[MemorySSA] Make the use of moveAllAfterMergeBlocks consistent.

Summary:
The rule for the moveAllAfterMergeBlocks API si for all instructions
from `From` to have been moved to `To`, while keeping the CFG edges (and
block terminators) unchanged.
Update all the callsites for moveAllAfterMergeBlocks to follow this.

Pending follow-up: since the same behavior is needed everytime, merge
all callsites into one. The common denominator may be the call to
`MergeBlockIntoPredecessor`.

Resolves PR43569.

Reviewers: george.burgess.iv

Subscribers: Prazek, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68659

llvm-svn: 374177

3 files changed, 42 insertions, 14 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
index 9938dd89c19..880f5389171 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -1629,15 +1629,27 @@ void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
           ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM,
                             MSSAU.get());
 
-        // If Succ has any successors with PHI nodes, update them to have
-        // entries coming from Pred instead of Succ.
-        Succ->replaceAllUsesWith(Pred);
+        Instruction *STI = Succ->getTerminator();
+        Instruction *Start = &*Succ->begin();
+        // If there's nothing to move, mark the starting instruction as the last
+        // instruction in the block.
+        if (Start == STI)
+          Start = BI;
 
         // Move all of the successor contents from Succ to Pred.
         Pred->getInstList().splice(BI->getIterator(), Succ->getInstList(),
-                                   Succ->begin(), Succ->end());
+                                   Succ->begin(), STI->getIterator());
         if (MSSAU)
-          MSSAU->moveAllAfterMergeBlocks(Succ, Pred, BI);
+          MSSAU->moveAllAfterMergeBlocks(Succ, Pred, Start);
+
+        // Move terminator instruction from Succ now, we're deleting BI below.
+        // FIXME: remove BI first might be more intuitive.
+        Pred->getInstList().splice(Pred->end(), Succ->getInstList());
+
+        // If Succ has any successors with PHI nodes, update them to have
+        // entries coming from Pred instead of Succ.
+        Succ->replaceAllUsesWith(Pred);
+
         LPM->deleteSimpleAnalysisValue(BI, L);
         RemoveFromWorklist(BI, Worklist);
         BI->eraseFromParent();
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 6e20ef21d9c..b3dd3c9d724 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -227,17 +227,29 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
     Updates.push_back({DominatorTree::Delete, PredBB, BB});
   }
 
-  if (MSSAU)
-    MSSAU->moveAllAfterMergeBlocks(BB, PredBB, &*(BB->begin()));
+  Instruction *PTI = PredBB->getTerminator();
+  Instruction *STI = BB->getTerminator();
+  Instruction *Start = &*BB->begin();
+  // If there's nothing to move, mark the starting instruction as the last
+  // instruction in the block.
+  if (Start == STI)
+    Start = PTI;
 
-  // Delete the unconditional branch from the predecessor...
-  PredBB->getInstList().pop_back();
+  // Move all definitions in the successor to the predecessor...
+  PredBB->getInstList().splice(PTI->getIterator(), BB->getInstList(),
+                               BB->begin(), STI->getIterator());
+
+  if (MSSAU)
+    MSSAU->moveAllAfterMergeBlocks(BB, PredBB, Start);
 
   // Make all PHI nodes that referred to BB now refer to Pred as their
   // source...
   BB->replaceAllUsesWith(PredBB);
 
-  // Move all definitions in the successor to the predecessor...
+  // Delete the unconditional branch from the predecessor...
+  PredBB->getInstList().pop_back();
+
+  // Move terminator instruction and add unreachable to now empty BB.
   PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
   new UnreachableInst(BB->getContext(), BB);
 
@@ -274,11 +286,10 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
            "applying corresponding DTU updates.");
     DTU->applyUpdatesPermissive(Updates);
     DTU->deleteBB(BB);
-  }
-
-  else {
+  } else {
     BB->eraseFromParent(); // Nuke BB if DTU is nullptr.
   }
+
   return true;
 }
 
diff --git a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
index 37389a695b4..4765aa115d1 100644
--- a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
@@ -615,8 +615,13 @@ bool LoopRotate::simplifyLoopLatch(Loop *L) {
   LLVM_DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
                     << LastExit->getName() << "\n");
 
+  Instruction *FirstLatchInst = &*Latch->begin();
+  // If there's nothing to move, mark the starting instruction as the last
+  // instruction in the block.
+  if (FirstLatchInst == Jmp)
+    FirstLatchInst = BI;
+
   // Hoist the instructions from Latch into LastExit.
-  Instruction *FirstLatchInst = &*(Latch->begin());
   LastExit->getInstList().splice(BI->getIterator(), Latch->getInstList(),
                                  Latch->begin(), Jmp->getIterator());