[FlattenCFG] Fix the miscompilation where phi nodes exist in the merge point (#81987)

When there are phi nodes in the merge point of the if-region, we cannot
do the merge.
Alive2: https://alive2.llvm.org/ce/z/DbgEan
Fixes #70900.

(cherry picked from commit f920b74)

Author: dtcxzyw

llvm/lib/Transforms/Utils/FlattenCFG.cpp

@@ -407,6 +407,10 @@ bool FlattenCFGOpt::CompareIfRegionBlock(BasicBlock *Block1, BasicBlock *Block2,
 /// form, by inverting the condition and the branch successors. The same
 /// approach goes for the opposite case.
 bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
+  // We cannot merge the if-region if the merge point has phi nodes.
+  if (isa<PHINode>(BB->front()))
+    return false;
+
   BasicBlock *IfTrue2, *IfFalse2;
   BranchInst *DomBI2 = GetIfCondition(BB, IfTrue2, IfFalse2);
   if (!DomBI2)
@@ -493,16 +497,6 @@ bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
   PBI->replaceUsesOfWith(PBI->getCondition(), NC);
   Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
 
-  // Handle PHI node to replace its predecessors to FirstEntryBlock.
-  for (BasicBlock *Succ : successors(PBI)) {
-    for (PHINode &Phi : Succ->phis()) {
-      for (unsigned i = 0, e = Phi.getNumIncomingValues(); i != e; ++i) {
-        if (Phi.getIncomingBlock(i) == SecondEntryBlock)
-          Phi.setIncomingBlock(i, FirstEntryBlock);
-      }
-    }
-  }
-
   // Remove IfTrue1
   if (IfTrue1 != FirstEntryBlock) {
     IfTrue1->dropAllReferences();

llvm/test/Transforms/Util/flattencfg.ll

@@ -77,13 +77,16 @@ define void @test_not_crash3(i32 %a) #0 {
 ; CHECK-SAME: (i32 [[A:%.*]]) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[A_EQ_0:%.*]] = icmp eq i32 [[A]], 0
+; CHECK-NEXT:    br i1 [[A_EQ_0]], label [[BB0:%.*]], label [[BB1:%.*]]
+; CHECK:       bb0:
+; CHECK-NEXT:    br label [[BB1]]
+; CHECK:       bb1:
 ; CHECK-NEXT:    [[A_EQ_1:%.*]] = icmp eq i32 [[A]], 1
-; CHECK-NEXT:    [[TMP0:%.*]] = or i1 [[A_EQ_0]], [[A_EQ_1]]
-; CHECK-NEXT:    br i1 [[TMP0]], label [[BB2:%.*]], label [[BB3:%.*]]
+; CHECK-NEXT:    br i1 [[A_EQ_1]], label [[BB2:%.*]], label [[BB3:%.*]]
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[CHECK_BADREF:%.*]] = phi i32 [ 17, [[ENTRY:%.*]] ], [ 11, [[BB2]] ]
+; CHECK-NEXT:    [[CHECK_BADREF:%.*]] = phi i32 [ 17, [[BB1]] ], [ 11, [[BB2]] ]
 ; CHECK-NEXT:    ret void
 ;
 entry:
@@ -278,9 +281,9 @@ define i1 @test_cond_multi_use(i32 %x, i32 %y, i32 %z) {
 ; CHECK-NEXT:  entry.x:
 ; CHECK-NEXT:    [[CMP_X:%.*]] = icmp ne i32 [[X]], 0
 ; CHECK-NEXT:    [[CMP_Y:%.*]] = icmp eq i32 [[Y]], 0
-; CHECK-NEXT:    [[TMP0:%.*]] = xor i1 [[CMP_Y]], true
-; CHECK-NEXT:    [[TMP1:%.*]] = or i1 [[CMP_X]], [[TMP0]]
-; CHECK-NEXT:    br i1 [[TMP1]], label [[IF_THEN_Y:%.*]], label [[EXIT:%.*]]
+; CHECK-NEXT:    [[CMP_Y_NOT:%.*]] = xor i1 [[CMP_Y]], true
+; CHECK-NEXT:    [[TMP0:%.*]] = or i1 [[CMP_X]], [[CMP_Y_NOT]]
+; CHECK-NEXT:    br i1 [[TMP0]], label [[IF_THEN_Y:%.*]], label [[EXIT:%.*]]
 ; CHECK:       if.then.y:
 ; CHECK-NEXT:    store i32 [[Z]], ptr @g, align 4
 ; CHECK-NEXT:    br label [[EXIT]]