[6.9] Failing to constant evaluate @dx.op.vectorReduce.*

[llvm-beanz]

Description
A user reported a shader that failed DXIL validation when targeting SM 6.9 for a shader that worked when targeting SM 6.8. The shader has a likely unnoticed logic error (a dead loop) that was optimized away in SM 6.8 due to evaluating scalarized constant operations. When targeting SM 6.9, DXC fails to constant-fold the vector reduce operation, which leaves the dead loop with no exit causing a DXIL validation error.

Steps to Reproduce

RWStructuredBuffer<uint> Out;

[numthreads(1, 1, 1)]
void main(uint3 dtid : SV_DispatchThreadID) {
  uint4 mask = uint4(0, 0, 0, 0);
  uint4 lane = uint4(1, 2, 4, 8);
  uint count = 0;
  [loop]
  while (any(mask & lane)) {
    mask ^= uint4(0, 0, 0, 0);
    count += 1;
  }
  Out[dtid.x] = count;
}

Compiler Explorer

Actual Behavior
While this code is clearly a logic error, it shouldn't generate invalid DXIL.

Environment

• DXC version: 1.10*
• Host Operating System: Any