diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -1458,10 +1458,8 @@
   /// uint64_t. The bitwidth must be <= 64 or the value must fit within a
   /// uint64_t. Otherwise an assertion will result.
   uint64_t getZExtValue() const {
-    if (isSingleWord()) {
-      assert(BitWidth && "zero width values not allowed");
+    if (isSingleWord())
       return U.VAL;
-    }
     assert(getActiveBits() <= 64 && "Too many bits for uint64_t");
     return U.pVal[0];
   }
diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
--- a/llvm/include/llvm/Support/MathExtras.h
+++ b/llvm/include/llvm/Support/MathExtras.h
@@ -757,34 +757,38 @@
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 32-bit integer.
-/// Requires 0 < B <= 32.
+/// Requires 0 <= B <= 32.  Sign extension of a zero-width value returns zero.
 template <unsigned B> constexpr inline int32_t SignExtend32(uint32_t X) {
-  static_assert(B > 0, "Bit width can't be 0.");
   static_assert(B <= 32, "Bit width out of range.");
+  if (!B)
+    return 0;
   return int32_t(X << (32 - B)) >> (32 - B);
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 32-bit integer.
-/// Requires 0 < B <= 32.
+/// Requires 0 <= B <= 32.  Sign extension of a zero-width value returns zero.
 inline int32_t SignExtend32(uint32_t X, unsigned B) {
-  assert(B > 0 && "Bit width can't be 0.");
   assert(B <= 32 && "Bit width out of range.");
+  if (!B)
+    return 0;
   return int32_t(X << (32 - B)) >> (32 - B);
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 64-bit integer.
-/// Requires 0 < B <= 64.
+/// Requires 0 <= B <= 64.  Sign extension of a zero-width value returns zero.
 template <unsigned B> constexpr inline int64_t SignExtend64(uint64_t x) {
-  static_assert(B > 0, "Bit width can't be 0.");
   static_assert(B <= 64, "Bit width out of range.");
+  if (!B)
+    return 0;
   return int64_t(x << (64 - B)) >> (64 - B);
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 64-bit integer.
-/// Requires 0 < B <= 64.
+/// Requires 0 <= B <= 64.  Sign extension of a zero-width value returns zero.
 inline int64_t SignExtend64(uint64_t X, unsigned B) {
-  assert(B > 0 && "Bit width can't be 0.");
   assert(B <= 64 && "Bit width out of range.");
+  if (!B)
+    return 0;
   return int64_t(X << (64 - B)) >> (64 - B);
 }