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vkd3d-shader/hlsl: Implement loop unrolling.

Victor Chiletto requested to merge vitorhnn/vkd3d:loop-unrolling-submit into master

The general idea is:

  • Run copy prop initially, save the copy prop state.
  • Unroll one iteration, substitute all jumps by stores to synthetic control vars, move all instructions after the jumps to checks to those control vars.
  • Run copy prop on the iteration, using the previous iteration's or the root's state as the parent state.
  • Probe the copy prop state to check if the "loop_broken" control variable was set, stop unrolling if it true, otherwise continue.

Some stuff I'm less sure about and would love comments on:

  • The way I deferred bounds checking.
  • Splitting the iter block from the body. This was needed because otherwise copy prop marks variable that are changed in iter blocks (e.g "i++") as invalidated and does not propagate them to subsequent iterations.
  • Messing around with copy prop internals. I attempted two other similar solutions that were less invasive: running copy prop on the entire block after each unroll and running copy prop once, after unrolling to max_iterations, both were too slow to be viable.

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