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Dean Galvin
d37463e76f
MaskedSelectEliminator walked forward from each select to see whether the
lanes its mask excludes are read. Since fda4aa90 the walks pass explicit-LOD
gathers, size queries and guarded image writes, so in a large pixel shader
most of them cover the rest of the program, and the cost grows with the
square of its size (#3504).
The pass now computes, for every value, the set of masks under which it is
unread, from its uses backwards: a use guarded by a condition contributes the
masks that condition implies, a lane-local use adds what its result is unread
under, and any other use contributes nothing. A select is dropped when its
mask is in its own set. Loops are solved as strongly connected components.
The rules for guards, lane-local uses, loops, whole quad masks, selected
execs, rebuilt mask words and guarded first-lane reads are the ones the walk
applied. A first-lane read passes the masks its read mask implies when its
own result is unread outside that read mask, which is solved once per read in
the same way. The walk's step limit, its 64-level implication depth and its
first-lane nesting depth are gone, so a proof no longer depends on the order
of the questions.