[SimplifyCFG] Transform for redirecting phis between unmergeable BB and SuccBB (#67275)

This patch extends function TryToSimplifyUncondBranchFromEmptyBlock to
handle the similar cases below.

```llvm
define i8 @src(i8 noundef %arg) {
start:
  switch i8 %arg, label %unreachable [
    i8 0, label %case012
    i8 1, label %case1
    i8 2, label %case2
    i8 3, label %end
  ]

unreachable:
  unreachable

case1:
  br label %case012

case2:
  br label %case012

case012:
  %phi1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [ 1, %start ]
  br label %end

end:
  %phi2 = phi i8 [ %phi1, %case012 ], [ 4, %start ]
  ret i8 %phi2
}
```
The phis here should be merged into one phi, so that we can better
optimize it:

```llvm
define i8 @tgt(i8 noundef %arg) {
start:
  switch i8 %arg, label %unreachable [
    i8 0, label %end
    i8 1, label %case1
    i8 2, label %case2
    i8 3, label %case3
  ]

unreachable:
  unreachable

case1:
  br label %end

case2:
  br label %end

case3:
  br label %end

end:
  %phi = phi i8 [ 4, %case3 ], [ 3, %case2 ], [ 2, %case1 ], [ 1, %start ]
  ret i8 %phi
}
```
Proof:
[normal](https://alive2.llvm.org/ce/z/vAWi88)
[multiple stages](https://alive2.llvm.org/ce/z/DDBQqp)
[multiple stages 2](https://alive2.llvm.org/ce/z/nGkeqN)
[multiple phi combinations](https://alive2.llvm.org/ce/z/VQeEdp)

And lookup table optimization should convert it into add %arg 1.
This patch just match similar CFG structure and merge the phis in
different cases.

Maybe such transform can be applied to other situations besides switch,
but I'm not sure whether it's better than not merging. Therefore, I only
try it in switch,

Related issue:
#63876

[Migrated](https://reviews.llvm.org/D155940)

Author: XChy

llvm/lib/Transforms/Utils/Local.cpp

@@ -847,17 +847,17 @@ static bool CanMergeValues(Value *First, Value *Second) {
 /// branch to Succ, into Succ.
 ///
 /// Assumption: Succ is the single successor for BB.
-static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
+static bool
+CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ,
+                                const SmallPtrSetImpl<BasicBlock *> &BBPreds) {
   assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
 
   LLVM_DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into "
                     << Succ->getName() << "\n");
   // Shortcut, if there is only a single predecessor it must be BB and merging
   // is always safe
-  if (Succ->getSinglePredecessor()) return true;
-
-  // Make a list of the predecessors of BB
-  SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
+  if (Succ->getSinglePredecessor())
+    return true;
 
   // Look at all the phi nodes in Succ, to see if they present a conflict when
   // merging these blocks
@@ -997,16 +997,47 @@ static void replaceUndefValuesInPhi(PHINode *PN,
   }
 }
 
+// Only when they shares a single common predecessor, return true.
+// Only handles cases when BB can't be merged while its predecessors can be
+// redirected.
+static bool
+CanRedirectPredsOfEmptyBBToSucc(BasicBlock *BB, BasicBlock *Succ,
+                                const SmallPtrSetImpl<BasicBlock *> &BBPreds,
+                                const SmallPtrSetImpl<BasicBlock *> &SuccPreds,
+                                BasicBlock *&CommonPred) {
+
+  // There must be phis in BB, otherwise BB will be merged into Succ directly
+  if (BB->phis().empty() || Succ->phis().empty())
+    return false;
+
+  // BB must have predecessors not shared that can be redirected to Succ
+  if (!BB->hasNPredecessorsOrMore(2))
+    return false;
+
+  // Get single common predecessors of both BB and Succ
+  for (BasicBlock *SuccPred : SuccPreds) {
+    if (BBPreds.count(SuccPred)) {
+      if (CommonPred)
+        return false;
+      CommonPred = SuccPred;
+    }
+  }
+
+  return true;
+}
+
 /// Replace a value flowing from a block to a phi with
 /// potentially multiple instances of that value flowing from the
 /// block's predecessors to the phi.
 ///
 /// \param BB The block with the value flowing into the phi.
 /// \param BBPreds The predecessors of BB.
 /// \param PN The phi that we are updating.
+/// \param CommonPred The common predecessor of BB and PN's BasicBlock
 static void redirectValuesFromPredecessorsToPhi(BasicBlock *BB,
                                                 const PredBlockVector &BBPreds,
-                                                PHINode *PN) {
+                                                PHINode *PN,
+                                                BasicBlock *CommonPred) {
   Value *OldVal = PN->removeIncomingValue(BB, false);
   assert(OldVal && "No entry in PHI for Pred BB!");
 
