Commits on Jan 3, 2021

1. [SSA] Fix bug in to_ssa.py …

   Fixes Issue sampsyo#108.
   
   The optimization in prune_phis() is not sound. phi nodes that are
   "partially undefined" cannot be removed. It is only illegal to read
   from the result if the second to last executed label corresponds to
   an undefined argument. `to_ssa.py` generated incorrect code for the
   two tests added, before the fix.
   
   In most cases, these phi nodes aren't read from, and will be removed
   by DCE. However, in the case where the phi nodes are read, a correct
   optimization would be:
   1. Detect partially undefined phi nodes whose value is always used.
      Let `undefined_labels` be the set of labels whose argument is
      undefined.
   2. Leverage the undefined behavior to say that `undefined_labels`
      are in fact not predecessors to the basic block.
      `preds = preds - undefined_labels`.
   Though, I'm not sure how useful that optimization would be in
   practice.

   

   terrelln committed Jan 3, 2021
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