[JumpThreading] Make sure UNDEF does not make us think a block can have multiple possible destinations

[trentxintong]

Summary

Make sure UNDEF does not make us think a block can have multiple possible destinations. This can prevent
folding.

[trentxintong]

I am having some trouble with arc. Created this diff accidentally. Its a duplicate of https://reviews.llvm.org/D48181

lib/Transforms/Scalar/JumpThreading.cpp

for (const auto &PredValue : PredValues) {
     } else {
       assert(isa<IndirectBrInst>(BB->getTerminator())
               && "Unexpected terminator");
       assert(isa<BlockAddress>(Val) && "Expecting a constant blockaddress");
       DestBB = cast<BlockAddress>(Val)->getBasicBlock();
     }
 
     // If we have exactly one destination, remember it for efficiency below.
+    // and we can ignore null destination.
+    if (DestBB) {
       if (PredToDestList.empty()) {
         OnlyDest = DestBB;
         OnlyVal = Val;
       } else {
         if (OnlyDest != DestBB)
           OnlyDest = MultipleDestSentinel;
-      // It possible we have same destination, but different value, e.g. default
-      // case in switchinst.
+        // It possible we have same destination, but different value, e.g.
+        // default case in switchinst.
         if (Val != OnlyVal)
           OnlyVal = MultipleVal;
       }
+    }
 
     // We know where this predecessor is going.
     ++PredWithKnownDest;
 
     // If the predecessor ends with an indirect goto, we can't change its
     // destination.
     if (isa<IndirectBrInst>(Pred->getTerminator()))
       continue;

test/Transforms/JumpThreading/fold-not-thread.ll

 L2:
   call void @f3()
   ret void
 
 L3:
   call void @f3()
   ret void
 }
+
+; Make sure we can fold this branch ... We will not be able to thread it as
+; L0 is too big to duplicate. LX is the unreachable block here, and it feeds
+; UNDEF to L2. We want to make sure the UNDEF does not make jump threading think
+; L2 has multiple destinations. 
+; CHECK-LABEL: @test_br_folding_not_threading_multiple_preds_with_undef(
+; CHECK: L3:
+; CHECK: call i32 @f2()
+; CHECK: call void @f3()
+; CHECK-NEXT: ret void
+define void @test_br_folding_not_threading_multiple_preds_with_undef(i1 %condx, i1 %cond) nounwind {
+L0:
+  br i1 %condx, label %L2, label %L1
+
+L1:
+  br i1 %cond, label %L2, label %L5
+
+LX:
+  br label %L2
+
+L2:
+  %p = phi i1 [ %condx, %L0 ], [ %cond, %L1 ], [ undef, %LX]
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  br i1 %p, label %L3, label %L4
+
+L3:
+  call void @f3()
+  ret void
+L4:
+  call void @f3()
+  ret void
+L5:
+  call void @f3()
+  ret void
+}