diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -249,7 +249,14 @@
       return countMinLeadingZeros();
     if (isNegative())
       return countMinLeadingOnes();
-    return 0;
+    // Every value has at least 1 sign bit.
+    return 1;
+  }
+
+  /// Returns the maximum number of bits needed to represent all possible
+  /// signed values with these known bits.
+  unsigned countMaxSignedBits() const {
+    return getBitWidth() - countMinSignBits() + 1;
   }
 
   /// Returns the maximum number of trailing zero bits possible.
diff --git a/llvm/unittests/Support/KnownBitsTest.cpp b/llvm/unittests/Support/KnownBitsTest.cpp
--- a/llvm/unittests/Support/KnownBitsTest.cpp
+++ b/llvm/unittests/Support/KnownBitsTest.cpp
@@ -442,6 +442,17 @@
   });
 }
 
+TEST(KnownBitsTest, CountMaxSignedBits) {
+  unsigned Bits = 4;
+  ForeachKnownBits(Bits, [&](const KnownBits &Known) {
+    unsigned Expected = 0;
+    ForeachNumInKnownBits(Known, [&](const APInt &N) {
+      Expected = std::max(Expected, N.getMinSignedBits());
+    });
+    EXPECT_EQ(Expected, Known.countMaxSignedBits());
+  });
+}
+
 TEST(KnownBitsTest, SExtOrTrunc) {
   const unsigned NarrowerSize = 4;
   const unsigned BaseSize = 6;