[LoopDeletion] Break backedge of loops when known not taken

The basic idea is that if SCEV can prove the backedge isn't taken, we can go ahead and get rid of the backedge (and thus the loop) while leaving the rest of the control in place. This nicely handles cases with dispatch between multiple exits and internal side effects.

Differential Revision: https://reviews.llvm.org/D93906

1 files changed, 31 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 96f1d42..4f98995 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -756,6 +756,37 @@ void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
   }
 }
 
+void llvm::breakLoopBackedge(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
+                             LoopInfo &LI, MemorySSA *MSSA) {
+
+  auto *Latch = L->getLoopLatch();
+  assert(Latch);
+  auto *Header = L->getHeader();
+
+  SE.forgetLoop(L);
+
+  // Note: By splitting the backedge, and then explicitly making it unreachable
+  // we gracefully handle corner cases such as non-bottom tested loops and the
+  // like.  We also have the benefit of being able to reuse existing well tested
+  // code.  It might be worth special casing the common bottom tested case at
+  // some point to avoid code churn.
+
+  std::unique_ptr<MemorySSAUpdater> MSSAU;
+  if (MSSA)
+    MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
+
+  auto *BackedgeBB = SplitEdge(Latch, Header, &DT, &LI, MSSAU.get());
+
+  DomTreeUpdater DTU(&DT, DomTreeUpdater::UpdateStrategy::Eager);
+  (void)changeToUnreachable(BackedgeBB->getTerminator(), /*UseTrap*/false,
+                            /*PreserveLCSSA*/true, &DTU, MSSAU.get());
+
+  // Erase (and destroy) this loop instance.  Handles relinking sub-loops
+  // and blocks within the loop as needed.
+  LI.erase(L);
+}
+
+
 /// Checks if \p L has single exit through latch block except possibly
 /// "deoptimizing" exits. Returns branch instruction terminating the loop
 /// latch if above check is successful, nullptr otherwise.