[SimpleLoopUnswitch] Make all checks before actual non-trivial unswitch

We should be able to make all relevant checks before we actually start the non-trivial
unswitching, so that we could guarantee that once we have started the transform,
it will always succeed.

Reviewed By: chandlerc
Differential Revision: https://reviews.llvm.org/D53747

llvm-svn: 345375

diff --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index 96db249584e82d231ea182165436ea045d347e24..e8f67a689f453f676e50d632559d3097143d5f1c 100644 (file)
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -1792,10 +1792,10 @@ void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
   } while (!DomWorklist.empty());
 }
 
-static bool unswitchNontrivialInvariants(
+static void unswitchNontrivialInvariants(
     Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
-    DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
-    function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
+    SmallVectorImpl<BasicBlock *> &ExitBlocks, DominatorTree &DT, LoopInfo &LI,
+    AssumptionCache &AC, function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
     ScalarEvolution *SE) {
   auto *ParentBB = TI.getParent();
   BranchInst *BI = dyn_cast<BranchInst>(&TI);
@@ -1851,17 +1851,6 @@ static bool unswitchNontrivialInvariants(
   // whatever reason).
   assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
 
-  SmallVector<BasicBlock *, 4> ExitBlocks;
-  L.getUniqueExitBlocks(ExitBlocks);
-
-  // We cannot unswitch if exit blocks contain a cleanuppad instruction as we
-  // don't know how to split those exit blocks.
-  // FIXME: We should teach SplitBlock to handle this and remove this
-  // restriction.
-  for (auto *ExitBB : ExitBlocks)
-    if (isa<CleanupPadInst>(ExitBB->getFirstNonPHI()))
-      return false;
-
   // Compute the parent loop now before we start hacking on things.
   Loop *ParentL = L.getParentLoop();
 
@@ -2145,7 +2134,6 @@ static bool unswitchNontrivialInvariants(
   UnswitchCB(IsStillLoop, SibLoops);
 
   ++NumBranches;
-  return true;
 }
 
 /// Recursively compute the cost of a dominator subtree based on the per-block
@@ -2241,6 +2229,19 @@ unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI,
   if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
     return false;
 
+  SmallVector<BasicBlock *, 4> ExitBlocks;
+  L.getUniqueExitBlocks(ExitBlocks);
+
+  // We cannot unswitch if exit blocks contain a cleanuppad instruction as we
+  // don't know how to split those exit blocks.
+  // FIXME: We should teach SplitBlock to handle this and remove this
+  // restriction.
+  for (auto *ExitBB : ExitBlocks)
+    if (isa<CleanupPadInst>(ExitBB->getFirstNonPHI())) {
+      dbgs() << "Cannot unswitch because of cleanuppad in exit block\n";
+      return false;
+    }
+
   LLVM_DEBUG(
       dbgs() << "Considering " << UnswitchCandidates.size()
              << " non-trivial loop invariant conditions for unswitching.\n");
@@ -2374,11 +2375,12 @@ unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI,
     return false;
   }
 
-  LLVM_DEBUG(dbgs() << "  Trying to unswitch non-trivial (cost = "
+  LLVM_DEBUG(dbgs() << "  Unswitching non-trivial (cost = "
                     << BestUnswitchCost << ") terminator: " << *BestUnswitchTI
                     << "\n");
-  return unswitchNontrivialInvariants(
-      L, *BestUnswitchTI, BestUnswitchInvariants, DT, LI, AC, UnswitchCB, SE);
+  unswitchNontrivialInvariants(L, *BestUnswitchTI, BestUnswitchInvariants,
+                               ExitBlocks, DT, LI, AC, UnswitchCB, SE);
+  return true;
 }
 
 /// Unswitch control flow predicated on loop invariant conditions.