Implementation design for `sycl::any_device_has` and `sycl::all_devices_have`¶

This design document describes the implementation of the SYCL 2020 device aspect traits any_device_has and all_devices_have as described in the SYCL 2020 Specification Rev. 6 Section 4.6.4.3.

In summary, any_device_has<aspect> and all_devices_have<aspect> must inherit from either std::true_type or std::false_type depending on whether the corresponding compilation environment can guarantee that any and all the supported devices support the aspect.

The design of these traits is inspired by the implementation of the sycl_ext_oneapi_device_if and sycl_ext_oneapi_device_architecture extensions as described in DeviceIf.md. Additionally, it leverages part of the design for optional kernel features, as described in OptionalDeviceFeatures.md.

Changes to the compiler driver¶

Using the -fsycl-targets options introduced in DeviceIf.md and the configuration file introduced in OptionalDeviceFeatures.md, the compiler driver finds the set of all aspects supported by each specified target. Note that in this section we refer to aspects as their integral representation as specified in the device headers rather than by the names specified in the SYCL 2020 specification.

For each target $t$ in -fsycl-targets, let $A^{any}_t$ be the set of aspects supported by any device supporting $t$ and let $A^{all}_t$ be the set of aspects supported by all devices supporting $t$. These sets are defined as follows:

If $t$ has an entry in the configuration file, $A^{all}_t$ is the same as the aspects list in that entry.
If $t$ does not have an entry in the configuration file, $A^{all}_t$ is empty.
If $t$ has an entry in the configuration file and the entry has a value may_support_other_aspects set to false, $A^{any}_t$ is the same as the aspects list in that entry.
If $t$ does not have an entry the configuration file or the entry has a value may_support_other_aspects set to true, $A^{any}_t$ is the set of all aspects.

For example, the target intel_gpu_dg1 is supported by a specific device (DG1) and as such would have an entry in the configuration file with aspects being the set of aspects that device supports. Likewise it would have may_support_other_aspects set to false as there will be no other devices supporting this target, meaning there will never be any devices supporting the target and supporting anything not in aspects. In contrast, the target nvidia_gpu_sm_80 is supported by CUDA devices with sm_80 architecture or newer, so its entry in the configuration file would have may_support_other_aspects set to true to indicate that there could be future devices that support aspects not in aspects, while it is known that all current and future devices must support the aspects in aspects. Lastly, the default JIT SPIR-V target (spir64) should not have an entry in the configuration file as it cannot guarantee anything about the devices supporting the target.

When compiling a SYCL program, where $[t1, t2, \ldots, tn]$ are the $n$ targets specified in -fsycl-targets including any targets implicitly added by the driver, the driver defines the following macros in both host and device compilation invocations:

__SYCL_ALL_DEVICES_HAVE_$aspectName_{i}$__ as 1 for all $i$ in ${\bigcap}^n_{k=1} A^{all}_{tk}$.
__SYCL_ANY_DEVICE_HAS_ANY_ASPECT__ as 1 if ${\bigcup}^n_{k=1} A^{any}_{tk}$ is the set of all aspects.
__SYCL_ANY_DEVICE_HAS_$aspectName_{j}$__ as 1 for all $j$ in ${\bigcup}^n_{k=1} A^{any}_{tk}$ if __SYCL_ANY_DEVICE_HAS_ANY_ASPECT__ was not defined.

Note that the need for the __SYCL_ANY_DEVICE_HAS_ANY_ASPECT__ macro is due to the special case where the driver finds no configuration for a target and must assume that there exists some device that supports any given aspect. Since the driver has no way of knowing all possible aspects, we use a catch-all macro to denote this case instead. This is not needed for $A^{all}_t$ for any target $t$, as it will always be a finite set of aspects.

Changes to the device headers¶

Using the macros defined by the driver, the device headers define the traits together with specializations for each aspect:

namespace sycl {
template <aspect Aspect> all_devices_have;
template<> all_devices_have<aspect::host> : std::bool_constant<__SYCL_ALL_DEVICES_HAVE_host__> {};
template<> all_devices_have<aspect::cpu> : std::bool_constant<__SYCL_ALL_DEVICES_HAVE_cpu__> {};
template<> all_devices_have<aspect::gpu> : std::bool_constant<__SYCL_ALL_DEVICES_HAVE_gpu__> {};
...

#ifdef __SYCL_ANY_DEVICE_HAS_ANY_ASPECT__
// Special case where any_device_has is trivially true.
template <aspect Aspect> any_device_has : std::true_type {};
#else
template <aspect Aspect> any_device_has;
template<> any_device_has<aspect::host> : std::bool_constant<__SYCL_ANY_DEVICE_HAS_host__> {};
template<> any_device_has<aspect::cpu> : std::bool_constant<__SYCL_ANY_DEVICE_HAS_cpu__> {};
template<> any_device_has<aspect::gpu> : std::bool_constant<__SYCL_ANY_DEVICE_HAS_gpu__> {};
...
#endif // __SYCL_ANY_DEVICE_HAS_ANY_ASPECT__

template <aspect Aspect> constexpr bool all_devices_have_v = all_devices_have<Aspect>::value;
template <aspect Aspect> constexpr bool any_device_has_v = any_device_has<Aspect>::value;
} // namespace sycl

Note that the driver may not define macros for all aspects as it only knows the specified subset from the configuration file. As such the device headers will have to define any undefined __SYCL_ANY_DEVICE_HAS_$aspectName_{i}$__ and __SYCL_ALL_DEVICES_HAVE_$aspectName_{i}$__ as 0 for all aspect values $i$.

Since the specializations need to be explicitly specified, there is a high probability of mistakes when new aspects are added. To avoid such mistakes, a SYCL unit-test uses the aspects.def file to generate test cases, ensuring that specializations exist for all aspects:

#define __SYCL_ASPECT(ASPECT, ASPECT_VAL)                                          \
  constexpr bool CheckAnyDeviceHas##ASPECT = any_devices_has_v<aspect::ASPECT>;    \
  constexpr bool CheckAllDevicesHave##ASPECT = all_devices_have_v<aspect::ASPECT>;

#include <sycl/info/aspects.def>

#undef __SYCL_ASPECT

This relies on the fact that unspecialized variants of any_device_has and all_devices_have are undefined.

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