[SYCL][InvokeSIMD] Add error for return type and subgroup size mismatch

[sarnex]

The spec doesn't allow this, but right now we don't error at all.

[sarnex]

Test PR: intel/llvm-test-suite#1679

[sarnex]

/summary:run

[v-klochkov]

The arguments need to pass similar check:
if sg-size is N, and one of operands is simd<T, N*2>, then it is the reportable error as well.

[sarnex]

Yeah I originally implemented that, but it turns out we already check for this when picking a subgroup size.

Example:

void SIMD_CALLEE( simd<float, VL> b,
                               simd<float, VL*2> c )  SYCL_ESIMD_FUNCTION {

Error:

error: static assertion failed due to requirement '__MP11_NS::integral_constant<unsigned long, 0>::value == 1'
    static_assert((__MP11_NS::mp_size<InvocableSgSizes>::value == 1) &&
                  "no or multiple invoke_simd targets found");

We didn't do the return type check there because we need the finalized subgroup size for the return type.

[v-klochkov]

I see, thank you for explanations.
The comment on the line 361 concerns me a bit.
It says 0 is fine. If N is 0 (i.e. all parameters are uniform), then verify_return_type_matches_sg_size would fail, right? If Yes, then this special case needs to be handled specially in the static_assert that was added in this PR.

[sarnex]

It shouldn't fail because in the case where all arguments are uniform but the return type is simd or simd mask, we pick the return type VL as subgroup size already. See the below code:

if constexpr (all_uniform_types<SpmdArgs...>()) {
    using SimdRet = std::invoke_result_t<SimdCallable, SpmdArgs...>;

    if constexpr (is_simd_or_mask_type<SimdRet>::value) {
      return simd_size<SimdRet>::value;
    } else {
      // fully uniform function - subgroup size does not matter
      return 0;
    }

So I don't think there is a way to hit the added static_assert because sg size will never be 0 if we go into the constexpr if I added