@@ -1034,26 +1065,39 @@ static void redirectValuesFromPredecessorsToPhi(BasicBlock *BB,
       // will trigger asserts if we try to clean it up now, without also
       // simplifying the corresponding conditional branch).
       BasicBlock *PredBB = OldValPN->getIncomingBlock(i);
+
+      if (PredBB == CommonPred)
+        continue;
+
       Value *PredVal = OldValPN->getIncomingValue(i);
-      Value *Selected = selectIncomingValueForBlock(PredVal, PredBB,
-                                                    IncomingValues);
+      Value *Selected =
+          selectIncomingValueForBlock(PredVal, PredBB, IncomingValues);
 
       // And add a new incoming value for this predecessor for the
       // newly retargeted branch.
       PN->addIncoming(Selected, PredBB);
     }
+    if (CommonPred)
+      PN->addIncoming(OldValPN->getIncomingValueForBlock(CommonPred), BB);
+
   } else {
     for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) {
       // Update existing incoming values in PN for this
       // predecessor of BB.
       BasicBlock *PredBB = BBPreds[i];
-      Value *Selected = selectIncomingValueForBlock(OldVal, PredBB,
-                                                    IncomingValues);
+
+      if (PredBB == CommonPred)
+        continue;
+
+      Value *Selected =
+          selectIncomingValueForBlock(OldVal, PredBB, IncomingValues);
 
       // And add a new incoming value for this predecessor for the
       // newly retargeted branch.
       PN->addIncoming(Selected, PredBB);
     }
+    if (CommonPred)
+      PN->addIncoming(OldVal, BB);
   }
 
   replaceUndefValuesInPhi(PN, IncomingValues);
@@ -1064,13 +1108,30 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   assert(BB != &BB->getParent()->getEntryBlock() &&
          "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!");
 
-  // We can't eliminate infinite loops.
+  // We can't simplify infinite loops.
   BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
-  if (BB == Succ) return false;
+  if (BB == Succ)
+    return false;
+
+  SmallPtrSet<BasicBlock *, 16> BBPreds(pred_begin(BB), pred_end(BB));
+  SmallPtrSet<BasicBlock *, 16> SuccPreds(pred_begin(Succ), pred_end(Succ));
 
-  // Check to see if merging these blocks would cause conflicts for any of the
-  // phi nodes in BB or Succ. If not, we can safely merge.
-  if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
+  // The single common predecessor of BB and Succ when BB cannot be killed
+  BasicBlock *CommonPred = nullptr;
+
+  bool BBKillable = CanPropagatePredecessorsForPHIs(BB, Succ, BBPreds);
+
+  // Even if we can not fold bB into Succ, we may be able to redirect the
+  // predecessors of BB to Succ.
+  bool BBPhisMergeable =
+      BBKillable ||
+      CanRedirectPredsOfEmptyBBToSucc(BB, Succ, BBPreds, SuccPreds, CommonPred);
+
+  if (!BBKillable && !BBPhisMergeable)
+    return false;
+
+  // Check to see if merging these blocks/phis would cause conflicts for any of
+  // the phi nodes in BB or Succ. If not, we can safely merge.
 
   // Check for cases where Succ has multiple predecessors and a PHI node in BB
   // has uses which will not disappear when the PHI nodes are merged.  It is
@@ -1099,6 +1160,11 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
     }
   }
 
+  if (BBPhisMergeable && CommonPred)
+    LLVM_DEBUG(dbgs() << "Found Common Predecessor between: " << BB->getName()
+                      << " and " << Succ->getName() << " : "
+                      << CommonPred->getName() << "\n");
+
   // 'BB' and 'BB->Pred' are loop latches, bail out to presrve inner loop
   // metadata.
   //
@@ -1171,25 +1237,37 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
           if (PredTI->hasMetadata(LLVMContext::MD_loop))
             return false;
 
-  LLVM_DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
+  if (BBKillable)
+    LLVM_DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
+  else if (BBPhisMergeable)
+    LLVM_DEBUG(dbgs() << "Merge Phis in Trivial BB: \n" << *BB);
 
