[SYCL][Doc] Add device code split feature design

sycl/doc/SYCLCompilerAndRuntimeDesign.md

@@ -394,6 +394,51 @@ llvm-no-spir-kernel host.bc
 
 It returns 0 if no kernels are present and 1 otherwise.
 
+#### Device code split
+
+Putting all device code into a single SPIRV module does not work well in the
+following cases:
+1. There are thousands of kernels defined and only small part of them is used at
+run-time. Having them all in one SPIR-V module significantly increases JIT time.
+2. Device code can be specialized for different devices. For example, kernels
+that are supposed to be executed only on FPGA can use extensions avaliable for
+FPGA only. This will cause JIT compilation failure on other devices even if this
+particular kernel is never called on them.
+
+To resolve these problems the compiler can split a single module into smaller
+ones. The following features is supported:
+* Emitting a separate module for source (translation unit)
+* Emitting a separate module for each kernel
+
+The current approach is:
+* Generate special meta-data with translation unit ID for each kernel in SYCL
+front-end. This ID will be used to group kernels on per-translation unit basis
+* Link all device LLVM modules using llvm-link
+* Perform split on a fully linked module
+* Generate a symbol table (list of kernels) for each produced device module for
+proper module selection in runtime
+* Perform SPIR-V translation and AOT compilation (if requested) on each produced
+module separately
+* Add information about presented kernels to a wrappring object for each device
+image
+
+Device code splitting process:
+![Device code splitting](images/DeviceCodeSplit.svg)
+
+The "split" box is implemented as functionality of the dedicated tool
+`sycl-post-link`. The tool runs a set of LLVM passes to split input module and
+generates a symbol table (list of kernels) for each produced device module.
+
+To enable device code split, a special option must be passed to the clang
+driver:
+
+`-fsycl-device-code-split=<value>`
+
+There are three possible values for this option:
+* `per_source` - enables emitting a separate module for each source (translation
+unit)
+* `per_kernel` - enables emitting a separate module for each kernel
+* `off` - disables device code split
 
 ### Integration with SPIR-V format