move</span><o:p></o:p></pre>
<pre><span lang="EN-US">the initialization code into a special dynamically generated object file</span><o:p></o:p></pre>
<pre><span lang="EN-US">(referred to as 'wrapper object' here onwards), which, besides the</span><o:p></o:p></pre>
<pre><span lang="EN-US">initialization code, will also contain embedded images for offload targets.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">The wrapper object file will be generated by the clang driver with the help of</span><o:p></o:p></pre>
<pre><span lang="EN-US">a new tool: clang-offload-wrapper. This tool will take offload target binaries</span><o:p></o:p></pre>
<pre><span lang="EN-US">as input and produce bitcode files containing offload initialization code and</span><o:p></o:p></pre>
<pre><span lang="EN-US">embedded target images. The output bitcode is then passed to the backend and</span><o:p></o:p></pre>
<pre><span lang="EN-US">assembler tools from the host toolchain to produce the wrapper object which is</span><o:p></o:p></pre>
<pre><span lang="EN-US">then added as an input to the linker for host linking.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">The offload action builder in the clang driver needs to be changed to use this</span><o:p></o:p></pre>
<pre><span lang="EN-US">tool while building the actions graph for OpenMP offload compilations.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Current status</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">A patch with initial implementation of the proposed changes has been uploaded to</span><o:p></o:p></pre>
<pre><span lang="EN-US">phabricator for review - </span><a href="https://reviews.llvm.org/D49510"><span lang="EN-US">https://reviews.llvm.org/D49510</span></a><span lang="EN-US">.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre><span lang="EN-US">Looking for a feedback for this proposal.</span><o:p></o:p></pre>
<pre><span lang="EN-US"> </span><o:p></o:p></pre>
<pre>Thanks,<o:p></o:p></pre>
<pre>Sergey<o:p></o:p></pre>
<pre> <o:p></o:p></pre>
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