diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index fa428fa108d..4317bcde91e 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -511,6 +511,10 @@ static void computeKnownBitsFromAssume(Value *V, APInt &KnownZero,
       return;
     }
 
+    // The remaining tests are all recursive, so bail out if we hit the limit.
+    if (Depth == MaxDepth)
+      continue;
+
     Value *A, *B;
     auto m_V = m_CombineOr(m_Specific(V),
                            m_CombineOr(m_PtrToInt(m_Specific(V)),
diff --git a/test/Transforms/InstCombine/pr21891.ll b/test/Transforms/InstCombine/pr21891.ll
new file mode 100644
index 00000000000..8194976b623
--- /dev/null
+++ b/test/Transforms/InstCombine/pr21891.ll
@@ -0,0 +1,18 @@
+; RUN: opt %s -instcombine
+
+define i32 @f(i32 %theNumber) {
+entry:
+  %cmp = icmp sgt i32 %theNumber, -1
+  call void @llvm.assume(i1 %cmp)
+  br i1 true, label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  %shl = shl nuw i32 %theNumber, 1
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %entry
+  %phi = phi i32 [ %shl, %if.then ], [ undef, %entry ]
+  ret i32 %phi
+}
+
+declare void @llvm.assume(i1)