[FlattenCFG] Replace branch condition with a created one correctly

[aleksandr.popov]

Summary

Fix case when FlattenCFGOpt combines the conditions but doesn't update
branch with a new combined condition. It happens when one of the
combinable condition is inverted and has more then 1 user

[mkazantsev]

Confirmed with alive2.
Bugged version: https://alive2.llvm.org/ce/z/L4ps9v
Fixed version: https://alive2.llvm.org/ce/z/QdrG5U

llvm/lib/Transforms/Utils/FlattenCFG.cpp

if (InvertCond2) {
     auto Cmp2 = dyn_cast<CmpInst>(CInst2);
     if (Cmp2 && Cmp2->hasOneUse())
       Cmp2->setPredicate(Cmp2->getInversePredicate());
     else
       CInst2 = cast<Instruction>(Builder.CreateNot(CInst2));
     PBI->swapSuccessors();
   }
   Value *NC = Builder.CreateBinOp(CombineOp, CInst1, CInst2);
-  PBI->replaceUsesOfWith(CInst2, NC);
+  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);

llvm/test/Transforms/Util/flattencfg.ll

bb_2:                                             ; preds = %bb_4
   br label %bb_4
 
 bb_4:                                             ; preds = %bb_4, %bb_2
   %val_i1_46 = call i1 @llvm.smax.i1(i1 %val_i1_5, i1 %val_i1_5)
   br i1 %val_i1_46, label %bb_4, label %bb_2
 }
 
 ; cmp.y has 2 users, but should be inverted. So that a new one cmp is created instead.
-; FIXME: Branch condition must be replaced with a new created combined condition
+; Branch condition must be replaced with a new created combined condition
 ; Proof of bug: https://alive2.llvm.org/ce/z/L4ps9v
+; Proof of fix: https://alive2.llvm.org/ce/z/QdrG5U
 define i1 @test_cond_multi_use(i32 %x, i32 %y, i32 %z) {
 ; CHECK-LABEL: define i1 @test_cond_multi_use
 ; CHECK-SAME: (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 [[CMP_Y]], label [[IF_THEN_Y:%.*]], label [[EXIT:%.*]]
+; CHECK-NEXT:    br i1 [[TMP1]], label [[IF_THEN_Y:%.*]], label [[EXIT:%.*]]
 ; CHECK:       if.then.y:
 ; CHECK-NEXT:    store i32 [[Z]], ptr @g, align 4
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret i1 [[CMP_Y]]
 ;
 entry.x:
   %cmp.x = icmp ne i32 %x, 0