[HLSL] `select` not resolving correctly for complex cases

[llvm-beanz]

HLSL currently allows much more flexible overloading of the select function. The following is an example of a case that DXC supports which Clang does not.

typedef uint32_t4 uint32_t8[2];

export void filter(inout uint32_t4 output, uint32_t predicate, uint32_t value) {
  output = select(uint4(0,1,2,3) == (uint4)predicate, value, output);
}

Compiler Explorer

[damyanp]

This likely needs us to rethink how select is implemented. Another option might be prohibit this.

[llvm-beanz]

I'm assigning this to myself to take into a design discussion and resolve.

My current position is that we don't need to do anything here, the code example provided is ambiguous and Clang is correctly reporting the ambiguity.

DXC's definition of the select intrinsic expects that the second and third arguments be the same type in all valid overloads:

$type2 [[rn]] select(in bool<> cond, in $match<1, 2> any<> t, in $type2 f);
$type2 [[rn]] select(in bool cond, in any_sampler t, in $type2 f);

The error reported by Clang is effectively that since the second and third arguments are not the same type all possible overloads are ambiguous.

We already determined that using a C++-like overload best match algorithm that reported ambiguity rather than DXC's scoring algorithm that disambiguated otherwise ambiguous cases was the correct approach for HLSL.

Any change we make here would either be revisiting that decision or deciding to treat select as somehow special to give it overload resolution behavior that is different from other functions.

The source updates required to make this code valid in both Clang and DXC is trivial, changing value to value.xxxx results in DXC and Clang both being able to compile the select call to the same correct unambiguous output.

[pow2clk]

I think select is distinct from other conversations about overload resolution because it is intended to replace ?:. These same arguments to select are allowed as operands for ?: in clang and DXC pre-2021: https://godbolt.org/z/hvWb37xoe