[PATCH] D114650: [SCEV] Construct SCEV iteratively (WIP).

[Philip Reames via Phabricator via llvm-commits]

reames added a comment.

Putting this here only because the bug database is still readonly.

Random thought: We're discussing this as a SCEV cornercase, and it's probably worth fixing the case this exposes, but it's also worth noting here that the unroller is producing a horrendously bad form here.  It might be worth considering whether the unroller should be applying a peephole as it unrolls to avoid generating this massive and utterly useless IR.

Specifically for this case: if generating an unrolled IV increment with no other uses of the IV, eagerly fold the step increments if constant.  That is, instead of producing:

  %a = gep i8, i8 %p, i64 1
  %b = gep i8, i8 %b, i64 1
  %c = gep i8, i8 %c, i64 1
  %d = gep i8, i8 %d, i64 1

produce:

  %a = gep i8, i8 %p, i64 1
  %b = gep i8, i8 %p, i64 2
  %c = gep i8, i8 %p, i64 3
  %d = gep i8, i8 %p, i64 4
  (All of which but the last are dead.)

Doing this might be a worthwhile compile time optimization on it's own.


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D114650