   SmallVector<DominatorTree::UpdateType, 32> Updates;
+
   if (DTU) {
     // To avoid processing the same predecessor more than once.
     SmallPtrSet<BasicBlock *, 8> SeenPreds;
-    // All predecessors of BB will be moved to Succ.
-    SmallPtrSet<BasicBlock *, 8> PredsOfSucc(pred_begin(Succ), pred_end(Succ));
+    // All predecessors of BB (except the common predecessor) will be moved to
+    // Succ.
     Updates.reserve(Updates.size() + 2 * pred_size(BB) + 1);
-    for (auto *PredOfBB : predecessors(BB))
-      // This predecessor of BB may already have Succ as a successor.
-      if (!PredsOfSucc.contains(PredOfBB))
+
+    for (auto *PredOfBB : predecessors(BB)) {
+      // Do not modify those common predecessors of BB and Succ
+      if (!SuccPreds.contains(PredOfBB))
         if (SeenPreds.insert(PredOfBB).second)
           Updates.push_back({DominatorTree::Insert, PredOfBB, Succ});
+    }
+
     SeenPreds.clear();
+
     for (auto *PredOfBB : predecessors(BB))
-      if (SeenPreds.insert(PredOfBB).second)
+      // When BB cannot be killed, do not remove the edge between BB and
+      // CommonPred.
+      if (SeenPreds.insert(PredOfBB).second && PredOfBB != CommonPred)
         Updates.push_back({DominatorTree::Delete, PredOfBB, BB});
-    Updates.push_back({DominatorTree::Delete, BB, Succ});
+
+    if (BBKillable)
+      Updates.push_back({DominatorTree::Delete, BB, Succ});
   }
 
   if (isa<PHINode>(Succ->begin())) {
@@ -1201,21 +1279,19 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
     // Loop over all of the PHI nodes in the successor of BB.
     for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
       PHINode *PN = cast<PHINode>(I);
-
-      redirectValuesFromPredecessorsToPhi(BB, BBPreds, PN);
+      redirectValuesFromPredecessorsToPhi(BB, BBPreds, PN, CommonPred);
     }
   }
 
   if (Succ->getSinglePredecessor()) {
     // BB is the only predecessor of Succ, so Succ will end up with exactly
     // the same predecessors BB had.
-
     // Copy over any phi, debug or lifetime instruction.
     BB->getTerminator()->eraseFromParent();
     Succ->splice(Succ->getFirstNonPHI()->getIterator(), BB);
   } else {
     while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
-      // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
+      // We explicitly check for such uses for merging phis.
       assert(PN->use_empty() && "There shouldn't be any uses here!");
       PN->eraseFromParent();
     }
@@ -1228,21 +1304,35 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
       for (BasicBlock *Pred : predecessors(BB))
         Pred->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopMD);
 
-  // Everything that jumped to BB now goes to Succ.
-  BB->replaceAllUsesWith(Succ);
-  if (!Succ->hasName()) Succ->takeName(BB);
-
-  // Clear the successor list of BB to match updates applying to DTU later.
-  if (BB->getTerminator())
-    BB->back().eraseFromParent();
-  new UnreachableInst(BB->getContext(), BB);
-  assert(succ_empty(BB) && "The successor list of BB isn't empty before "
-                           "applying corresponding DTU updates.");
+  if (BBKillable) {
+    // Everything that jumped to BB now goes to Succ.
+    BB->replaceAllUsesWith(Succ);
+
+    if (!Succ->hasName())
+      Succ->takeName(BB);
+
+    // Clear the successor list of BB to match updates applying to DTU later.
+    if (BB->getTerminator())
+      BB->back().eraseFromParent();
+
+    new UnreachableInst(BB->getContext(), BB);
+    assert(succ_empty(BB) && "The successor list of BB isn't empty before "
+                             "applying corresponding DTU updates.");
+  } else if (BBPhisMergeable) {
+    //  Everything except CommonPred that jumped to BB now goes to Succ.
+    BB->replaceUsesWithIf(Succ, [BBPreds, CommonPred](Use &U) -> bool {
+      if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser()))
+        return UseInst->getParent() != CommonPred &&
+               BBPreds.contains(UseInst->getParent());
+      return false;
+    });
+  }
 
   if (DTU)
     DTU->applyUpdates(Updates);
 
-  DeleteDeadBlock(BB, DTU);
+  if (BBKillable)
+    DeleteDeadBlock(BB, DTU);
 
   return true;
 }

llvm/test/CodeGen/ARM/jump-table-islands.ll

@@ -2,6 +2,8 @@
 
 %BigInt = type i8500
 
+declare void @use(%BigInt)
+
 define %BigInt @test_moved_jumptable(i1 %tst, i32 %sw, %BigInt %l) {
 ; CHECK-LABEL: test_moved_jumptable:
 
