diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1457,12 +1457,12 @@
         // Ok, we have a PHI of the form L op= R. Check for low
         // zero bits.
         RecQ.CxtI = RInst;
-        computeKnownBits(R, Known2, Depth + 1, RecQ);
+        computeKnownBits(R, DemandedElts, Known2, Depth + 1, RecQ);
 
         // We need to take the minimum number of known bits
         KnownBits Known3(BitWidth);
         RecQ.CxtI = LInst;
-        computeKnownBits(L, Known3, Depth + 1, RecQ);
+        computeKnownBits(L, DemandedElts, Known3, Depth + 1, RecQ);
 
         Known.Zero.setLowBits(std::min(Known2.countMinTrailingZeros(),
                                        Known3.countMinTrailingZeros()));
diff --git a/llvm/test/Transforms/InstCombine/phi-known-bits-demandedelts.ll b/llvm/test/Transforms/InstCombine/phi-known-bits-demandedelts.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/phi-known-bits-demandedelts.ll
@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+declare void @foo4x32(<4 x i32>)
+
+define i1 @test(i32 %count) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[BODY:%.*]]
+; CHECK:       body:
+; CHECK-NEXT:    call void @foo4x32(<4 x i32> <i32 0, i32 1, i32 2, i32 3>)
+; CHECK-NEXT:    br i1 true, label [[TAKEN:%.*]], label [[NOTTAKEN:%.*]]
+; CHECK:       taken:
+; CHECK-NEXT:    ret i1 true
+; CHECK:       nottaken:
+; CHECK-NEXT:    br i1 undef, label [[END:%.*]], label [[BODY]]
+; CHECK:       end:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  br label %body
+
+body:
+  %index = phi i32 [ 0, %entry ], [ %index.next, %nottaken ]
+  %vec.ind = phi <4 x i32> [ <i32 0, i32 1, i32 2, i32 3>, %entry ], [ %vec.ind.next, %nottaken ]
+  call void @foo4x32(<4 x i32> %vec.ind)
+  %a = extractelement <4 x i32> %vec.ind, i32 0
+  %b = and i32 %a, 3
+  %cmp1 = icmp eq i32 %b, 0                    ;; should be always true
+  br i1 %cmp1, label %taken, label %nottaken
+
+taken:
+  ret i1 true
+
+nottaken:
+  %index.next = add i32 %index, 4
+  %vec.ind.next = add <4 x i32> %vec.ind, <i32 4, i32 4, i32 4, i32 4>
+  %cmp2 = icmp eq i32 %index.next, %count
+  br i1 %cmp2, label %end, label %body
+
+end:
+  ret i1 false
+}