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[NFC] Describe sycl-post-link support for dynamic linking #14337

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[NFC] Describe sycl-post-link support for dynamic linking
Jun 27, 2024
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Describe sycl-post-link changes more clearly
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32 changes: 27 additions & 5 deletions sycl/doc/design/SharedLibraries.md
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Expand Up @@ -148,12 +148,34 @@ For this purpose DPC++ front-end generates `module-id` attribute on each

### sycl-post-link changes

In order to support dynamic linking of device code, `sycl-post-link` performs
2 main tasks:
When device code splitting is performed during a typical compilation of a SYCL offloading program,
all dependent device code is brought into a split image in order to construct a complete image.
When device code splitting is performed during the compilation flow to generate a dynamic library,
construction of a complete device image is not possible since the dynamic library may rely on
functions that are not defined in the library. Thus, when supporting dynamic linking a dependency
to a `canBeImportedFunction` will not cause the dependent function to be added to the image.

`canBeImportedFunction` is a boolean function accepting a Function input and returns true
if the Function "can be imported". A `canBeImportedFunction` is:

1. Not an intrinsic
2. Name does not start with "__"
3. Demangled name does not start with "__"
4. Must be a `SYCL_EXTERNAL` function

More information about `SYCL_EXTERNAL` can be found in:
https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#subsec:syclexternal

In order to support dynamic linking of device code, `sycl-post-link` has
the following modifications:

- Device images that have a dependency to a `canBeImportedFunction` do not include the
function. Instead the dependency is recorded in the imported symbols property list.
- An image that provides a `canBeImportedFunction` has the symbol recorded in the exported
symbols property list.
- All functions symbols that are not `canBeImportedFunction` are internalized


- Supplies device images containing exports with an information about exported
symbols
- Supplies device images with an information about imported symbols

In addition, `SYCL_EXTERNAL` functions as well as kernels are considered as entry
points during device code split.
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