@@ -34,6 +36,8 @@ other:
 
 end:
   %val = phi %BigInt [ %l, %complex ], [ -1, %simple ]
+; Prevent SimplifyCFG from simplifying the phi node above.
+  call void @use(%BigInt %val)
   ret %BigInt %val
 }
 

llvm/test/Transforms/JumpThreading/codesize-loop.ll

@@ -42,16 +42,14 @@ define i32 @test_minsize(i32 %argc, ptr nocapture readonly %argv) local_unnamed_
 ; OVERIDE-NEXT:    [[TMP3:%.*]] = mul i32 [[CALL]], [[TMP2]]
 ; OVERIDE-NEXT:    [[TMP4:%.*]] = icmp sgt i32 [[CALL]], 0
 ; OVERIDE-NEXT:    [[COND_FR:%.*]] = freeze i1 [[TMP4]]
-; OVERIDE-NEXT:    br i1 [[COND_FR]], label [[COND_END_THREAD]], label [[TMP6:%.*]]
+; OVERIDE-NEXT:    br i1 [[COND_FR]], label [[TMP5:%.*]], label [[COND_END_THREAD]]
+; OVERIDE:       5:
+; OVERIDE-NEXT:    br label [[COND_END_THREAD]]
 ; OVERIDE:       cond.end.thread:
-; OVERIDE-NEXT:    [[TMP5:%.*]] = phi i32 [ [[TMP3]], [[COND_END]] ], [ 205962976, [[ENTRY:%.*]] ]
-; OVERIDE-NEXT:    [[COND3:%.*]] = phi i32 [ [[CALL]], [[COND_END]] ], [ 46, [[ENTRY]] ]
-; OVERIDE-NEXT:    br label [[TMP6]]
-; OVERIDE:       6:
-; OVERIDE-NEXT:    [[TMP7:%.*]] = phi i32 [ [[TMP5]], [[COND_END_THREAD]] ], [ [[TMP3]], [[COND_END]] ]
-; OVERIDE-NEXT:    [[TMP8:%.*]] = phi i32 [ [[COND3]], [[COND_END_THREAD]] ], [ 0, [[COND_END]] ]
-; OVERIDE-NEXT:    [[TMP9:%.*]] = mul i32 [[TMP7]], [[TMP8]]
-; OVERIDE-NEXT:    [[CALL33:%.*]] = tail call i32 (ptr, ...) @printf(ptr nonnull dereferenceable(1) @.str, i32 [[TMP9]])
+; OVERIDE-NEXT:    [[TMP6:%.*]] = phi i32 [ [[TMP3]], [[COND_END]] ], [ [[TMP3]], [[TMP5]] ], [ 205962976, [[ENTRY:%.*]] ]
+; OVERIDE-NEXT:    [[TMP7:%.*]] = phi i32 [ 0, [[COND_END]] ], [ [[CALL]], [[TMP5]] ], [ 46, [[ENTRY]] ]
+; OVERIDE-NEXT:    [[TMP8:%.*]] = mul i32 [[TMP6]], [[TMP7]]
+; OVERIDE-NEXT:    [[CALL33:%.*]] = tail call i32 (ptr, ...) @printf(ptr nonnull dereferenceable(1) @.str, i32 [[TMP8]])
 ; OVERIDE-NEXT:    ret i32 0
 ;
 entry:
@@ -90,16 +88,14 @@ define i32 @test_optsize(i32 %argc, ptr nocapture readonly %argv) local_unnamed_
 ; DEFAULT-NEXT:    [[TMP3:%.*]] = mul i32 [[CALL]], [[TMP2]]
 ; DEFAULT-NEXT:    [[TMP4:%.*]] = icmp sgt i32 [[CALL]], 0
 ; DEFAULT-NEXT:    [[COND_FR:%.*]] = freeze i1 [[TMP4]]
-; DEFAULT-NEXT:    br i1 [[COND_FR]], label [[COND_END_THREAD]], label [[TMP6:%.*]]
+; DEFAULT-NEXT:    br i1 [[COND_FR]], label [[TMP5:%.*]], label [[COND_END_THREAD]]
+; DEFAULT:       5:
+; DEFAULT-NEXT:    br label [[COND_END_THREAD]]
 ; DEFAULT:       cond.end.thread:
-; DEFAULT-NEXT:    [[TMP5:%.*]] = phi i32 [ [[TMP3]], [[COND_END]] ], [ 205962976, [[ENTRY:%.*]] ]
-; DEFAULT-NEXT:    [[COND3:%.*]] = phi i32 [ [[CALL]], [[COND_END]] ], [ 46, [[ENTRY]] ]
