Fix required_memory_mb (Qiskit#1881)

* Fix required_memory_mb

* add release note

---------

Co-authored-by: Hiroshi Horii <hhorii@users.noreply.github.com>

releasenotes/notes/fix_required_memory_mb-7aeafa0fe553b85a.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    requried_memory_mb function for statevector returns wrong value when number
+    of qubits is very large because of overflow of 64 bits integer.
+    Now it returns SIZE_MAX value when number of qubits is too large so that
+    Qiskit Aer can know memory is not sufficient for
+    statvector/unitary/density matrix methods.

src/simulators/density_matrix/densitymatrix_state.hpp

@@ -391,7 +391,6 @@ template <class densmat_t>
 size_t State<densmat_t>::required_memory_mb(
     uint_t num_qubits, const std::vector<Operations::Op> &ops) const {
   (void)ops; // avoid unused variable compiler warning
-  (void)ops; // avoid unused variable compiler warning
   densmat_t tmp;
   return tmp.required_memory_mb(2 * num_qubits);
 }

src/simulators/statevector/qubitvector.hpp

@@ -971,6 +971,8 @@ size_t QubitVector<data_t>::required_memory_mb(uint_t num_qubits) const {
 
   size_t unit = std::log2(sizeof(std::complex<data_t>));
   size_t shift_mb = std::max<int_t>(0, num_qubits + unit - 20);
+  if (shift_mb >= 63)
+    return SIZE_MAX;
   size_t mem_mb = 1ULL << shift_mb;
   return mem_mb;
 }

src/simulators/statevector/qubitvector_thrust.hpp

@@ -959,8 +959,9 @@ size_t QubitVectorThrust<data_t>::required_memory_mb(uint_t num_qubits) const {
 
   size_t unit = std::log2(sizeof(std::complex<data_t>));
   size_t shift_mb = std::max<int_t>(0, num_qubits + unit - 20);
+  if (shift_mb >= 63)
+    return SIZE_MAX;
   size_t mem_mb = 1ULL << shift_mb;
-
   return mem_mb;
 }
 

src/simulators/unitary/unitary_state.hpp

@@ -321,12 +321,9 @@ void State<unitary_matrix_t>::apply_op(const Operations::Op &op,
 template <class unitary_matrix_t>
 size_t State<unitary_matrix_t>::required_memory_mb(
     uint_t num_qubits, const std::vector<Operations::Op> &ops) const {
-  // An n-qubit unitary as 2^2n complex doubles
-  // where each complex double is 16 bytes
   (void)ops; // avoid unused variable compiler warning
-  size_t shift_mb = std::max<int_t>(0, num_qubits + 4 - 20);
-  size_t mem_mb = 1ULL << (2 * shift_mb);
-  return mem_mb;
+  unitary_matrix_t tmp;
+  return tmp.required_memory_mb(2 * num_qubits);
 }
 
 template <class unitary_matrix_t>