[SimplifyCFG] Don't limit the number of simultaneous forwards from switch condition

Author: dianqk

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -5818,8 +5818,6 @@ static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
   for (auto &ForwardingNode : ForwardingNodes) {
     PHINode *Phi = ForwardingNode.first;
     SmallVectorImpl<int> &Indexes = ForwardingNode.second;
-    if (Indexes.size() < 2)
-      continue;
 
     for (int Index : Indexes)
       Phi->setIncomingValue(Index, SI->getCondition());

llvm/test/Transforms/SimplifyCFG/ForwardSwitchConditionToPHI.ll

@@ -97,7 +97,7 @@ define i32 @forward_one(i32 %m) {
 ; FWD:       sw.bb4:
 ; FWD-NEXT:    br label [[RETURN]]
 ; FWD:       return:
-; FWD-NEXT:    [[RETVAL_0:%.*]] = phi i32 [ 4, [[SW_BB4]] ], [ 5, [[SW_BB3]] ], [ 6, [[SW_BB2]] ], [ 1, [[SW_BB1]] ], [ 8, [[ENTRY:%.*]] ]
+; FWD-NEXT:    [[RETVAL_0:%.*]] = phi i32 [ 4, [[SW_BB4]] ], [ 5, [[SW_BB3]] ], [ 6, [[SW_BB2]] ], [ [[M]], [[SW_BB1]] ], [ 8, [[ENTRY:%.*]] ]
 ; FWD-NEXT:    ret i32 [[RETVAL_0]]
 ;
 entry:
@@ -207,19 +207,15 @@ define { i64, i64 } @PR95919(i64 noundef %arg, i64 noundef %arg1) {
 ;
 ; FWD-LABEL: @PR95919(
 ; FWD-NEXT:  bb:
-; FWD-NEXT:    switch i64 [[ARG1:%.*]], label [[BB3:%.*]] [
-; FWD-NEXT:      i64 0, label [[BB5:%.*]]
-; FWD-NEXT:      i64 1, label [[BB2:%.*]]
-; FWD-NEXT:    ]
-; FWD:       bb2:
-; FWD-NEXT:    br label [[BB5]]
+; FWD-NEXT:    [[SWITCH:%.*]] = icmp ult i64 [[ARG1:%.*]], 2
+; FWD-NEXT:    br i1 [[SWITCH]], label [[BB5:%.*]], label [[BB3:%.*]]
 ; FWD:       bb3:
 ; FWD-NEXT:    [[I:%.*]] = udiv i64 [[ARG:%.*]], [[ARG1]]
 ; FWD-NEXT:    [[I4:%.*]] = shl nuw i64 [[I]], 1
 ; FWD-NEXT:    br label [[BB5]]
 ; FWD:       bb5:
-; FWD-NEXT:    [[I6:%.*]] = phi i64 [ [[I4]], [[BB3]] ], [ [[ARG]], [[BB2]] ], [ undef, [[BB:%.*]] ]
-; FWD-NEXT:    [[I7:%.*]] = phi i64 [ 1, [[BB3]] ], [ 1, [[BB2]] ], [ [[ARG1]], [[BB]] ]
+; FWD-NEXT:    [[I6:%.*]] = phi i64 [ [[I4]], [[BB3]] ], [ [[ARG]], [[BB:%.*]] ]
+; FWD-NEXT:    [[I7:%.*]] = phi i64 [ 1, [[BB3]] ], [ [[ARG1]], [[BB]] ]
 ; FWD-NEXT:    [[I8:%.*]] = insertvalue { i64, i64 } poison, i64 [[I7]], 0
 ; FWD-NEXT:    [[I9:%.*]] = insertvalue { i64, i64 } [[I8]], i64 [[I6]], 1
 ; FWD-NEXT:    ret { i64, i64 } [[I9]]