-; DEFAULT-NEXT:    br label [[TMP6]]
-; DEFAULT:       6:
-; DEFAULT-NEXT:    [[TMP7:%.*]] = phi i32 [ [[TMP5]], [[COND_END_THREAD]] ], [ [[TMP3]], [[COND_END]] ]
-; DEFAULT-NEXT:    [[TMP8:%.*]] = phi i32 [ [[COND3]], [[COND_END_THREAD]] ], [ 0, [[COND_END]] ]
-; DEFAULT-NEXT:    [[TMP9:%.*]] = mul i32 [[TMP7]], [[TMP8]]
-; DEFAULT-NEXT:    [[CALL33:%.*]] = tail call i32 (ptr, ...) @printf(ptr nonnull dereferenceable(1) @.str, i32 [[TMP9]])
+; DEFAULT-NEXT:    [[TMP6:%.*]] = phi i32 [ [[TMP3]], [[COND_END]] ], [ [[TMP3]], [[TMP5]] ], [ 205962976, [[ENTRY:%.*]] ]
+; DEFAULT-NEXT:    [[TMP7:%.*]] = phi i32 [ 0, [[COND_END]] ], [ [[CALL]], [[TMP5]] ], [ 46, [[ENTRY]] ]
+; DEFAULT-NEXT:    [[TMP8:%.*]] = mul i32 [[TMP6]], [[TMP7]]
+; DEFAULT-NEXT:    [[CALL33:%.*]] = tail call i32 (ptr, ...) @printf(ptr nonnull dereferenceable(1) @.str, i32 [[TMP8]])
 ; DEFAULT-NEXT:    ret i32 0
 ;
 ; OVERIDE-LABEL: @test_optsize(
@@ -115,16 +111,14 @@ define i32 @test_optsize(i32 %argc, ptr nocapture readonly %argv) local_unnamed_
 ; OVERIDE-NEXT:    [[TMP3:%.*]] = mul i32 [[CALL]], [[TMP2]]
 ; OVERIDE-NEXT:    [[TMP4:%.*]] = icmp sgt i32 [[CALL]], 0
 ; OVERIDE-NEXT:    [[COND_FR:%.*]] = freeze i1 [[TMP4]]
-; OVERIDE-NEXT:    br i1 [[COND_FR]], label [[COND_END_THREAD]], label [[TMP6:%.*]]
+; OVERIDE-NEXT:    br i1 [[COND_FR]], label [[TMP5:%.*]], label [[COND_END_THREAD]]
+; OVERIDE:       5:
+; OVERIDE-NEXT:    br label [[COND_END_THREAD]]
 ; OVERIDE:       cond.end.thread:
-; OVERIDE-NEXT:    [[TMP5:%.*]] = phi i32 [ [[TMP3]], [[COND_END]] ], [ 205962976, [[ENTRY:%.*]] ]
-; OVERIDE-NEXT:    [[COND3:%.*]] = phi i32 [ [[CALL]], [[COND_END]] ], [ 46, [[ENTRY]] ]
-; OVERIDE-NEXT:    br label [[TMP6]]
-; OVERIDE:       6:
-; OVERIDE-NEXT:    [[TMP7:%.*]] = phi i32 [ [[TMP5]], [[COND_END_THREAD]] ], [ [[TMP3]], [[COND_END]] ]
-; OVERIDE-NEXT:    [[TMP8:%.*]] = phi i32 [ [[COND3]], [[COND_END_THREAD]] ], [ 0, [[COND_END]] ]
-; OVERIDE-NEXT:    [[TMP9:%.*]] = mul i32 [[TMP7]], [[TMP8]]
-; OVERIDE-NEXT:    [[CALL33:%.*]] = tail call i32 (ptr, ...) @printf(ptr nonnull dereferenceable(1) @.str, i32 [[TMP9]])
+; OVERIDE-NEXT:    [[TMP6:%.*]] = phi i32 [ [[TMP3]], [[COND_END]] ], [ [[TMP3]], [[TMP5]] ], [ 205962976, [[ENTRY:%.*]] ]
+; OVERIDE-NEXT:    [[TMP7:%.*]] = phi i32 [ 0, [[COND_END]] ], [ [[CALL]], [[TMP5]] ], [ 46, [[ENTRY]] ]
+; OVERIDE-NEXT:    [[TMP8:%.*]] = mul i32 [[TMP6]], [[TMP7]]
+; OVERIDE-NEXT:    [[CALL33:%.*]] = tail call i32 (ptr, ...) @printf(ptr nonnull dereferenceable(1) @.str, i32 [[TMP8]])
 ; OVERIDE-NEXT:    ret i32 0
 ;
 entry: