[PATCH] D21720: Unroll for uncountable loops

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

Currently, there's a profitability check hidden in llvm::UnrollLoop: if the "Force" parameter is false, it will refuse to unroll any loop unless it can make the loop latch in the unrolled iterations an unconditional branch.  (See the debug message "Wont unroll; remainder loop could not be generated" in lib/Transforms/Utils/LoopUnroll.cpp.)  This prevents the unroll pass from going crazy unrolling every loop in the program, but it's also not a very good way to measure profitability.

Ideally, we want to tie unrolling to a dynamic cost discount.  If the unrolled loop is substantially cheaper than the original, we should unroll, whether or not the savings come from eliminating the conditional branch in the latch.  Conversely, unrolling a large loop just to eliminate one branch is probably a bad idea.

This patch is trying to solve the problem for certain cases... but the approach is kind of awkward: rather than actually moving the profitability check, it adds a special case to set the "Force" bit for certain loops.


Repository:
  rL LLVM

https://reviews.llvm.org/D21720