[RFC] SYCL Driver enhancements


Support for the SYCL programming model requires a number of adjustments to the
compiler driver. These changes involve introducing enabling command line
options and adjusting the compilation toolchains to provide device-specific
compilations for corresponding host compilations. The behaviors are centered
around the compilation of source files. Consumption of objects and libraries that
contain SYCL kernels will be done by the existing clang-linker-wrapper that is
used for other languages with offloading support like OpenMP.

Command line options for enabling

A number of command line options will be introduced to enable the offloading
compilation for SYCL.

  • -fsycl-targets=<target>
    • Enables device compilation for each listed comma-separated target. Targets
      can consist of target triples and other device-specific representations.
      When not provided the default target is spir64.
  • -fsycl-device-only
    • Enables the ability to create only device code.
  • -fsycl-host-only
    • Enables the ability to create only host code.
  • -fsycl-host-compiler=<compiler>
    • Provide the ability to use a 3rd party compiler to perform the host
      compilation step.

The above options are to be used with the existing -fsycl option.

Any additional options that are useful for SYCL offloading compilation will be
prefixed with -fsycl-.

Introduction of SYCL specific offload action builder

An additional offloading target toolchain will be created to support the SYCL
device compilations. This will be unique to the existing OpenMP and other
offloading targets and will allow for the creation of unique toolchains specific
to each SYCL target.

Compilation behaviors

Using the -fsycl option will enable the multi-pass compilation approach. This
consists of a device-specific compilation and a corresponding host compilation.
The device compilation will also produce an integration header and integration
footer that used during the subsequent host compilation. Additional information
detailing the integration header and footer can be found in “[RFC] SYCL Host Compiler Integration Header and Footer”.

The ability to use a 3rd party compiler is available via the
-fsycl-host-compiler option. The argument provided points to the 3rd party
compiler that would be used for the host-specific portion of the compile. The
driver is responsible for constructing the compilation call to the 3rd party
compiler to contain the generated integration header and footer.

The -fsycl-device-only option provides the user the ability to create only
the device binary as opposed to generating a multi-targeted object as stated
above. Although the generated object file cannot be executed, it is useful to
allow developers to examine the device code for the offload regions.

Adversely, the -fsycl-host-only option creates the host binary as it would
be compiled in the multi-target aspect. This binary will be created with the
generated integration header and footer from the device side.

Additional details

Generation of the object in the two-step compilation scenario is considered a
multi-targeted object that will be consumed by the clang-linker-wrapper. See
[RFC] Offloading design for SYCL offload kind and SPIR targets” for details.

The integration will be using the existing clang-linker-wrapper functionality to
perform final device and host link steps.

Based on discussions on the other threads for SYCL, we might wish to table this discussion until the over-arching architecture (particularly around host/device separation) are complete.

That certainly seems reasonable. Most of that discussion is happening at [RFC] SYCL Host Compiler Integration Header and Footer.

As the discussion in [RFC] SYCL Host Compiler Integration Header and Footer is still continuing, I think some aspects of this RFC are still on hold for the time being. However, I believe we now have consensus against arbitrary host compiler support and so -fsycl-host-compiler= can be removed from this RFC.