main reads target.vertexSubgroups at request version 18 since #4470, so
reading the pixel primitive class at 18 as well read a version 18
request written by main one byte short. The class is now written and
read at version 19. agc_shader_memory loads a version 18 request as
main writes it, and it fails with "truncated data" when the class is
read at 18.
The composed word no longer reads the Layer builtin for bits 16-28.
Draws render to a single layer and guest layer exports are ignored, so
no stage writes Layer, and AMD's Windows driver drops the writes of a
fragment shader that reads it unwritten (#4800). The field is the
constant 0 and pixel_ancillary checks that the composed program has no
Layer read. The debt entry names the bits that have no source and are
assumed zero (2-7, 12-15, 29-31), and says that a change which makes
draws export a layer must restore the read.
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