ValueTracking: Don't recurse too deeply in computeKnownBitsFromAssume

Respect the MaxDepth recursion limit, doing otherwise will trigger an
assert in computeKnownBits.

This fixes PR21891.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224168 91177308-0d34-0410-b5e6-96231b3b80d8

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)),

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)