[LoopUnroll] Eliminate PreserveCondBr parameter and fix a bug in the process

This builds on D103584. The change eliminates the coupling between unroll heuristic and implementation w.r.t. knowing when the passed in trip count is an exact trip count or a max trip count. In theory the new code is slightly less powerful (since it relies on exact computable trip counts), but in practice, it appears to cover all the same cases. It can also be extended if needed.

The test change shows what appears to be a bug in the existing code around the interaction of peeling and unrolling. The original loop only ran 8 iterations. The previous output had the loop peeled by 2, and then an exact unroll of 8. This meant the loop ran a total of 10 iterations which appears to have been a miscompile.

Differential Revision: https://reviews.llvm.org/D103620

1 files changed, 1 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
index cc85fb6..767b072 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -986,8 +986,7 @@ bool llvm::UnrollRuntimeLoopRemainder(
         UnrollLoop(remainderLoop,
                    {/*Count*/ Count - 1, /*TripCount*/ Count - 1,
                     /*Force*/ false, /*AllowRuntime*/ false,
-                    /*AllowExpensiveTripCount*/ false, /*PreserveCondBr*/ true,
-                    /*TripMultiple*/ 1,
+                    /*AllowExpensiveTripCount*/ false, /*TripMultiple*/ 1,
                     /*PeelCount*/ 0, /*UnrollRemainder*/ false, ForgetAllSCEV},
                    LI, SE, DT, AC, TTI, /*ORE*/ nullptr, PreserveLCSSA);
   }