A select of exec and zero gives a mask that is the same for every lane. Reading
it back went through a lane number, which Metal does not have in a vertex
shader, so MoltenVK rejected the shader. The select now stores the lane bit and
marks the pair as a mask, a constant mask word reads as a constant, and the
constant folder removes the bit test on an all-zero or all-one word.
agc_scalar_select_mask runs the translated IR over a wave and pins the lane
values for scc set and clear, a source that differs per lane, the two wave64
halves differing, a destination that is also a source, and constant and
literal sources.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Morten Holm