Which proposal does this relate to?

https://github.com/microsoft/hlsl-specs/blob/main/proposals/0027-shader-execution-reordering.md#maybereorderthread

Describe the issue or outstanding question.

This is an implementation issue that needs a spec change. Quoting #479:

One problem is that an undef value could arise from a coding error or optimization problem, and end up looking like a legitimate call to the intrinsic without the HitObject, rather than being caught by the validator as an undef value when a valid value should have been provided.

Another issue is that undef values are replaced with poison values in latest LLVM, and these are supposed to indicate a bad value that should not be used. We shouldn't be determining whether or not an operation uses a value (and does something different) on the basis of whether or not the value in question is poison.

Solution

Rephrasing to something like the below. This is the consensus solution for #479.

Operation that reorders the current thread based on the supplied hints and HitObject. The HLSL overload without HitObject is lowered by passing a NOP HitObject (HitObject_MakeNop). The HLSL overload without coherence hit is lowered by passing 0 for num of coherence hint bits from LSB.