The current TPF validator enforces that for each register involved
in a DCL_INDEX_RANGE instruction there must be a signature element
for that register and the DCL_INDEX_RANGE write mask. This is an
excessively strong request, and causes some shaders from The Falconeer
to be invalidly rejected.
The excessively strong check was needed to avoid triggering a bug
in the I/O normaliser. Since that bug is now solved, the check
can be relaxed.
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