[SimpleLoopUnswitch] Form dedicated exits after trivial unswitches.

Summary:
Form dedicated exits after trivial unswitches.
Fixes PR38737, PR38283.

Reviewers: chandlerc, fedor.sergeev

Subscribers: sanjoy, jlebar, uabelho, llvm-commits

Differential Revision: https://reviews.llvm.org/D51375

llvm-svn: 340871

1 files changed, 8 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index 5834b619046..8b67c470a35 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -302,10 +302,11 @@ static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
     formLCSSA(*OldContainingL, DT, &LI, nullptr);
 
     // We shouldn't need to form dedicated exits because the exit introduced
-    // here is the (just split by unswitching) preheader. As such, it is
-    // necessarily dedicated.
-    assert(OldContainingL->hasDedicatedExits() &&
-           "Unexpected predecessor of hoisted loop preheader!");
+    // here is the (just split by unswitching) preheader. However, after trivial
+    // unswitching it is possible to get new non-dedicated exits out of parent
+    // loop so let's conservatively form dedicated exit blocks and figure out
+    // if we can optimize later.
+    formDedicatedExitBlocks(OldContainingL, &DT, &LI, /*PreserveLCSSA*/ true);
   }
 }
 
@@ -482,6 +483,7 @@ static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
   if (FullUnswitch)
     hoistLoopToNewParent(L, *NewPH, DT, LI);
 
+  LLVM_DEBUG(dbgs() << "    done: unswitching trivial branch...\n");
   ++NumTrivial;
   ++NumBranches;
   return true;
@@ -539,7 +541,7 @@ static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
   else if (ExitCaseIndices.empty())
     return false;
 
-  LLVM_DEBUG(dbgs() << "    unswitching trivial cases...\n");
+  LLVM_DEBUG(dbgs() << "    unswitching trivial switch...\n");
 
   // We may need to invalidate SCEVs for the outermost loop reached by any of
   // the exits.
@@ -739,6 +741,7 @@ static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
 
   ++NumTrivial;
   ++NumSwitches;
+  LLVM_DEBUG(dbgs() << "    done: unswitching trivial switch...\n");
   return true;
 }