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[LinkerWrapper] Support relocatable linking for offloading (#80066)
Summary: The standard GPU compilation process embeds each intermediate object file into the host file at the `.llvm.offloading` section so it can be linked later. We also use a special section called something like `omp_offloading_entries` to store all the globals that need to be registered by the runtime. The linker-wrapper's job is to link the embedded device code stored at this section and then emit code to register the linked image and the kernels and globals in the offloading entry section. One downside to RDC linking is that it can become quite big for very large projects that wish to make use of static linking. This patch changes the support for relocatable linking via `-r` to support a kind of "partial" RDC compilation for offloading languages. This primarily requires manually editing the embedded data in the output object file for the relocatable link. We need to rename the output section to make it distinct from the input sections that will be merged. We then delete the old embedded object code so it won't be linked further. We then need to rename the old offloading section so that it is private to the module. A runtime solution could also be done to defer entries that don't belong to the given GPU executable, but this is easier. Note that this does not work with COFF linking, only the ELF method for handling offloading entries, that could be made to work similarly. Given this support, the following compilation path should produce two distinct images for OpenMP offloading. ``` $ clang foo.c -fopenmp --offload-arch=native -c $ clang foo.c -lomptarget.devicertl --offload-link -r -o merged.o $ clang main.c merged.o -fopenmp --offload-arch=native $ ./a.out ``` Or similarly for HIP to effectively perform non-RDC mode compilation for a subset of files. ``` $ clang -x hip foo.c --offload-arch=native --offload-new-driver -fgpu-rdc -c $ clang -x hip foo.c -lomptarget.devicertl --offload-link -r -o merged.o $ clang -x hip main.c merged.o --offload-arch=native --offload-new-driver -fgpu-rdc $ ./a.out ``` One question is whether or not this should be the default behavior of `-r` when run through the linker-wrapper or a special option. Standard `-r` behavior is still possible if used without invoking the linker-wrapper and it guaranteed to be correct.
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